Remove [Courses] from lesson names/post subjects

[mbox2moodle.git] / source / perl

From alice@schiffwood.demon.co.uk  Mon Feb 10 03:57:43 2003
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From: Alice Wood <alice@schiffwood.demon.co.uk>
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On Saturday 08 February 2003 16:44, Sachin wrote:
> Dear mam,
>                                                     It seems that lots =
of
> people are interested in learning it(which includes mine).Exactly when =
are
> you going to start.
>
>
> Sachin.R.Athreya

Wow!!!

I cannot believe the response my post has had. I would like to thank ever=
yone=20
who has offered help. So yes I will be going ahead with this. I would lik=
e to=20
start on March 1st, and post lessons/exercises every second Saturday. I=20
realise that some people will think this is a little slow at the beginnin=
g. I=20
tend to agree, but given that the course will get more difficult and peop=
le=20
cannot always make weekly deadlines, two weeks seems more practical. Also=
, I=20
want to start March 1st as I haven't written anything yet and this gives =
time=20
to sort things out.

I would like to use Learning Perl as the course book (ISBN:0-596-00132-0)=
,=20
this is generally known as the Llama book, and Randal Schwartz has given=20
permission to use bits and pieces out of this. If people do not want to g=
o to=20
the expense of buying a book, there is information online at=20
http://learn.perl.org where there is a downloadable version of Beginning =
Perl=20
which should be useful (note this is not the same book as Learning Perl),=
=20
also there are notes at http://www.perltraining.com.au. which are very=20
helpful. It is not absolutely necessary to have the Learning Perl book, b=
ut=20
if you haven't got any material at all, this is where I would start.

My aim is to send an email to the courses list every two weeks with a=20
tutorial, exercises and useful links in it. I will also hope to post to a=
=20
permanent site, at the same time, an HTML version which may be slightly=20
expanded, so if you miss a post it should be easy to catch up.

Any comments / questions please to me at the addy above, otherwise I'll s=
ee=20
you bright and early March 1st for the first lesson.

Best wishes,

Alice Wood

From admin@surgo.net  Mon Feb 10 04:04:03 2003
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I'm definately interested in learning. I was supposed to get the llama book 
for Xmas, but the post office lost it *mutter*.

Morgon
-- 
grab my public key from http://www.surgo.net/pubkey.asc



From suzo@bigpond.net.au  Sun Feb 23 15:54:33 2003
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From: "Sue Stones" <suzo@bigpond.net.au>
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How much difference is there between the 2nd and 3rd editions?

sue


----- Original Message -----
From: "Alice Wood" <alice@schiffwood.demon.co.uk>
To: <programming@linuxchix.org>; <courses@linuxchix.org>
Sent: Monday, February 10, 2003 3:51 AM
Subject: Re: [prog] Perl course


On Saturday 08 February 2003 16:44, Sachin wrote:
> Dear mam,
>                                                     It seems that lots of
> people are interested in learning it(which includes mine).Exactly when are
> you going to start.
>
>
> Sachin.R.Athreya

Wow!!!

I cannot believe the response my post has had. I would like to thank
everyone
who has offered help. So yes I will be going ahead with this. I would like
to
start on March 1st, and post lessons/exercises every second Saturday. I
realise that some people will think this is a little slow at the beginning.
I
tend to agree, but given that the course will get more difficult and people
cannot always make weekly deadlines, two weeks seems more practical. Also, I
want to start March 1st as I haven't written anything yet and this gives
time
to sort things out.

I would like to use Learning Perl as the course book (ISBN:0-596-00132-0),
this is generally known as the Llama book, and Randal Schwartz has given
permission to use bits and pieces out of this. If people do not want to go
to
the expense of buying a book, there is information online at
http://learn.perl.org where there is a downloadable version of Beginning
Perl
which should be useful (note this is not the same book as Learning Perl),
also there are notes at http://www.perltraining.com.au. which are very
helpful. It is not absolutely necessary to have the Learning Perl book, but
if you haven't got any material at all, this is where I would start.

My aim is to send an email to the courses list every two weeks with a
tutorial, exercises and useful links in it. I will also hope to post to a
permanent site, at the same time, an HTML version which may be slightly
expanded, so if you miss a post it should be easy to catch up.

Any comments / questions please to me at the addy above, otherwise I'll see
you bright and early March 1st for the first lesson.

Best wishes,

Alice Wood
_______________________________________________
Programming mailing list
Programming@linuxchix.org
http://mailman.linuxchix.org/mailman/listinfo/programming




From alice@schiffwood.demon.co.uk  Mon Feb 24 07:26:53 2003
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Subject: Re: [Courses] Re: [prog] Perl course
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On Sunday 23 February 2003 04:54, Sue Stones wrote:
> How much difference is there between the 2nd and 3rd editions?
>
> sue

Hi,

Sorry it has taken so long to reply to you, I got too involved in my=20
decorating.

The answer to the above question seems to be quite a lot. I have an elect=
ronic=20
version of 2 and a paper version of 3. Below is a summary of the differen=
ces=20
as noted in the preface to 3.

Completely new text, taken from teaching the Perl course (it has to be sa=
id=20
that the text does occasionally suffer from a lack of clear explanation, =
2=20
may help with this, along with LinuxChix)

The regular expressions section has been split into three parts to make i=
t=20
more managable.

The artificial control structures chapter has gone and parts have been=20
incorporated into other chapters.

Subroutines are brought in earlier.

Element syntax is before aggregate syntax for both arrays and hashes.

The exercises are different, although not for the first couple of chapter=
s.

Information is included on use strict, warnings and modules.

Book 3 is also address more to general programmers which tends to make it=
 more=20
friendly.

Hope this helps.

I will be basing the course on book 3 obviously, but as I have access to =
a=20
book 2 I will try to flag up different pages if I can.

See you Saturday,

Alice

--=20
Alice Wood

Project Gutenberg newsletter volunteer

From giljacobs@woza.co.za  Tue Feb 11 21:49:40 2003
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I'm looking forward to it! Just bought the "Llama" book.

Gil jacobs

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<DIV><FONT face=3DArial size=3D2>I'm looking forward to it! Just bought =
the "Llama"=20
book.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2></FONT>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>Gil jacobs</FONT></DIV><BR><PRE>
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From jscalf@pegasus.rutgers.edu  Sat Feb 15 02:21:29 2003
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From: Jennifer Scalf <jscalf@pegasus.rutgers.edu>
Cc: Courses@linuxchix.org
Subject: Re: [Courses] Perl Course
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I also just got the llama book and will be watching for your course.

Great idea!

Jennifer

On Tue, 11 Feb 2003, Gil Jacobs wrote:

> I'm looking forward to it! Just bought the "Llama" book.
>
> Gil jacobs
>
> This email was scanned by the ICL anti-virus solution


From alice@schiffwood.demon.co.uk  Sat Mar  1 22:47:46 2003
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From: Alice Wood <alice@schiffwood.demon.co.uk>
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Subject: [Courses] Perl course part 1
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LinuxChix Perl Course Part 1: Getting Started

Contents

1) Shebang/Hashbang line
2) Documentation and help
3) Exercises
4) Where to find out more...

            -----------------------------------

1) Shebang/Hashbang line

# (aka "octothorp", or "sharp" or "pound sign" or "hash")
normally begins a comment.  The comment is from the #
to the end of the line.

#i.e. This would be a single-line comment.
   #So is this
<code goes here>#And so is this.

=3D comment

This is
a
multi-line comment

=3Dcut


There is one exception to this comment assignment and that is
when the comment is followed immediately by a ! ( aka
"exclamation mark", or "bang").  The combination #! is known as
the "she-bang" (a contraction of sharp-bang)or "hash-bang" and it
is the top line of the script in both Perl and shell scripts.
It tells the interpreter where to find the program that should
be used for interpreting this script.

To find out what your she-bang line should read, you need to go to
the command line and type "whence perl" (korn shell), "type
perl" or "which perl" (bash).

Typing "type perl" into my command line gives me

perl is /usr/bin/perl

Then create your she-bang line accordingly:

#!/usr/bin/perl
or
#!/usr/bin/local/perl

This means that I need to use #!/usr/bin/perl as my top line.

Notice that, unlike other lines of Perl code, this one does NOT
end in a semi-colon.

           -----------------------------------------

2) Documentation and help

There are various methods of getting help with Perl, two of them are
supplied with Perl on your machine. The first is the series of 'man' page=
s.=20
Typing "man perl" on the command line or in a terminal with give you a 30=
2=20
line
introduction page with help on how to get to various types of information
The second is the 'perldoc' series which can be accessed by typing=20
"perldoc perldoc". Perldoc is aimed at helping with functions and modules=
=2E
To start the 'man' series is probably more useful, and can be helpful rea=
ding.
However, this is not for the faint-hearted as they are extremely long.

Updated versions and other documentation can be found at
http://www.perldoc.com

Note: Some Linux installations come with tkman, this is a browser style
version of the man pages.

           -----------------------------------------

3) Exercises

1) Try "type perl" in your command line to establish what your top line
of code should be.

2) Use your answer to the above to try the traditional Hello World progra=
m.
Type the following in using any text editor. When you come to save the pr=
ogram=20
you will need to save it with .pl as the extension.

e.g.

<hello.pl>

#!/usr/bin/perl
print "Hello, world!\n";

To run your program type

perl <programname.pl>

3) Use the man pages to find out what Perl stands for.


          -----------------------------------------

4) Where to find out more...

An archive of lessons is being created at

http://www.schiffwood.demon.co.uk/index.html

Here there are links to useful websites and other books.

Part 2 of the course will follow on 15th March 2003
          -----------------------------------------

Note: The author uses Perl version 5.6.1. All exercises should therefore=20
work on later versions. If you wish to find out which version of Perl you=
 are
running type "perl -v" into your command line.

If you have any comments regarding this perl lesson, suggestions for futu=
re=20
topics, useful website addresses, useful reference material or you just w=
ant=20
to say what you thought, please send your email to=20

alice at schiffwood.demon.co.uk

I will be pleased to hear from you.

Credits and acknowledgements

Thanks to Meryll Larkin and Jacinta Richardson for their help with puttin=
g
together this material.=20
Sources of material include Learning Perl, Randal Schwartx & Tom Phoenix,
& Perl Training Australia (http://www.perltraining.com.au)

From rachel@xtreme.com  Sat Mar 15 09:20:33 2003
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From: "Rachel McConnell" <rachel@xtreme.com>
To: <courses@linuxchix.org>
Subject: RE: [Courses] Perl course part 1
Date: Fri, 14 Mar 2003 14:18:35 -0800
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Hello Perl Course Teacher and Fellow Students,

I have done my part 1 coursework just in time for part 2, and I'm very
excited about moving forward.  I have a perl script that prints, not
merely Hello World, but "Hello world from my badass self!" - a much
superior phrasing, as you will all agree.  I won't post it unless asked,
I'm sure everyone else has one just like it.

However I couldn't find what P E R L stood for on the man pages -  There
are a LOT of man pages... however I found, in perlfaq1:

"never write 'PERL', because perl isn't really an acronym, apocryphal
folklore and post-facto expansions notwithstanding."

So i cheated & googled it.  the post-facto expansion i found was,
Practical Expression and Reporting Language.  anyone find something
else?

Teacher, I ran into discussions of CPAN which appears to be something
like rpm's, only for just perl... will you discuss CPAN at some point
where appropriate?

Rachel

PS - I haven't taken a course on this list before (and haven't checked
the archives, Bad Me!) so am not sure if it's Entirely Appropriate to
post this message, but what the hell!

From sarah@ph34rbob.org  Sat Mar 15 09:37:52 2003
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Subject: RE: [Courses] [Perl] course part 1
From: Sarah Huffman <sarah@ph34rbob.org>
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On Fri, 2003-03-14 at 14:18, Rachel McConnell wrote:

> However I couldn't find what P E R L stood for on the man pages -  There
> are a LOT of man pages... however I found, in perlfaq1:

Larry Wall has something to say about it, which can be found here-
http://www.linuxjournal.com/article.php?sid=3394.

-Sarah


From alice@schiffwood.demon.co.uk  Sat Mar  1 22:47:45 2003
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Subject: [Courses] Perl course overview
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Hell o everyone,

Welcome to the Perl course. You should receive two mails today, this one =
as an=20
introduction and a second containing part one of the course. I am writing=
=20
this to give you an idea of how the course will (hopefully) plan out  and=
=20
what I hope to do in future installments.

The course will roughly follow the Learning Perl book, and I will refer t=
o=20
that where appropriate in the mails I send. The first few lessons are pla=
nned=20
as:

1) Getting started
2) Scalar data and chomp
3) Control structures including if and while
4) Arrays: foreach
5) Arrays: reverse and sort

Given the amount of material we have to cover this may extend.

I am publishing the lessons on http://www.schiffwood.demon.co.uk as they =
are=20
put onto the mailing list and there should be a mirror on linuxchix soon.=
 The=20
website also includes helpful websites and a list of useful books is also=
=20
developing, if you have any suggestions then please put them forward.

Please feel free to let me know your thoughts on the course. As the exerc=
ises=20
develop I hope to include different versions of answers in the lessons, s=
o if=20
you come up with something different then send it in. I look forward to=20
getting to know everyone during the course.

Best wishes,

Alice
--=20
Alice Wood

Project Gutenberg newsletter editor

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Subject: RE: [Courses] [Perl] course part 1
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On Fri, 2003-03-14 at 14:18, Rachel McConnell wrote:

> However I couldn't find what P E R L stood for on the man pages -  There
> are a LOT of man pages... however I found, in perlfaq1:

Larry Wall has something to say about it, which can be found here-
http://www.linuxjournal.com/article.php?sid=3394.

-Sarah


From alice@schiffwood.demon.co.uk  Sat Mar 15 23:04:09 2003
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From: Alice Wood <alice@schiffwood.demon.co.uk>
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Subject: [Courses] LinuxChix Perl Course Part 2
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LinuxChix Perl Course Part 2: Scalar Data

Contents

1) Good programming guide
2) Scalar data
3) Exercises
4) Where to find out more...

            -----------------------------------

1) Good Programming Guide

comments
white space
-w switch
use strict

As beginners it is always a good idea to try and start off with good habi=
ts so=20
that as we become more experienced, we are regularly building into a prog=
ram=20
pointers to help us when we have a problem. Good programs will be=20
appropriately commented and have lots of white space (gaps between sectio=
ns,=20
and longer lines).

It is also possible to use tools that are built into Perl to help you spo=
t=20
potential problems. The first is an addition to the top line.

#!usr/bin/perl -w

The -w switch allows Perl to print warnings if the variables in programs =
are=20
_probably_ not being used correctly, if there is a function infinitely de=
ep=20
in recursion or when it spots something suspicious in the program. There =
are=20
many switches available for use in Perl. further information about these =
can=20
be found on the Perlrun manpage.

use strict;

It is useful to put this in as the second line to a program (see below).=20

#!usr/bin/perl -w
use strict;

This will enforce good programming rules throughout your program. For exa=
mple,=20
if you declare

my $suzy =3D 7;

then later do

$Suzy +=3D1;

Your mistake will be caught automatically.

            ----------------------------------

2) Scalar Data

There are four basic data types that Perl uses.
These are:
scalars - A single chunk of data
arrays - A bunch of scalars, indexed by a number
hashes - A bunch of scalars, indexed by another scalar known as a key
handles - A pointer enabling the opening of resources from the operating=20
system (files, directories, etc.)

A scalar is denoted by $. It may be used to represent integers, strings,=20
floats, any block of information. The syntax for creating a scalar is

$variable =3D <value>;

Where the value may be numeric, string, reference or boolean.
There is no size restriction on a scalar, so you may make your scalar as =
large=20
as necessary. Perl is similar to shell scripting when assigning values to=
 a=20
variable.

Below are some examples of scalars

$scalar1 =3D 'this is a string';=09# simple scalar assigned 'this is a st=
ring'
$value =3D 10.0;=09=09=09# simple scalar assigned 10 (number is rounded)
$value2 =3D '10.0'=09=09# number that acts as a string, number is not rou=
nded
$scalar2 =3D "this is $value";=09# interpolation, assigned 'this is 10.0'
$scalar3 =3D 'this is $value';=09# non-interpolation, assigned 'this is $=
value'
$empty =3D '';=09=09=09# empty string

Note the use of quotes here. The single quote (') indicates that the text=
 will=20
be used verbatim and the double quote (") indicates the text will be=20
interpreted.
It is possible to operate on scalars.
For example

$suzy =3D $luigi + 4;

$suzy =3D $suzy - 3; # alternatively, $suzy -=3D 3;

The scalars may also be operated on as strings.
For example

$suzy =3D $suzy x 3; # this will print suzysuzysuzy

$suzy =3D $suzy . 'space'; # this will append 'space' to the string held =
in=20
$suzy.

            ---------------------------------

3) Exercises

a) Create a program 'newline.pl' containing the following:

print"Hi there.\nThis is my second program.\n"

Run this and then replace the \n with a space or an Enter and compare the=
=20
results.

What is the purpose of \n?

b) Write a program called 'average.pl' to calculate the average of the=20
following numbers:

23, 28, 31, 17, 1

(Remember an average is where you add the numbers together and divide by =
the=20
amount you have, this is learning perl not maths 101! Although, the maths=
 is=20
normally the hardest part.)

c) Modify the program called 'repetition.pl' to work more efficiently.=20
(Remember there's more than one way to do it!)

!#usr/bin/perl -w
use strict;

print("Suzy\n");
print("Suzy\n");
print("Suzy\n");
print("Suzy\n");
print("Suzy\n")

            ----------------------------------

Answers to previous exercises

1) You should have found that your top line was something like as stated =
in=20
part 1 of the lesson.

2) 'Hello World' should have appeared on your screen.

3) Practical Extraction and Report Language, or Pathologically Eclectic=20
Rubbish Lister depending on your point of view.

            -----------------------------------

4) Where to find out more...

An archive of lessons is being created at

http://www.schiffwood.demon.co.uk/index.html

Also, there are links to useful websites and other books.

The archive should appear on the LinuxChix website soon.

Part 3 of the course will follow on 29th March 2003

          -----------------------------------------

Note: The author uses Perl version 5.6.1. All exercises should therefore=20
work on later versions. If you wish to find out which version of Perl you=
 are
running type "perl -v" into your command line.

If you have any comments regarding this perl lesson, suggestions for futu=
re=20
topics, useful website addresses, useful reference material or you just w=
ant=20
to say what you thought, please send your email to=20

alice at schiffwood.demon.co.uk

I will be pleased to hear from you.

Credits and acknowledgements

Thanks to Meryll Larkin and Jacinta Richardson for their help with puttin=
g
together this material.=20
Sources of material include Learning Perl, Randal Schwartx & Tom Phoenix,
Perl Training Australia (http://www.perltraining.com.au),
Perl 5 Developer's Guide, Peschko & DeWolfe

Suzy is the name of my cat.

--=20
Alice Wood

Project Gutenberg newsletter editor
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Date: Sat, 15 Mar 2003 11:35:39 -0500
From: Morgon Kanter <morgon@surgo.net>
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Subject: [Courses] [Perl] A bit of clarification on what Alice said about
        quotes
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> Note the use of quotes here. The single quote (') indicates that the text will 
> be used verbatim and the double quote (") indicates the text will be 
> interpreted.

This would be a bit confusing for me, so let me clarify.

A string enclosed ' ' means it is literally what you put it. Any character 
other than a single quote or a backslash-single-quote (\') stands for itself 
inside of the string.

$var = ' '      # A string with one space
$var = '\''     # A string with a single quote inside of it
$var = 'yo\n'   # A string with yo followed by a \n
$var = 'te
st'             # A string that reads te NEWLINE st

Now on the other hand, in a double-quoted string, you can use escape characters 
etc like in C and other languages. Here is a brief list.

\n      # Newline
\r      # Return
\t      # Tab
\f      # Form feed (AKA page breaks on some printers)
\b      # Backspace
\a      # Rings the bell!
\e      # Escape (think of the escape key on your keyboard)
\\      # Backslash
\"      # Double quotes
\'      # Single quote in a single-quoted string
\l      # Lowercase next letter
\L      # Lowercase all the following letters until a \E
\u      # Uppercase next letter
\U      # Same as \L but uppercases
\Q      # Quote non-word characters by adding a \ until a \E is read
\E      # Terminates \L, \U and \Q
\xyz    # Any character's octal value
\xFF    # Any character's hex value
\cC     # A so-called "control" character, in this example ^C (ctrl-C)

so...
$var = "yo"     # The string reads yo
$var = "yo\n"   # The string reads yo NEWLINE

I hope that cleared some things up for any that were confused

Morgon
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Date: Sat, 15 Mar 2003 11:38:39 -0500
From: Morgon Kanter <morgon@surgo.net>
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1) As I said in my previous post, its to put a newline on the screen

2)
$var = 23 + 28 + 31 + 17 + 1;
$var /= 5;
print $var . "\n";

3)
print "Suzy\n" x 5;

That's all folks
Morgon
From laurel@sdf.lonestar.org  Tue Mar 18 05:05:32 2003
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From: Laurel Fan <laurel@sdf.lonestar.org>
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Subject: Re: [Courses] [Perl] exercise answers...
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On the subject of 'verbatim' and "interpreted" strings:

On Sat, Mar 15, 2003 at 11:38:39AM -0500, Morgon Kanter wrote:
> 2)
> $var = 23 + 28 + 31 + 17 + 1;
> $var /= 5;
> print $var . "\n";

Here you could also do:

print "$var\n";

because $variables within "double quote strings" are also interpreted.

-- 
laurel@sdf.lonestar.org
http://dreadnought.gorgorg.org
From alice@schiffwood.demon.co.uk  Sat Mar 29 07:52:01 2003
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Hi,

Due to a small family emergency I am nowhere near my usual computer at th=
e=20
moment and so will be unable to post this weeks lesson / episode. I will =
post=20
the lesson as soon as I am able to do so, and will aim to be back on trac=
k=20
April 12th.

Many apologies

Alice
--=20
Alice Wood

Project Gutenberg newsletter editor
From alice@schiffwood.co.uk  Sat Mar 29 06:33:56 2003
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Hi,

Due to a small family emergency I am nowhere near my usual computer at th=
e=20
moment and so will be unable to post this weeks lesson / episode. I will =
post=20
the lesson as soon as I am able to do so, and will aim to be back on trac=
k=20
April 12th.

Many apologies

Alice
--=20
Alice Wood

Project Gutenberg newsletter editor
From suzo@bigpond.net.au  Sun Mar 30 18:55:23 2003
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From: Sue Stones <suzo@bigpond.net.au>
Organization: Vyette's House
To: alice@schiffwood.demon.co.uk, courses@linuxchix.org
Subject: Re: [Courses] [Perl] Important Information
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That's cool.

sue

On Sat, 29 Mar 2003 07:52 am, Alice Wood wrote:
> Hi,
>
> Due to a small family emergency I am nowhere near my usual computer at the
> moment and so will be unable to post this weeks lesson / episode. I will
> post the lesson as soon as I am able to do so, and will aim to be back on
> track April 12th.
>
> Many apologies
>
> Alice

From jscalf@pegasus.rutgers.edu  Mon Mar 31 23:57:33 2003
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From: Jennifer Scalf <jscalf@pegasus.rutgers.edu>
To: Alice Wood <alice@schiffwood.co.uk>
Subject: Re: [Courses] [Perl] Important Information
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It's ok, just take care of what you need to we'll still be here :) I hope
everything is ok soon.

Jennifer

On Fri, 28 Mar 2003, Alice Wood wrote:

> Hi,
>
> Due to a small family emergency I am nowhere near my usual computer at the
> moment and so will be unable to post this weeks lesson / episode. I will post
> the lesson as soon as I am able to do so, and will aim to be back on track
> April 12th.
>
> Many apologies
>
> Alice
> --
> Alice Wood
>
> Project Gutenberg newsletter editor
> _______________________________________________
> Courses mailing list
> Courses@linuxchix.org
> http://mailman.linuxchix.org/mailman/listinfo/courses
>

From alice@schiffwood.co.uk  Sat Apr 12 23:53:46 2003
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From: Alice Wood <alice@schiffwood.co.uk>
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Subject: [Courses] [Perl] Perl course part 3
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LinuxChix Perl Course Part 3: User Input, chomp and chop.

Contents

1) User input
2) Chomp and chop
3) Exercises
4) Where to find out more...

           -----------------------------------------

1) User input from the keyboard:

The line input operator <>, (also known as the diamond operator) allows t=
he=20
program to read data (or even
futher code) from file handles.  In particular the special filehandle
STDIN can be used to read input from the keyboard.  For example:

$line =3D <STDIN>;

Each time we use this construct the next complete line of text is read in=
=2E
The definition of a "complete line of text" depends on the value of $/
which is newline by default. Read the INPUT_RECORD_SEPARATOR
section of "perldoc perlvar" for more information about $/.

Example:

Type the following into a file called mycat.pl

#!/usr/bin/perl -w
use strict;

my $line =3D <STDIN>;
print "You wrote: $line";


and execute it from the command line:

mylogin$  ./mycat.pl

You'll notice that the program "hangs" rather that printing and then
returning. This means it is waiting for some user input. Type
something and then press enter and see what happens.

mylogin$  ./mycat.pl
Let me add some input
You wrote: Let me add some input
mylogin$

           -----------------------------------------

2) Chomp

This is a command that is worth learning early as it is so useful
later on. Chomp can be performed on both scalar variables (ones starting
with $ signs) and lists. When used, chomp removes any trailing string
that corresponds to the current value of $/ (also known as the
INPUT_RECORD_SEPARATOR which you can read more about in "perldoc
perlvar"). Usually $/ is set to newline, but we can change it if we
want to.

Chomp is often used to remove the trailing charadcter from input you've
just read in. This is best demonstrated with an example:

Type the following into a file called mycat.pl

                #!/usr/bin/perl -w
                use strict;

                my $line =3D <STDIN>;
                print "You wrote: [$line]\n";


and execute it from the command line, you should get something like:

                mylogin$  ./mycat.pl
                Let me add some input
                You wrote: [Let me add some input
=09=09]
                mylogin$


As you can see when we read in the line, we read in the newline as well. =
We
can remove this newline by using chomp:

Change your program so that it reads as follows:

                #!/usr/bin/perl -w
                use strict;

                my $line =3D <STDIN>;
=09=09chomp $line;
                print "You wrote: [$line]\n";


and execute it from the command line, this time you should get:

                mylogin$  ./mycat.pl
                Let me add some input
                You wrote: [Let me add some input]
                mylogin$


"chomp" is also called the "safe chop"

chop will remove any single final character.
chomp will remove only a new line character (i.e. the enter from
the keyboard).

Also, this is the common usage of chomp when it is combined with
accepting the input in a single line of code:

    chomp(my $input =3D <STDIN>);


           -----------------------------------------
3) Exercises

a) Try adding the following line to your program, before my $line =3D <ST=
DIN=3D
>;

$/ ",";

What happens now?  Try entering a line without a comma in it followed by =
one
with a comma in it but not as the last character.  Can you explain what i=
s
happening?

b) Write a currency conversion program to calculate the exchange value of=
=20
100GBP in your
local currency (or if you are in the UK, use the Euro exchange
rate). your program should ask for the exchange rate.

Answers: All missing answers will be sorted out next time and also
published via the website.

           -----------------------------------------

4) Where to find out more...

An archive of lessons is being created at

http://www.schiffwood.demon.co.uk/index.html

Here there are links to useful websites and other books.

Part 4 of the course will follow on 26th April 2003
          -----------------------------------------

Note: The author uses Perl version 5.6.1. All exercises should therefore=20
work on later versions. If you wish to find out which version of Perl you=
 are
running type "perl -v" into your command line.

If you have any comments regarding this Perl lesson, suggestions for futu=
re=20
topics, useful website addresses, useful reference material or you just w=
ant=20
to say what you thought, please send your email to=20

alice at schiffwood.co.uk

I will be pleased to hear from you.

Credits and acknowledgements

Sources of material include:
Learning Perl, Randal Schwartx & Tom Phoenix,
Perl Training Australia (http://www.perltraining.com.au)
Perl 5 Developer's guide, Peschko & DeWolfe
http://learn.perl.org
With very great thanks to Jacinta Richardson and Meryll Larkin for the
course help, and Trish Fraser for the webpages and consumer testing.
--=20
Alice Wood
Project Gutenberg newsletter editor
From morgon@surgo.net  Sun Apr 13 12:14:30 2003
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Date: Sat, 12 Apr 2003 22:13:42 -0400
From: Morgon Kanter <morgon@surgo.net>
To: courses@linuxchix.org
Subject: Re: [Courses] [Perl] Perl course part 3
Message-Id: <20030412221342.6c8a8a3f.morgon@surgo.net>
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> a) Try adding the following line to your program, before my $line = <STDIN=>;
> 
> $/ ",";

I suppose you mean $/ = ","; because above won't work without the 
assignment operator

-- 
Morgon Kanter <morgon@surgo.net> http://www.surgo.net
GPG key ID: 297CEA5B
Please don't CC me on mailing lists, I read them!
From jarich@perltraining.com.au  Sun Apr 13 15:59:38 2003
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Date: Sun, 13 Apr 2003 15:59:36 +1000 (EST)
From: Jacinta Richardson <jarich@perltraining.com.au>
To: Alice Wood <alice@schiffwood.co.uk>
Subject: Re: [Courses] [Perl] Perl course part 3 (some corrections)
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On Sun, 13 Apr 2003, Alice Wood wrote:

A very good part of a Perl course, but in need of a minor correction or
two..

> LinuxChix Perl Course Part 3: User Input, chomp and chop.

> 2) Chomp
....
> "chomp" is also called the "safe chop"
> 
> chop will remove any single final character.
> chomp will remove only a new line character (i.e. the enter from
> the keyboard).

Chomp will remove any string specified in $/.  Hence if $/ were equal to
"this", then:

$/ = "this";
$fred = "let's say we have a string like this";
chomp $fred;
print $fred;

would print "let's say we have a string like ";  

The key is, that if $/ is set to "\n" as it is by default then calling
chomp on our string would ONLY remove the trailing newline, if it
existed.  If there is not trailing newline, then chomp will remove
nothing.  Chomp is not restricited to only removing a single character,
neither is $/ restricted to being set to a single character.

....

> 3) Exercises
> 
> a) Try adding the following line to your program, before my $line = <STDIN=
> >;
> 
> $/ ",";

This most certainly should be:
$/ = ",";

note the missing = character.




Hope they help.

        Jacinta

--
   ("`-''-/").___..--''"`-._          |  Jacinta Richardson         |
    `6_ 6  )   `-.  (     ).`-.__.`)  |  Perl Training Australia    |
    (_Y_.)'  ._   )  `._ `. ``-..-'   |      +613 9354 6001         |  
  _..`--'_..-_/  /--'_.' ,'           | contact@perltraining.com.au |
(il),-''  (li),'  ((!.-'              |   www.perltraining.com.au   |

From alice@schiffwood.co.uk  Tue Apr 29 04:46:51 2003
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From: Alice Wood <alice@schiffwood.co.uk>
To: courses@linuxchix.org
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Subject: [Courses] [Perl] Perl course Part 4
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LinuxChix Perl Course Part 4: Control structures - If, else, while.

Contents

1) If, else
2) While
3) Exercises
4) Where to find out more...

           -----------------------------------------

1) If, else

Conditional statements in Perl tell your program which way to proceed
through your code.

One of the most useful (along with the for loop) is the if statement.

The basic setup for an if statement is

 if (conditional statement 1) {  #Note the curly bracket here

=09=09=09    <Action code>
}

Example:

 if ($money <0){
=09=09print "You are overdrawn!\n";
}


The code can be left in this manner, if the condition is not met then
the program will simply ignore that particular block of code.


An alternative to this could be.

if (conditional statement 1) {

=09=09=09    <Action code 1>

} if (conditional statement 2){

     <Action code 2>

} if (conditional statement 3){

     <Action code 3>

}


An alternative block can be added to the code as follows

if (conditional statement 1){

=09=09=09=09<Action code 1>
} else {

=09<Action code 2>

}

 if ($money < 0) {
=09=09 print "You are overdrawn!"\n;
} else {
       print "You are in credit."\n;
}


# There is an elsif command which may also be used in this situation,
 we will meet that later.

           -----------------------------------------

2) While

A while loop will repeat a block of code when a condition is true.

  while (conditional statement){

=09=09=09=09Block of code

}

Example #$money must be greater than 0 to begin with for this example to =
work.

 while ($money > 0){
=09=09=09print"You are in credit.\n";
=09=09=09$money--; # -- is the autodecrement
}

#If we are testing for something negative, than the unless condition
may be more helpful, we will meet this later.


The importance of the use brackets to denote blocks of code cannot be
over emphasised here.=20

In C this is acceptable:


  if condition do_this();


in Perl, you must do:

  if condition {do_this();}


The only change to this would be where the block is a single line, where =
you=20
could have:

  do_this() if $condition;

           -----------------------------------------

3) Exercise

Write a program that will ask the user to guess a number. The number
will be predefined at the beginning of your program.

Example

#!usr/bin/perl -w
use strict;

my $target =3D 17;

#Rest of your program

If you get into trouble please feel free to mail me for hints.
---------------------------

Answers to previous exercises

Part 2

a) The purpose of \n is to force a newline. (The game is given away by
the program title)

b) Here is a version of average.pl

#!/usr/bin/perl -w
use strict;

my $sum =3D 23+28+17+31+1;

my $average =3D $sum/5;

print "The average of the five numbers is: $average\n";

# Note: This is not the most efficient program that could be written,
  however I have tried to show the steps involved.

c) This is a version of repetition.pl

#!/usr/bin/perl -w
use strict;

print "Suzy\n" x 6;

All futher answers will be published via the website by the first
weekend in May.

           -----------------------------------------

4) Where to find out more...

An archive of lessons is being created at

http://www.tyneside.pm.org/courseindex.html

Here there are links to useful websites and other books.

Part 5 of the course will follow on 10th May 2003
          -----------------------------------------

Note: The author uses Perl version 5.6.1. All exercises should therefore=20
work on later versions. If you wish to find out which version of Perl you=
 are
running type "perl -v" into your command line.

If you have any comments regarding this Perl lesson, suggestions for futu=
re=20
topics, useful website addresses, useful reference material or you just w=
ant=20
to say what you thought, please send your email to=20

alice at schiffwood dot co dot uk

I will be pleased to hear from you.

Credits and acknowledgements

Sources of material include:
Learning Perl, Randal Schwartx & Tom Phoenix,
Perl Training Australia (http://www.perltraining.com.au)
Perl 5 Developer's guide, Peschko & DeWolfe
http://learn.perl.org
With very great thanks to Jacinta Richardson and Meryll Larkin for the
course help, and Trish Fraser for the webpages and consumer testing.
--=20
Alice Wood
Project Gutenberg newsletter editor
From morgon@surgo.net  Tue Apr 29 05:45:41 2003
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Date: Mon, 28 Apr 2003 15:42:42 -0400
From: Morgon Kanter <morgon@surgo.net>
To: courses@linuxchix.org
Subject: Re: [Courses] [Perl] Perl course Part 4
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This one time, at band camp, Alice Wood <alice@schiffwood.co.uk> wrote:
> 3) Exercise
> 
> Write a program that will ask the user to guess a number. The number
> will be predefined at the beginning of your program.
> 
> Example
> 
> #!usr/bin/perl -w
> use strict;
> 
> my $target = 17;
> 
> #Rest of your program

Here's the answer (don't look down if you don't want to be spoiled)
--------------- begin number.pl ----------------
use strict;

my $number = 17;

while(1) {
        print "Enter your guess: ";
        my $guess = <STDIN>;
        if($guess == $number) {
                print "Correct!\n";
                exit 0;
        } else {
                print "Wrong, try again :(\n";
        }
}
------------- end number.pl ---------------
venthorn@surgo:~$ perl number.pl
Enter your guess: 4
Wrong, try again :(
Enter your guess: 5
Wrong, try again :(
Enter your guess: 17
Correct!
venthorn@surgo:~$

Morgon
--
You said homosexuals form a small percentage of the population.  So
do Jews.  Is that a reason to deny someone equality?
 - Richard Marceau
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LinuxChix Perl Course Part 4.5: Review

Since this course has encountered a three-month interuption, I thought we
should start back up with a refresher lesson. Refer back to the original
lessons if you get lost, or ask for help from the list.

Note that there's a little new material at the end.

             -----------------------------------

Contents

1) Getting Started
2) Scalar Data
3) User Input
4) Control structures
5) New Stuff
6) Exercises

             -----------------------------------

1) Getting Started       Complete Lesson:
         http://linuxchix.org/pipermail/courses/2003-March/001147.html

Your Perl files should start with a "shebang" line. It should look something
like this:

   #!/usr/bin/perl -w

The /usr/bin/perl is the location of the Perl interpreter (you should set it
to wherever Perl is on your system). The "-w" switch means "warn me if I do
something that looks like a mistake."

Although this technically comes from lesson two, I thought I would mention
here that the second line of your Perl programs should always be "use
strict;". This has a function similar to the "-w" switch, but "use strict;"
and "-w" cover different mistakes, so you should use them both.

Here is a Hello World program:

   #!/usr/bin/perl -w
   use strict;

   print "Hello, world!\n";

The "\n" at the end causes a newline to be printed, because "print" doesn't
automatically output a newline.

             -----------------------------------

2) Scalar Data       Complete Lesson:
         http://linuxchix.org/pipermail/courses/2003-March/001153.html

Now we're going to look at "scalar" variables. Scalar is the simplest type of
variable: it always refers to a single "thing", such as a number or a piece of
text (which can be multiple words). We'll see a more complicated type of
variable when we deal with arrays.

Scalar variables are preceded by a dollar sign, like this:

   my $foo;

The word "my" declares the variable. In other words, it says, "make this
variable magically come into existance". You should declare a variable before
using it. You can declare a variable and give it a value at the same time,
like this:

   my $foo = 'some text';      # Set $foo to be the words "some text".

Here we use strings, but we can also use numbers:

   my $minutes = 12;
   my $seconds = $minutes * 60;
   print $seconds;            # Output is 720.

Perl includes several operators on strings (text), including "."
(concatenation) and "x" (multiplication):

   my $text = 'abc';
   $text = $text . 'xyz';   # Now $text = 'abcxyz'
   my $longtext = $text x 3;
   # Now $longtext = 'abcxyzabcxyzabcxyz'

Don't use a plus sign to concatenate strings! The plus sign means numerical
addition only!

Single and double quotes have different meanings. Inside of double quotes,
variables (e.g., beginning with "$") and special characters (beginning with
"\") are interpreted (replaced with appropriate values). Inside of single
quotes, variables and "\" are not interpolated. (Actually, "\" is interpolated
ONLY if followed by a single quote or another "\".)

   my $value = 'fish';
   my $foo = "Eat $value\n";   # Result is: Eat fish [newline]
   my $bar = 'Eat $value\n';   # Result is: Eat $value\n

             -----------------------------------

3) User Input       Complete Lesson:
       http://linuxchix.org/pipermail/courses/2003-April/001170.html

The "diamond operator", or "<>", is placed around a filehandle to read a line
from it. You can define your own filehandles, but the special filehandle STDIN
is always defined for you:

   #!/usr/bin/perl -w
   use strict;

   my $line = <STDIN>;
   print "You wrote: [$line]\n";

Note that the input includes the newline character at the end of the line. Use
"chomp" to remove it:

   #!/usr/bin/perl -w
   use strict;

   my $line = <STDIN>;
   chomp($line);
   print "You wrote: [$line]\n";

You can't write "$line = chomp(<STDIN>)" because "chomp" alters the value
passed in rather than returning the changed input. If you want to be really
tricky, do this:

   chomp(my $input = <STDIN>);

The record separator ("$/") controls how input is broken into readable pieces,
and "chomp" always removes $/ from the end of the line. You can set it as
follows:

   $/ = ",";

Now "<STDIN>" will read text up to (and including) the first comma, and
"chomp" will cut that comma off the end of the input.

             -----------------------------------

4) Control Structures       Complete Lesson:
       http://linuxchix.org/pipermail/courses/2003-April/001184.html

Like pretty much every other (imperitive) programming language, Perl uses the
"if" control structure.

   if ( <condition> ) {
     # Do something here.
   }

Or:

   if ( <condition> ) {
     # Do something here.
   }
   else {
     # Do something else here.
   }

Perl also has the innovative "unless" keyword. "unless ( <condition> )" has
the same meaning as "if ( not <condition> )".

   unless ( $number == 5 ) {
     print "The number is not 5.\n";
   }

You can also add an "else" after "unless", but only a truly twisted soul would
do that!

The "while" keyword causes commands to be executed as long as a condition is
true:

   #!/usr/bin/perl -w
   use strict;

   # Count to ten.
   my $x = 1;
   while ( $x <= 10 ) {
     print "x = $x\n";
     $x = $x + 1;
   }

In contrast to "while", "until" loops if the condition is false:

   #!/usr/bin/perl -w
   use strict;

   # Count to ten (again).
   my $x = 1;
   until ( $x > 10 ) {
     print "x = $x\n";
     $x = $x + 1;
   }

Both "if" and "unless" can be written after the statement, too:

   print "Your account is overdrawn.\n" if $money < 0;

The same is true of "while" and "until".

All of these keywords ("if", "unless", "while" and "until") require braces and
parentheses unless used after the statement.

             -----------------------------------

5) New Stuff

I'd like to point out a few things that haven't been mentioned out before.

Perl allows you to make the same type of "if-assignment" as C:

   if ( $x = 10 ) { ... }    # Uh oh...

Do you see the error? It should be "==" instead of "=":

   if ( $x == 10 ) { ... }   # Much better.

If you have faithfully turned on warnings, Perl will warn you of that one, but
not this one:

   if ( $x = $y ) { ... }    # Possible mistake, but no warning.

Perl doesn't warn you because assignment may in fact be what you want,
especially in a loop:

   #!/usr/bin/perl -w
   use strict;

   my $line;
   while ( defined($line = <STDIN>) ) {
     chomp($line);
     print "$line\n";
   }

The while loop terminates when there's no more input.

(If you execute this program, enter a few lines and then hit Ctrl+D [hold down
Control and hit D] to indicate that you don't want to input anymore. There's a
chance that Perl will wait for you to hit Ctrl+D before it begins repeating
what you input.)

One other thing that wasn't pointed out before that I think worthy of noting:
even though numbers and text can be stored in the same type of variable, they
use different comparison operators. Operators like "<", ">" and "==" are for
numbers. For text, use:
   eq (equal)
   ne (not equal)
   lt (less than)
   le (less than or equal to)
   gt (greater than)
   ge (greater than or equal to)

For example:

   my $x = 'Larry Wall';
   my $y = 'Alex Nobrain';
   if ( $x gt $y ) {
     print "$x is greater than than $y\n";
   }

If you're wondering why this is, consider this comparison:

   my $a = '   25  ';
   my $b = 25;
   if ( $a == $b ) { ... }

Perl is happy to strip the white space off of $a and treat it as a number, but
maybe you intended to treat $b as a string instead. So you have to explicitly
specify whether you want a string comparison or a numerical comparison.

             -----------------------------------

6) Exercise

This exercise is taken from previous material, but made slightly more
difficult.

Write a program that reads numbers from the keyboard (one number per line)
until there's no more to read, then outputs the average of all those numbers.

Here are some things to remember:
a) Use "while ( defined($line = <STDIN>) )" to loop until there's no more
input.
b) When entering information, hit Ctrl+D to signal the end of the data.
c) Don't forget "use strict;" and "-w"!

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Subject: [Courses] [Perl] Perl course Part 5: the "tr///" operator
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LinuxChix Perl Course Part 5: the "tr///" operator

Contents
1) Introduction
2) The tr/// operator
3) Options
4) Answers to Previous Exercises
5) Exercises
6) Past Information

             -----------------------------------

1) Introduction

It seems that the original plan for this course was to start with features
common to every language, then perhaps get into features unique to Perl.
After thinking long and hard, I have decided that it would be better to do
the opposite: start with features that are unique to Perl. This is surely the
best thing for those of you who already program in other languages, and I
don't think it's detrimental to anyone who is learning Perl as a first
language. (Though learning Perl as a first language might in itself be
detrimental!)

So now we're going to look at a Perl language construct that doesn't exist in
most other languages, called "tr". It's generally written with trailing
slashes, for reasons that we will see shortly. Some people call this language
construct a "function," but I believe that "operator" is the correct term,
and it is the term that I will use.

             -----------------------------------

2) The tr/// operator

Try running the following Perl program:

   #!/usr/bin/perl -w
   use strict;

   my $text = 'some cheese';
   $text =~ tr/ce/XY/;
   print "$text\n";

What happened to $text?

The "tr///" operator performs a substitution on the individual characters in
a string.

Examples:

   $x =~ tr/a/b/;    # Replace each "a" with a "b".
   $x =~ tr/ /_/;    # Convert spaces to underlines.
   $x =~ tr/aeiou/AEIOU/;  # Capitalise vowels.
   $x =~ tr/79/97/;  # Exchange "7" and "9".

The only characters that have special meaning to "tr///" are the backslash
and the dash. The latter indicates a range of characters:

   $x =~ tr/0-9/QERTYUIOPX/; # Digits to letters.
   $x =~ tr/A-Z/a-z/;        # Convert to lowercase.

Actually, the slash also has a special meaning to "tr///". The slash is
called the "delimiter", because it indicates the "limit" on the list of
characters to substitute. However, we can use most kinds of punctuation in
place of the slash. For example:

   $x =~ tr!aeiou!AEIOU!;
   $x =~ tr:aeiou:AEIOU:;

Note that we can also use parentheses, but the syntax changes a little
because parentheses include the idea of containment:

   $x =~ tr(aeiou)(AEIOU);
   $x =~ tr<aeiou><AEIOU>;

The semantics (meaning) don't change; only the syntax (way of writing it)
changes. But even though the delimiter is abitrary, we still talk about it as
"tr///".

"tr///" returns the number of replacements it made:

   my $salary = '$1,000,000.00';   # Dollar sign: use single quote!
   my $ego = ($salary =~ tr/0/0/); # Count the zeros in salary.

One more thing: "tr///" has an alias: "y///". This is to please users of the
program "sed", which uses the "y///" command do to basically what "tr///"
does. In Perl, "tr///" and "y///" do exactly the same thing; use whichever
you like. Remember: there is more than one way to do it (TIMTOWTDI).

   $text =~ tr/0-9/A-J/;   # Convert digits to letters.
   $text =~  y/0-9/A-J/;   # Does exactly the same thing.

             -----------------------------------

3) Options

"tr///" can take the following options:
   c   Complement (invert) the searchlist.
   d   Delete found but unreplaced characters.
   s   Squash duplicate replaced characters.

These options are specified after the final delimiter, like this:

   $x =~ tr/abc/xyz/s;    # Note the "s" at the end.
   $x =~ tr(abc)(xyz)s;   # Same thing, but with parentheses.
   $x =~ tr/abc/xy/ds;    # Multiple options.
   # In the last case, the "z" is missing. You'll see why shortly.

In the last example, we specified both the "d" and the "s" options. The order
of the options isn't important: we could have used "sd" instead of "ds".

Examples:

   #!/usr/bin/perl -w
   use strict;

   ##### The "s" option #####
   my $text = 'good cheese';
   $text =~ tr/eo/eu/s;
   print "$text\n";
   # Output is: gud chese

   ##### The "d" option #####
   my $big = 'vowels are useful';
   $big =~ tr/aeiou/AEI/d;
   print "$big\n";
   # The first three vowels are made uppercase.
   # The other two, which have no replacement
   # character, are deleted because of the "d".

We won't demonstrate the last option, "c", because it's rather complicated.
You can learn more about it using "man perlop" (look under "tr").

             -----------------------------------

4) Answers to Previous Exercises

There was only one previous exercise: a program that reads numbers and
outputs the average. Here is one such program:

   #!/usr/bin/perl -w
   use strict;

   my $line;
   my $sum = 0;
   my $n = 0;
   while ( defined($line = <STDIN>) ) {
     $sum += $line;
     $n++;
   }

   my $average = $sum / $n;

   print "The average is $sum/$n = $average\n";

             -----------------------------------

5) Exercises

a) I constantly get spam for "V1agra" or "un1vers1ty dimpl0mas". Write a
program that helps my spam filter by converting all 1's to i's and all 0's to
o's.

b) Julius Ceasar is said to have used the famous Ceasar Cypher to encrypt his
communications with Rome. When encrypting, the Ceasar Cypher substitutes
letter-for-letter like this:
   A -> C
   B -> D
   C -> E
    ...
   X -> Z
   Y -> A
   Z -> B

Write a program that performs a Ceasar Cypher on its input.

As a test, if you feed the program "Ceasar Cypher", you should get "Egcuct
Earjgt". Be especially careful that the "y" in "cypher" maps to an "a".

             -----------------------------------

6) Past Information

Part 1: Getting Started
         http://linuxchix.org/pipermail/courses/2003-March/001147.html

Part 2: Scalar Data
         http://linuxchix.org/pipermail/courses/2003-March/001153.html

Part 3: User Input
         http://linuxchix.org/pipermail/courses/2003-April/001170.html

Part 4: Control Structures
         http://linuxchix.org/pipermail/courses/2003-April/001184.html

Part 4.5, a review with a little new information at the end:
         http://linuxchix.org/pipermail/courses/2003-July/001297.html

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LinuxChix Perl Course Part 6: The "m//" Operator

Contents
1) Introduction
2) Introducing the m// Operator
3) A Crash Course on Perl Regular Expressions
4) Back to "m//"
5) Exercise
6) Answers to Previous Exercises
7) Licensing Announcement
8) Past Information
9) Credits

             -----------------------------------

1) Introduction

Having seen the "tr///" operator, we are now going to look at the "m//"
operator, which stands for "match". Because this requires an understanding of
regular expressions, we give a crash course on that. If you're already
familiar with regular expressions, you should still skim over the crash
course because Perl regular expressions have handy shortcuts that you may not
be aware of.

As you may have noticed, Part 6 is twice as long as Part 5, so you may want
to take a break around the beginning of section 4. Consider yourself lucky:
Part 6 was originally about a third longer, but I finally caved in and moved
some material to Part 7.

             -----------------------------------

2) Introducing the m// operator

Try running this program:

   #!/usr/bin/perl -w
   use strict;

   my $a = 'I talked to Alice this morning.';
   my $b = 'I talked to Bobby this morning.';
   print "'$a' matches /Bob/ \n"    if   $a =~ m/Bob/;
   print "'$b' matches /Bob/ \n"    if   $b =~ m/Bob/;

What happens?

This example makes it look as though "m//" indicates whether one piece of
text contains another. In fact, "m//" performs a "regular expression" match
(or "pattern match"), which is much, much more powerful. We haven't seen
regular expressions yet, but we're going to look at them in a moment.

By the way, Perl is big on avoiding redundancy. If you use a slash as the
delimiter, you can omit the letter "m" entirely! So the last two statements
of the above program could have been written like this:

   print "'$a' matches /Bob/ \n"    if   $a =~ /Bob/;
   print "'$b' matches /Bob/ \n"    if   $b =~ /Bob/;

In fact, most Perl programmers prefer to write it that way. But remember: it
only works if your delimiter is a slash!

Now we're going to look at regular expressions in more detail. But keep these
four things in mind:

a) "m//" is used to test whether a given string matches or does not match a
certain regular expression.

b) The match does not need to be a separate word: in the previous example,
/Bob/ matched "Bobby".

c) The syntax of "m//" is similar to the syntax of "tr///". So we could have
used "m!Bob!" or "m<Bob>" instead of "m/Bob/".

d) You can omit the "m" if your delimiter is a slash.

             -----------------------------------

3) A Crash Course on Perl Regular Expressions

Regular expressions are well-known in Unix circles, though they're not unique
to Unix and they certainly don't require Unix to work. Beware that different
Unix tools (like grep and sed) use different "flavors" of regular
expressions, so the patterns you use in Perl probably aren't directly
portable to other tools.

In Perl regular expressions, the following characters have special meaning:
   . * + ? | ^ $ @ ( ) [ ] { } \
   and the delimiter (usually slash)

They can all be escaped with a backslash.

Okay, let's start with the easiest special character: the period (dot). The
period matches any single character (except a newline, but we'll see more
about that in Part 7). So /a.c/ matches "abc", "aNc" or "a%c". If you want to
match exactly "a.c", escape the period with a backslash: /a\.c/

Square brackets indicate a list of possible choices:
   /a[123]b/  matches exactly three strings: "a1b", "a2b" and "a3b".

A dash inside square brackets indicates a range, just as in "tr///":
   /a[1-9]b/  matches "a1b", "a2b", "a3b", ..., "a9b".

To include the dash itself in a list of choices, place the dash at the
beginning or end of the list:
   /a[12-]b/  matches "a1b", "a2b" and "a-b".

A dash OUTSIDE square brackets has no special meaning; it's just a literal
dash.

A caret at the beginning of a choice negates (inverts) it:
   /a[^A-Z]b/ matches "a1b" or "a%b", but NOT "aMb".

A caret anywhere else in a choice means a literal caret:
   /a[A^Z]b/  matches "aAb", "a^b" or "aZb".

The characters "* + ?" don't stand for any character, but instead indicate
that the previous character should appear a certain number of times:
   *   previous character appears zero or more times
   +   previous character appears one or more times
   ?   previous character appears zero or one times

Examples:
   /ab*c/     matches "ac", "abc", "abbc" and "abbbbbbbbbc"
   /ab+c/     same as /a*b/ except doesn't match "ac"
   /ab?c/     matches "ac" and "abc", but NOT "abbc"

Note that if "*" or "+" follows a range or a dot, the match is interpreted
"broadly":
   /a[123]*b/ matches "a22b" and "a22222b", but also "a12312322321b"
   /a.+b/     matches an "a" followed by a "b", with anything in between

Also note that most special characters lose their special meaning when inside
a range. For example, /[.*]/ matches a literal period or a literal astrisk.

The characters "^" and "$" indicate the beginning and end of the string,
respectively:
   /^a/       an "a" followed by anything (or nothing)
   /a$/       anything ending in "a"
   /^a$/      matches only "a"
   /^a[^z]*b$/   matches anything starting with "a", ending with "b" and not
                 containing "z".
Remember that unless you explicitly specify a "^" or a "$", the match can
include any part of the string. So, for example, /ab*c/ matches "abbc", but
also "123abbcXYZ".

In addition to representing the end of the string, the character "$" can also
be used for variable interpolation:
   my $pattern = '[aeiou]';
   print "Found vowels\n" if $str =~ m/$pattern/;

Parentheses are used for grouping:
   /(abc)+/  matches "abc" and "abcabc", but NOT "abcc" or "aabbcc"
   /a(bc)?d/ matches "ad" and "abcd", but NOT "abd" or "acd".

The vertical bar means "either but not both":
   /a|b/     matches "a" and "b".
   /ab|cd/   matches "ab" and "cd"

The vertical bar has low precidence, meaning that /AB|CD/ means "AB or CD"
rather than "A, then either B or C, then D". To override this, use
parentheses:
   /a(b|c)d/ matches "abd" or "acd" but NOT "abcd"

Finally, Perl includes many useful escape-sequences that only have meaning in
regular expressions, such as:
   \s   whitespace (same as [ \t\n\r])
   \w   a "word" character (same as [A-Za-z0-9_])
   \d   a digit (same as [0-9])

Example: /^\s*\d+\s*$/ means "beginning of string, possible leading
whitespace, one or more digits, possible trailing whitespace, end of string".
In other words, it means a non-negative integer.

Capitalizing each of these means "the opposite":
   \S   non-whitespace
   \W   non-word characters
   \D   non-digits

In addition, here is a very handy "zero-length" escape sequence:
   \b   word boundry
and its opposite:
   \B   not a word boundry

They are zero-length because they don't represent a character, just the
boundry between characters. This may not sound important right now, but you
will find it useful with time.

A word boundry is defined as a \w next to a \W (in either order). For
example, /\Bing\b/ matches "playing" and "king", but not "jingle" (doesn't
match \b) or "abc ing" (doesn't match \B).

             -----------------------------------

4) Back to "m//"

Now that we have seen the details of regular expressions, we can use "m//"
more appropriately.

Regular expressions provide a very concise way of describing whether a piece
of text is "what you want". For example:

   print "Unix\n" if $os =~ m/Linux|Unix|(Free|Open)?BSD|Solaris/;

Remember: you can omit the "m" if you use a slash as the delimiter:

   print "Unix\n" if $os =~ /Linux|Unix|(Free|Open)?BSD|Solaris/;

In Perl, you can surround a part of a regular expression with parentheses so
as to extract the information:

   # Match digits-colon-digits-colon-digits
   if ( $time =~ /(\d+):(\d+):(\d+)/ ) {
     $hour = $1;     # First set of parentheses
     $minute = $2;   # Second set of parentheses
     $second = $3;   # Third set of parentheses
   }

We can compress the above into one line:

   ($hour, $minute, $second) = ( $time =~ /(\d+):(\d+):(\d+)/ );

The parentheses are necessary because "m//" returns the list ($1,$2,$3).
We'll learn more about lists at a later date.

If the parentheses in a regular expression are nested, it's the order of the
LEFT parenthesis that counts:

   # Same as the previous example, but takes into account
   # the possibility that the seconds are not specified.
   if ( $time =~ /(\d+):(\d+)(:(\d+))?/ ) {
     $hour = $1;     # First set of parentheses
     $minute = $2;   # Second set of parentheses
     $second = $4;   # Fourth set of parentheses
   }

In the above example, the third set of parentheses is used to associate both
the colon and the digits with "?" - either both should be specified, or
neither, but not one or the other. We could have set something equal to $3 as
well, but that would be something like ":59", which probably isn't very
useful.

By the way, since our regular expression explicitly allows the user to not
specify the number of seconds, we should anticipate the possibility that the
seconds are excluded. We can test this using the "defined" function on "$4"
or "$second", but a quicker way is to do this:

   $second = $4 || 0;

This will set "$second" to zero if $4 is undefined or zero.

If you have enabled warnings (and you should have), Perl will complain if you
use an undefined value for most operations, so be sure to anticipate possible
undefined values in regular expressions.

             -----------------------------------

5) Exercise

Many URIs[*] (but not all) can be divided into protocol, authority and path
(in that order). For example, consider the following URIs:

   ftp://example.com/foo/bar
   gopher://abc.example.com/xyz
   telnet://example.com/

The protocol is the part before the colon, the authority is the part in the
middle (the domain name), and the path is whatever comes after the authority.
For example, in the first URI, the protocol is "ftp", the authority is
"example.com", and the path is "/foo/bar". Note that the "://" between the
protocol and the authority is not part of the protocol or the authority.

Write a Perl program that uses "m//" to split a URI into protocol, authority
and path, or outputs a message saying that you gave it a URI that it can't
parse.

[*] According to RFC 2396, URLs are structural (they tell you how to get what
you want) whereas URNs are logical (a permanent, abstract mapping of strings
to resources), and URIs include both URLs and URNs. The consensus in Perl
seems to be to use the term "URI".

             -----------------------------------

6) Answers to Previous Exercises

a) A program to convert all 1's to i's and all 0's to o's:

   #!/usr/bin/perl -w
   use strict;

   while ( defined(my $line = <STDIN>) ) {
     $line =~ tr/01/oi/;
     print $line;
   }


b) A Ceasar Cypher:

   #!/usr/bin/perl -w
   use strict;

   while ( defined(my $line = <STDIN>) ) {
     $line =~ tr/A-Za-z/C-ZABc-zab/;
     print $line;
   }

             -----------------------------------

7) Licensing Announcement

This course material, including the previously released parts, is copyright
Alice Wood and Dan Richter, and is retroactively released under the same
license as Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your Perl
code/documentation into their own projects. It is not generally used in
projects not related to Perl.

By the way, Alice Wood is the author of the first four parts of this course,
and I got her permission to release those parts under that license as well.

             -----------------------------------

8) Past Information

Part 1: Getting Started
         http://linuxchix.org/pipermail/courses/2003-March/001147.html

Part 2: Scalar Data
         http://linuxchix.org/pipermail/courses/2003-March/001153.html

Part 3: User Input
         http://linuxchix.org/pipermail/courses/2003-April/001170.html

Part 4: Control Structures
         http://linuxchix.org/pipermail/courses/2003-April/001184.html

Part 4.5, a review with a little new information at the end:
         http://linuxchix.org/pipermail/courses/2003-July/001297.html

Part 5: The "tr///" Operator:
         http://linuxchix.org/pipermail/courses/2003-July/001302.html

             -----------------------------------

9) Credits

Works cited: "man perlre" and "man perlop"

Thanks to Jacinta Richardson for fact checking.

From sneha@vit.ac.in  Tue Aug 12 19:30:17 2003
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Subject: Re: [Courses] [Perl] Part 6: The "m//" Operator
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**Hi ,
      My Answer :) =20

***********************************************************
 #! /usr/bin/perl -w
use strict;
my $url =3D 'ftp://example.com/foo/bar';
my $protocol;
my $authority;
my $path;
($protocol,$authority,$path)=3D($url=3D~m#(\w*)://(.*\.\w*)/(.*)#);
print "Protocol   =3D > $protocol\n";
print "Authority =3D > $authority\n";
print "Path         =3D > $path\n";
************************************************************             =
                     =20
friendly
sneha :o)

----- Original Message -----=20
From: Dan Richter=20
To: courses@linuxchix.org=20
Sent: Friday, August 08, 2003 4:18 PM
Subject: [Courses] [Perl] Part 6: The "m//" Operator


LinuxChix Perl Course Part 6: The "m//" Operator

Contents
1) Introduction
2) Introducing the m// Operator
3) A Crash Course on Perl Regular Expressions
4) Back to "m//"
5) Exercise
6) Answers to Previous Exercises
7) Licensing Announcement
8) Past Information
9) Credits

             -----------------------------------

1) Introduction

Having seen the "tr///" operator, we are now going to look at the "m//"
operator, which stands for "match". Because this requires an =
understanding of
regular expressions, we give a crash course on that. If you're already
familiar with regular expressions, you should still skim over the crash
course because Perl regular expressions have handy shortcuts that you =
may not
be aware of.

As you may have noticed, Part 6 is twice as long as Part 5, so you may =
want
to take a break around the beginning of section 4. Consider yourself =
lucky:
Part 6 was originally about a third longer, but I finally caved in and =
moved
some material to Part 7.

             -----------------------------------

2) Introducing the m// operator

Try running this program:

   #!/usr/bin/perl -w
   use strict;

   my $a =3D 'I talked to Alice this morning.';
   my $b =3D 'I talked to Bobby this morning.';
   print "'$a' matches /Bob/ \n"    if   $a =3D~ m/Bob/;
   print "'$b' matches /Bob/ \n"    if   $b =3D~ m/Bob/;

What happens?

This example makes it look as though "m//" indicates whether one piece =
of
text contains another. In fact, "m//" performs a "regular expression" =
match
(or "pattern match"), which is much, much more powerful. We haven't seen
regular expressions yet, but we're going to look at them in a moment.

By the way, Perl is big on avoiding redundancy. If you use a slash as =
the
delimiter, you can omit the letter "m" entirely! So the last two =
statements
of the above program could have been written like this:

   print "'$a' matches /Bob/ \n"    if   $a =3D~ /Bob/;
   print "'$b' matches /Bob/ \n"    if   $b =3D~ /Bob/;

In fact, most Perl programmers prefer to write it that way. But =
remember: it
only works if your delimiter is a slash!

Now we're going to look at regular expressions in more detail. But keep =
these
four things in mind:

a) "m//" is used to test whether a given string matches or does not =
match a
certain regular expression.

b) The match does not need to be a separate word: in the previous =
example,
/Bob/ matched "Bobby".

c) The syntax of "m//" is similar to the syntax of "tr///". So we could =
have
used "m!Bob!" or "m<Bob>" instead of "m/Bob/".

d) You can omit the "m" if your delimiter is a slash.

             -----------------------------------

3) A Crash Course on Perl Regular Expressions

Regular expressions are well-known in Unix circles, though they're not =
unique
to Unix and they certainly don't require Unix to work. Beware that =
different
Unix tools (like grep and sed) use different "flavors" of regular
expressions, so the patterns you use in Perl probably aren't directly
portable to other tools.

In Perl regular expressions, the following characters have special =
meaning:
   . * + ? | ^ $ @ ( ) [ ] { } \
   and the delimiter (usually slash)

They can all be escaped with a backslash.

Okay, let's start with the easiest special character: the period (dot). =
The
period matches any single character (except a newline, but we'll see =
more
about that in Part 7). So /a.c/ matches "abc", "aNc" or "a%c". If you =
want to
match exactly "a.c", escape the period with a backslash: /a\.c/

Square brackets indicate a list of possible choices:
   /a[123]b/  matches exactly three strings: "a1b", "a2b" and "a3b".

A dash inside square brackets indicates a range, just as in "tr///":
   /a[1-9]b/  matches "a1b", "a2b", "a3b", ..., "a9b".

To include the dash itself in a list of choices, place the dash at the
beginning or end of the list:
   /a[12-]b/  matches "a1b", "a2b" and "a-b".

A dash OUTSIDE square brackets has no special meaning; it's just a =
literal
dash.

A caret at the beginning of a choice negates (inverts) it:
   /a[^A-Z]b/ matches "a1b" or "a%b", but NOT "aMb".

A caret anywhere else in a choice means a literal caret:
   /a[A^Z]b/  matches "aAb", "a^b" or "aZb".

The characters "* + ?" don't stand for any character, but instead =
indicate
that the previous character should appear a certain number of times:
   *   previous character appears zero or more times
   +   previous character appears one or more times
   ?   previous character appears zero or one times

Examples:
   /ab*c/     matches "ac", "abc", "abbc" and "abbbbbbbbbc"
   /ab+c/     same as /a*b/ except doesn't match "ac"
   /ab?c/     matches "ac" and "abc", but NOT "abbc"

Note that if "*" or "+" follows a range or a dot, the match is =
interpreted
"broadly":
   /a[123]*b/ matches "a22b" and "a22222b", but also "a12312322321b"
   /a.+b/     matches an "a" followed by a "b", with anything in between

Also note that most special characters lose their special meaning when =
inside
a range. For example, /[.*]/ matches a literal period or a literal =
astrisk.

The characters "^" and "$" indicate the beginning and end of the string,
respectively:
   /^a/       an "a" followed by anything (or nothing)
   /a$/       anything ending in "a"
   /^a$/      matches only "a"
   /^a[^z]*b$/   matches anything starting with "a", ending with "b" and =
not
                 containing "z".
Remember that unless you explicitly specify a "^" or a "$", the match =
can
include any part of the string. So, for example, /ab*c/ matches "abbc", =
but
also "123abbcXYZ".

In addition to representing the end of the string, the character "$" can =
also
be used for variable interpolation:
   my $pattern =3D '[aeiou]';
   print "Found vowels\n" if $str =3D~ m/$pattern/;

Parentheses are used for grouping:
   /(abc)+/  matches "abc" and "abcabc", but NOT "abcc" or "aabbcc"
   /a(bc)?d/ matches "ad" and "abcd", but NOT "abd" or "acd".

The vertical bar means "either but not both":
   /a|b/     matches "a" and "b".
   /ab|cd/   matches "ab" and "cd"

The vertical bar has low precidence, meaning that /AB|CD/ means "AB or =
CD"
rather than "A, then either B or C, then D". To override this, use
parentheses:
   /a(b|c)d/ matches "abd" or "acd" but NOT "abcd"

Finally, Perl includes many useful escape-sequences that only have =
meaning in
regular expressions, such as:
   \s   whitespace (same as [ \t\n\r])
   \w   a "word" character (same as [A-Za-z0-9_])
   \d   a digit (same as [0-9])

Example: /^\s*\d+\s*$/ means "beginning of string, possible leading
whitespace, one or more digits, possible trailing whitespace, end of =
string".
In other words, it means a non-negative integer.

Capitalizing each of these means "the opposite":
   \S   non-whitespace
   \W   non-word characters
   \D   non-digits

In addition, here is a very handy "zero-length" escape sequence:
   \b   word boundry
and its opposite:
   \B   not a word boundry

They are zero-length because they don't represent a character, just the
boundry between characters. This may not sound important right now, but =
you
will find it useful with time.

A word boundry is defined as a \w next to a \W (in either order). For
example, /\Bing\b/ matches "playing" and "king", but not "jingle" =
(doesn't
match \b) or "abc ing" (doesn't match \B).

             -----------------------------------

4) Back to "m//"

Now that we have seen the details of regular expressions, we can use =
"m//"
more appropriately.

Regular expressions provide a very concise way of describing whether a =
piece
of text is "what you want". For example:

   print "Unix\n" if $os =3D~ m/Linux|Unix|(Free|Open)?BSD|Solaris/;

Remember: you can omit the "m" if you use a slash as the delimiter:

   print "Unix\n" if $os =3D~ /Linux|Unix|(Free|Open)?BSD|Solaris/;

In Perl, you can surround a part of a regular expression with =
parentheses so
as to extract the information:

   # Match digits-colon-digits-colon-digits
   if ( $time =3D~ /(\d+):(\d+):(\d+)/ ) {
     $hour =3D $1;     # First set of parentheses
     $minute =3D $2;   # Second set of parentheses
     $second =3D $3;   # Third set of parentheses
   }

We can compress the above into one line:

   ($hour, $minute, $second) =3D ( $time =3D~ /(\d+):(\d+):(\d+)/ );

The parentheses are necessary because "m//" returns the list ($1,$2,$3).
We'll learn more about lists at a later date.

If the parentheses in a regular expression are nested, it's the order of =
the
LEFT parenthesis that counts:

   # Same as the previous example, but takes into account
   # the possibility that the seconds are not specified.
   if ( $time =3D~ /(\d+):(\d+)(:(\d+))?/ ) {
     $hour =3D $1;     # First set of parentheses
     $minute =3D $2;   # Second set of parentheses
     $second =3D $4;   # Fourth set of parentheses
   }

In the above example, the third set of parentheses is used to associate =
both
the colon and the digits with "?" - either both should be specified, or
neither, but not one or the other. We could have set something equal to =
$3 as
well, but that would be something like ":59", which probably isn't very
useful.

By the way, since our regular expression explicitly allows the user to =
not
specify the number of seconds, we should anticipate the possibility that =
the
seconds are excluded. We can test this using the "defined" function on =
"$4"
or "$second", but a quicker way is to do this:

   $second =3D $4 || 0;

This will set "$second" to zero if $4 is undefined or zero.

If you have enabled warnings (and you should have), Perl will complain =
if you
use an undefined value for most operations, so be sure to anticipate =
possible
undefined values in regular expressions.

             -----------------------------------

5) Exercise

Many URIs[*] (but not all) can be divided into protocol, authority and =
path
(in that order). For example, consider the following URIs:

   ftp://example.com/foo/bar
   gopher://abc.example.com/xyz
   telnet://example.com/

The protocol is the part before the colon, the authority is the part in =
the
middle (the domain name), and the path is whatever comes after the =
authority.
For example, in the first URI, the protocol is "ftp", the authority is
"example.com", and the path is "/foo/bar". Note that the "://" between =
the
protocol and the authority is not part of the protocol or the authority.

Write a Perl program that uses "m//" to split a URI into protocol, =
authority
and path, or outputs a message saying that you gave it a URI that it =
can't
parse.

[*] According to RFC 2396, URLs are structural (they tell you how to get =
what
you want) whereas URNs are logical (a permanent, abstract mapping of =
strings
to resources), and URIs include both URLs and URNs. The consensus in =
Perl
seems to be to use the term "URI".

             -----------------------------------

6) Answers to Previous Exercises

a) A program to convert all 1's to i's and all 0's to o's:

   #!/usr/bin/perl -w
   use strict;

   while ( defined(my $line =3D <STDIN>) ) {
     $line =3D~ tr/01/oi/;
     print $line;
   }


b) A Ceasar Cypher:

   #!/usr/bin/perl -w
   use strict;

   while ( defined(my $line =3D <STDIN>) ) {
     $line =3D~ tr/A-Za-z/C-ZABc-zab/;
     print $line;
   }

             -----------------------------------

7) Licensing Announcement

This course material, including the previously released parts, is =
copyright
Alice Wood and Dan Richter, and is retroactively released under the same
license as Perl itself (Artistic License or GPL, your choice). This is =
the
license of choice to make it easy for other people to integrate your =
Perl
code/documentation into their own projects. It is not generally used in
projects not related to Perl.

By the way, Alice Wood is the author of the first four parts of this =
course,
and I got her permission to release those parts under that license as =
well.

             -----------------------------------

8) Past Information

Part 1: Getting Started
         http://linuxchix.org/pipermail/courses/2003-March/001147.html

Part 2: Scalar Data
         http://linuxchix.org/pipermail/courses/2003-March/001153.html

Part 3: User Input
         http://linuxchix.org/pipermail/courses/2003-April/001170.html

Part 4: Control Structures
         http://linuxchix.org/pipermail/courses/2003-April/001184.html

Part 4.5, a review with a little new information at the end:
         http://linuxchix.org/pipermail/courses/2003-July/001297.html

Part 5: The "tr///" Operator:
         http://linuxchix.org/pipermail/courses/2003-July/001302.html

             -----------------------------------

9) Credits

Works cited: "man perlre" and "man perlop"

Thanks to Jacinta Richardson for fact checking.

_______________________________________________
Courses mailing list
Courses@linuxchix.org
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From daniel.richter@wimba.com  Tue Aug 12 22:40:07 2003
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Date: Tue, 12 Aug 2003 14:55:17 +0200
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From: Dan Richter <daniel.richter@wimba.com>
Subject: Re: [Courses] [Perl] Part 6: The "m//" Operator
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Hi Sneha!

>($protocol,$authority,$path)=($url=~m#(\w*)://(.*\.\w*)/(.*)#);

Okay, three comments.

First, for the benefit of other readers, I mention that the delimiter in 
this case is a hash. That's perfectly legal, and many Perl users prefer to 
do that, both because hashes are easy to see and because they're not used 
very often in patterns. Just make sure you don't put a space between the 
"m" and the hash.    :-)

Second, there's a minor problem in this pattern. I foolishly neglected to 
give diverse example cases. Try using this program on the URI 
<ftp://example.com/foo.bar/baz>. This is a problem caused by "greediness", 
meaning that each part of the regular expression will match as much as it 
can before moving on. We'll see more about this later.

Third, I would have put a caret at the beginning and a dollar sign at the 
end to make sure the whole string matches. But of course that depends on 
exactly how the program is to be used; greediness assures that nothing will 
be left out.

Finally, let me say that I am quite pleased to see your response, because 
up until now I have seen no response whatsoever, and I was beginning to 
wonder if anyone at all was benefiting from the course!

-- 
    Nothing is sadder than the murder of a beautiful theory
    by a gang of ugly facts.
        - Jeffrey Zeldman, in A List Apart

From sneha@vit.ac.in  Tue Aug 12 23:38:38 2003
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Subject: Re: [Courses] [Perl] Part 6: The "m//" Operator
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** Hi ,
        I hope, there r lotza readers making use of this course, I am =
one of them.
        I appreciate ur work on this. Wish to learn more :).
        The following is a small correction I made in my code...after I =
received ur mail...
       =20
ftp://example.com/foo.bar/baz
($protocol,$authority,$path)=3D($url=3D~m#^(\w*)://(.[^/]*)(.*)$#);=20

friendly
sneha :o)

----- Original Message -----=20
From: Dan Richter=20
To: courses@linuxchix.org=20
Sent: Tuesday, August 12, 2003 6:25 PM
Subject: Re: [Courses] [Perl] Part 6: The "m//" Operator


Hi Sneha!

>($protocol,$authority,$path)=3D($url=3D~m#(\w*)://(.*\.\w*)/(.*)#);

Okay, three comments.

First, for the benefit of other readers, I mention that the delimiter in =

this case is a hash. That's perfectly legal, and many Perl users prefer =
to=20
do that, both because hashes are easy to see and because they're not =
used=20
very often in patterns. Just make sure you don't put a space between the =

"m" and the hash.    :-)

Second, there's a minor problem in this pattern. I foolishly neglected =
to=20
give diverse example cases. Try using this program on the URI=20
<ftp://example.com/foo.bar/baz>. This is a problem caused by =
"greediness",=20
meaning that each part of the regular expression will match as much as =
it=20
can before moving on. We'll see more about this later.

Third, I would have put a caret at the beginning and a dollar sign at =
the=20
end to make sure the whole string matches. But of course that depends on =

exactly how the program is to be used; greediness assures that nothing =
will=20
be left out.

Finally, let me say that I am quite pleased to see your response, =
because=20
up until now I have seen no response whatsoever, and I was beginning to=20
wonder if anyone at all was benefiting from the course!

--=20
    Nothing is sadder than the murder of a beautiful theory
    by a gang of ugly facts.
        - Jeffrey Zeldman, in A List Apart

_______________________________________________
Courses mailing list
Courses@linuxchix.org
http://mailman.linuxchix.org/mailman/listinfo/courses
From stolar@numericable.fr  Wed Aug 13 03:49:23 2003
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From: "Isabelle & Zohar Stolar" <stolar@numericable.fr>
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Subject: Re: [Courses] [Perl] Part 6: The "m//" Operator
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>
> Finally, let me say that I am quite pleased to see your response, because
> up until now I have seen no response whatsoever, and I was beginning to
> wonder if anyone at all was benefiting from the course!
>

Don't be sad Dan - You have you're faithful audience :-)

Here's my solution:
--------------------------------------------------------------
my ($prot, $auth, $path) ;
while ( defined( my $input = <STDIN> )) {
    if ($input =~ /(\s*[a-z]*):\/\/([\w.]+)\/(.*)/) {
        ( $prot, $auth, $path ) = ( $input =~
/(\s*\w*):\/\/([\w.]+)\/(.*)/ ) ;  # it's not a double-u it's "\ / \ /"
        print "protocol = $prot\n" ;
        print "authority = $auth\n" ;
        print "path = $path\n" ;
    }
    else {
        print "$input cannot be parsed" ;
    }
}
---------------------------------------------------------------

Thanks for your lessons,
Zohar.


From daniel.richter@wimba.com  Wed Aug 13 18:53:59 2003
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Subject: Re: [Courses] [Perl] Part 6: The "m//" Operator
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>Don't be sad Dan - You have you're faithful audience :-)

Well, I'm glad to hear it.   :-)

>Here's my solution:

This is good. There is a minor bug though: domain names can include a few 
characters not included by \w, such as hyphens (dashes).

># it's not a double-u it's "\ / \ /"

That's very good: the delimiter can be escaped just like any other special 
character.

Some people call that LTS: Leaning Toothpick Syndrome.   :-)

-- 
   They had a big meeting, drank some beer and had some pizza and
   decided 'A' would be 65.
        - Jim Greenly, professor at Georgia Institute of Technology

From zohar@numericable.fr  Wed Aug 13 05:14:02 2003
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Subject: Re: [Courses] [Perl] Part 6: The "m//" Operator
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>
> Finally, let me say that I am quite pleased to see your response, because
> up until now I have seen no response whatsoever, and I was beginning to
> wonder if anyone at all was benefiting from the course!
>

Don't be sad Dan - You have you're faithful audience :-)

Here's my solution:
--------------------------------------------------------------
my ($prot, $auth, $path) ;
while ( defined( my $input = <STDIN> )) {
    if ($input =~ /(\s*[a-z]*):\/\/([\w.]+)\/(.*)/) {
        ( $prot, $auth, $path ) = ( $input =~
/(\s*\w*):\/\/([\w.]+)\/(.*)/ ) ;  # it's not a double-u it's "\ / \ /"
        print "protocol = $prot\n" ;
        print "authority = $auth\n" ;
        print "path = $path\n" ;
    }
    else {
        print "$input cannot be parsed" ;
    }
}
---------------------------------------------------------------

Thanks for your lessons,
Zohar.


From katie@katie-and-rob.org  Fri Aug 15 00:03:42 2003
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Date: Thu, 14 Aug 2003 10:00:29 -0400
From: Katie Bechtold <katie@katie-and-rob.org>
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Subject: Re: [Courses] [Perl] Part 6: The "m//" Operator
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On Fri, Aug 08, 2003 at 12:48:50PM +0200, Dan Richter wrote:
> Write a Perl program that uses "m//" to split a URI into protocol, authority
> and path, or outputs a message saying that you gave it a URI that it can't
> parse.

Here's my answer (written before I looked at any of the offered
answers):

#!/usr/bin/perl -w                                                                                                     
use strict;   

print "Input a URI: ";
chomp(my $uri = <STDIN>);

if ( $uri =~ /([^:]+):\/\/(([^\/]+))(\/.*)/ ) {                                                                          
     
    my $protocol = $1;  
    my $authority = $3;
    my $path = $4;

    print "Protocol: " . $protocol . "\n";
    print "Authority: " . $authority . "\n"; 
    print "Path: " . $path . "\n";
}     
else {
    print "Invalid URI.\n";
}

-- 
Katie Bechtold         http://katie-and-rob.org/

Backward conditioning:
        Putting saliva in a dog's mouth in an attempt to make a bell ring.
From daniel.richter@wimba.com  Fri Aug 15 00:40:18 2003
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Date: Thu, 14 Aug 2003 16:55:01 +0200
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Subject: [Courses] [Perl] Part 7: More About "m//"
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LinuxChix Perl Course Part 7: More About "m//"

1) Introduction
2) Options
3) Anticipating Failed Matches
4) Using Open Patterns
5) Exercise
6) Answer to Previous Exercise
7) Past Information
8) Credits
9) Licensing

             -----------------------------------

1) Introduction

I realise that Part 6 was quite a bit to handle. Regular expressions can be a
lot to get your head around -- and there's no good way to split them up into
short lessons -- but once you've mastered them they're very powerful.

Fortunately, Part 7 has less actual content to memorise -- just a lot of
warnings designed to help you avoid bugs.

             -----------------------------------

2) Options for "m//"

The "m//" operator takes several options, but we will only discuss four of them
here:
   i   Do case-insensitive pattern matching.
   m   Treat string as multiple lines (let /^/ and /$/ match "\n").
   s   Treat string as single line (let /./ match "\n").
   g   Match globally, i.e., find all occurrences.
To see the full list, consult "man perlop".

The easiest option to understand is "i" (case insensitive):

   print "February\n" if $date =~ /feb/i;   # Match "Feb", "FEB", etc.

The "s" option (treat string as single line) is only slightly trickier.
Normally /./ matches any character EXCEPT a newline. This is often what you
want, but if you want /./ to match a newline as well, use "s":

   my($head,$body) = ( $html =~ m!<head>(.*)</head>.*<body>(.*)</body>!si );

We use the "s" option here because line breaks shouldn't be taken into account.

The "m" option (treat string a multiple lines) looks like it would be the
opposite of the "s" option, but in fact the two can be used together because
they affect the meanings of different characters. "m" causes /^/ and /$/ to
match the beginning and end of a "line" in the middle of the string in addition
to their normal meaning (the beginning or end of the string itself). The
"lines" in a string are separated by the newline character "\n".

   my($subject) = ( $email =~ /^Subject: (.*)$/m );

Note that even with the "m" option set, /^/ can only be used at the beginning
of the pattern and /$/ can only be used at the end. The purpose of the "m"
option is to allow you to identify a match without worrying about whether it's
the beginning of the string or just the beginning of the line. If you're
looking for a line break in the middle of a pattern, you know it's a newline,
so use /\n/ instead:

   if ( $foo =~ /^one line^two lines$/m ) { ... }    # Wrong
   if ( $foo =~ /^one line$two lines$/m ) { ... }    # Wrong
   if ( $foo =~ /^one line$^two lines$/m ) { ... }   # Wrong
   if ( $foo =~ /^one line\ntwo lines$/m ) { ... }   # Right

Finally, the "g" option causes a "global" match. This doesn't change WHETHER a
string matches; rather, it returns all matches at once:

   # Find up to three words that contain the letter "q".
   my($a,$b,$c) = ( $list =~ /[a-z]*q[a-z]*/g );

This isn't very useful right now, but it will be more helpful when we start
using arrays.

             -----------------------------------

3) Anticipating Failed Matches

The "m//" operator is a great way to extract information from a string, but you
should always anticipate the possibility that the string won't match your
pattern.

For example, consider the following code:

   #!/usr/bin/perl -w
   use strict;

   my($name, $blood);

   while ( 1 ) {   # Loop forever.
     print "Enter patient name:   ";
     chomp($name = <STDIN>);
     print "Enter blood type:     ";
     chomp($blood = <STDIN>);
     $blood =~ m/(AB|A|B|O)/;
     print "Transfusing type $1 blood into $name\n\n";
   }

Now let's try it:

   Enter patient name:   Bob
   Enter blood type:     type AB
   Transfusing type AB blood into Bob

So far, so good, but now the nurse accidentally hits <Enter> before giving a
blood type:

   Enter patient name:   Jack
   Enter blood type:
   Transfusing type AB blood into Jack

Uh oh: since this match didn't work, "$1" maintained its value from the
previous match. If Jack's blood type isn't AB, this could be fatal.

So let's try writing that middle part differently:

   chomp($blood = <STDIN>);
   my($type) = ($blood =~ m/(AB|A|B|O)/);
   print "Transfusing type $type blood into $name\n\n";

Okay, let's run it now:

   Enter patient name:   Bob
   Enter blood type:     type AB
   Transfusing type AB blood into Bob

   Enter patient name:   Jack
   Enter blood type:
   Use of uninitialized value in concatenation (.) or string at [...]
   Transfusing type  blood into Jack

Still not good.

The second-to-last line will only appear if you turned on warnings (which you
should have). You see, "$type" only has a meaningful value if the user entered
meaningful input; otherwise the match fails and "$type" has the special value
"undef". So Jack is getting undefined blood. I don't know if that carries any
risk but I wouldn't want to try it.

Let's try again:

   my $type;
   if ( $blood =~ m/(AB|A|B|O)/ ) {
     $type = $1;
   }
   print "Transfusing type $type blood into $name\n\n";

This "fix" has the same result as the previous one: "$type" is only set if the
blood type is valid, but the transfusion occurs anyway.

So let's do this right:

   #!/usr/bin/perl -w
   use strict;

   my($name, $blood);

   while ( 1 ) {   # Loop forever.
     print "Enter patient name:   ";
     chomp($name = <STDIN>);
     print "Enter blood type:     ";
     chomp($blood = <STDIN>);
     if ( $blood =~ m/(AB|A|B|O)/ ) {
       print "Transfusing type $1 blood into $name\n\n";
     }
     else {
       print "Wrong blood type; try again.\n\n";
     }
   }

Of course, we don't have to use "$1": we could do this instead:

     if ( my($type) = ($blood =~ m/(AB|A|B|O)/) ) {
       print "Transfusing type $type blood into $name\n\n";
     }
     else {
       print "Wrong blood type; try again.\n\n";
     }

The issue isn't whether or not you use "$1". What's important is that you
always plan for a failed match.

             -----------------------------------

4) Using Open Patterns

It's great to check your input, but do it properly. For example, you often want
a user to enter an e-mail address, so you test to make sure the e-mail address
is valid. You might write a test like this:

   if ( $address =~ /^[A-Za-z0-9.]+\@[A-Za-z0-9.]+$/ ) { ... }

But can an e-mail address include quotes? What about slashes?

It turns out that an e-mail address can contain almost any character (at least
on the left side of the "@"). So the following are valid e-mail addresses:
   a/b@example.com
   ***@example.com
   1+2=3@example.com

And let's not forget that not every top-level domain has just two or three
letters. If you doubt this, go to:
   http://index.museum/

For more information on e-mail addresses, consult RFCs 822 and 2822:
   http://www.faqs.org/rfcs/rfc822.html
   http://www.faqs.org/rfcs/rfc2822.html

Another example: suppose you want someone to enter his last name (sirname). If
you just write the validation without thinking about it, you might forget that
a last name can contain dashes, apostrophes (think "O'Conner") spaces (yes,
some people have compound last names) and accented letters (non-English names).

So remember: check your input, but do it properly. Don't assume you know more
than you do, and give the user some leeway.

Note: some of the examples in this tutorial are deliberately simplified. For
example, in some places we operate on the "Subject" field of an e-mail as
though it was a single line, but RFC 822 says that e-mail headers can be
"folded" into multiple lines. A real program would have to be a little more
rigorous.

             -----------------------------------

5) Exercise

Computers make very bad poets. However, a computer program might be able to
help a poet find a good rhyme.

Write a program that accepts lines of poetry (one at a time) and looks for
rhymes in a pre-programmed list of lines of poetry. The lines in the pre-
programmed list should be separated by newlines.

Of course, it's hard for a computer to determine whether two words rhyme. For
this exercise we will use a very simple mechanism: we will assume that two
words rhyme if their last three letters are the same. For extra credit, try to
find a better mechanism.

Your program should look like this:

   #!/usr/bin/perl -w
   use strict;

   # The following is a multi-line string.
   # (It's the beginning of a poem by A. E. Housman.)

   my $possibilities = "Terence, this is stupid stuff:
   You eat your victuals fast enough;
   There can't be much amiss, 'tis clear,
   to see the rate you drink your beer.
   But oh, good Lord, the verse you make,
   It gives a chap the belly-ache.";

   while ( defined( my $line = <STDIN> ) ) {
     if ( my($last_letters) = ( $line =~ [** regular expression 1 **] ) ) {
       if ( my($match) = ( $possibilities =~ [** regular expression 2 **] ) ) {
         print "That rhymes with: $match\n";
       }
       else {
         print "I can't think of anything that rhymes with that.\n";
       }
     }
     else {
       print "Sorry, I couldn't even begin to rhyme that one.\n";
     }
   }

All you have to do is fill in the parts in [** brackets **].

Some hints:
a) Remember that you only want to match letters; punctuation should be ignored.
b) Use variable interpolation in the second regular expression. That is,
    regular expression 2 should contain '$last_letters'. Doing that can
    sometimes lead to unexpected results (e.g., if $last_letters contained a
    period it would be interpreted as a wild card), but since we're only
    matching letters we shouldn't have any problems.
c) Make sure your match is case-insensitive: "poodle" rhymes with "NOODLE".
d) Test for matches at the beginning, middle and end of $possibilities (that
    is: the first line, the last line, and some other line).

In case you're wondering, I chose the poem "Terence, this is stupid stuff" for
two reasons. First, the rhyming words are spelled very differently, so whether
you want to rhyme with "fluff" or "rough", you will find a rhyme. Second, it's
what people will say to YOU if you let your computer write your poetry!

             -----------------------------------

6) Answer to Previous Exercise

The previous exercise was to write a program that splits a URI into protocol,
host and path. Here is mine:

   #!/usr/bin/perl -w
   use strict;

   while ( defined(my $line = <STDIN>) ) {
     chomp($line);
     if ( my($protocol,$host,$path) =
                       ($line =~ m<^([^:/]+)://([^/]+)(/?.*)$> ) ) {
       print "protocol=$protocol, host=$host, path=$path\n";
     }
     else {
       print "Sorry: I can't parse that URI.\n";
     }
   }

Yours will probably be different. For example, I was very loose on the
hostname; you might have stipulated that it must not contain certain
characters, such as spaces.

             -----------------------------------

7) Past Information

Part 1: Getting Started
         http://linuxchix.org/pipermail/courses/2003-March/001147.html

Part 2: Scalar Data
         http://linuxchix.org/pipermail/courses/2003-March/001153.html

Part 3: User Input
         http://linuxchix.org/pipermail/courses/2003-April/001170.html

Part 4: Control Structures
         http://linuxchix.org/pipermail/courses/2003-April/001184.html

Part 4.5, a review with a little new information at the end:
         http://linuxchix.org/pipermail/courses/2003-July/001297.html

Part 5: The "tr///" Operator:
         http://linuxchix.org/pipermail/courses/2003-July/001302.html

Part 6: Part 6: The "m//" Operator
         http://linuxchix.org/pipermail/courses/2003-August/001305.html

             -----------------------------------

8) Credits

Works cited:
a) "man perlop"
b) Kirrily Robert, Paul Fenwick and Jacinta Richardson's "Intermedia Perl",
    which you can find (along with their "Introduction to Perl") at:
    http://www.perltraining.com.au/notes.html

Thanks to Jacinta Richardson for fact checking.

             -----------------------------------

9) Licensing

This course (i.e., all parts of it) is copyright Alice Wood and Dan Richter,
and is released under the same license as Perl itself (Artistic License or GPL,
your choice). This is the license of choice to make it easy for other people to
integrate your Perl code/documentation into their own projects. It is not
generally used in projects not related to Perl.

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LinuxChix Perl Course Part 8: The "s///" Operator

1) Introduction
2) The "s///" Operator
2) Options
3) Greediness
4) Exercise
5) Answer to Previous Exercise
6) Past Information
7) Credits
8) Licensing

             -----------------------------------

1) Introduction

We're getting to the best part now. The "s///" operator is one of the most
powerful operators in Perl. You'll find yourself using it again and again.

Let me also mention that I'll be on vacation for the next two weeks, so you
won't hear from me again until September 12. But I encourage you to send your
exercises to the mailing list and to give suggestions on other people's
exercises.

             -----------------------------------

2) The "s///" Operator

The "s///" operator performs a regular expression substitution. For example,
try the following program:

   #!/usr/bin/perl -w
   use strict;

   my $text = 'food';
   $text =~ s/foo/bear/;
   print "$text\n";

The command "s/X/Y/" replaces the first instance of the regular expression X
with the text Y. (We'll see how to replace all instances in a moment.) If the
regular expression X is not found anywhere in the string, the command is
silently ignored.

A common use for "s///" is to remove text entirely (by specifying that the
text is to be replaced by nothing):

   $witness_data =~ s/Name: \w+//;      # Anonymise data by removing name.

As you have probably guessed, "s///" can use any delimiter, just like "m//"
and "tr///":

   $text =~ s/foo/bar/;
   $text =~ s!foo!bar!;
   $text =~ s<foo><bar>;

The "s///" operator can also use parentheses just like "m//":

   # Munge e-mail address. Note the "escaped at": @ is a special character.
   $address =~ s/(.+)\@([^@]+)/$1 at $2/;
   # Not a very logical way to do it; just for demonstration.

Note: you may see "\1" used instead of "$1". Both work, but "$1" is the
preferred method.

The "s///" operator accepts all of the options we discussed for "m//", and
they have the same meaning:
   i   Do case-insensitive pattern matching.
   m   Treat string as multiple lines (let /^/ and /$/ match "\n").
   s   Treat string as single line (let /./ match "\n").
   g   Match globally, i.e., change all occurrences.
We will also discuss an additional option:
   e   Evaluate the replacement as Perl code.

Examples:

   # Convert "foo", "Foo" or "FOO" to "bar".
   $text =~ s/foo/bar/i;

   # Reply to an e-mail. /^/ and /$/ mean beginning and end of line.
   $headers =~ s/^Subject: (.*)$/Subject: Re: $1/m;

The "g" option still means "global", but with "s///" it means "replace all
instances" rather than just "match all instances":

   # Convert all C++ comments to C comments.
   $C_code =~ s#//(.*)$#/*$1*/#mg;

Note that we used the "$1" variable even though there are multiple matches; $1
is re-evaluated for each match.

You won't use the "e" option very often, but it comes in very handy when you
do need it. For example:

   # Convert Japanese yen to euros.
   $price_list =~ s/([0-9.]+) yen/ ($1 * $conversion_factor) . ' euros' /ge;

             -----------------------------------

3) Greediness

Regular expressions in Perl are "greedy" by default, meaning that * and + will
match as many characters as possible. For example, in the following regular
expression:

   my($answer) = ( $foo =~ /The answer is: (.+)/ );

the /.+/ will match the whole answer, even though it would theoretically be
correct to only match the first character of the answer, or the first few
characters.

Usually you want to be greedy, but not always. If you don't want to be greedy,
add a question mark after the * or +. This doesn't change WHETHER there's a
match; it only changes the CONTENT of the match.

For example, if you want to strip HTML tags:

   $html =~ s/<.*>//sg;       # Oops: greediness got us here.

That doesn't work because Perl sees "<html>...</html>" as one big /<.*>/ (it
starts with "<" and ends with ">", right?). So it performs one replacement on
the entire HTML document, replacing the whole document with nothing! We need
to turn off the greediness:

   $html =~ s/<.*?>//sg;      # Much better.

This causes the /.*/ to be as short as possible while still making a match.

Of course, we could do this instead:

   $html =~ s/<[^>]*>//sg;    # Also works.

But it's not always that easy. Consider this code to strip C-style comments
from your C code:

   # Remove /* ... */ comments from your C code.
   $C_code =~ s</\*(.*?)\*/><>sg;
   # (Asterisks are escaped. Parentheses are provided only for clarity.
   # The <> is the delimiter.)

Once you get your head around that regular expression, you'll see that no
greedy regular expression would do.

             -----------------------------------

4) Exercises

a) Harvard cardiologist Thomas Michel writes in his "Guide to Politically
Correct Cardiology" about the importance of using inoffensive medical terms.
For example, he suggests saying "metabolically different" instead of the
highly offensive "dead"[*]. Write a Perl program that reads a medical
diagnosis (or any other input) and uses "s///" to change the word "dead" to
"metabolically different". Don't forget to use the "g" option to replace ALL
matches, and to use /\b/ (word break) to avoid false matches such as "deaden"
or "deadly".

[*] Yes, the paper is real and the scientist is real. Of course, he wasn't
taking himself too seriously when he wrote the paper.

b) American English is slightly different from British English in several
respects, one of which is spelling[*]. For example, words ending in "ise" in
England generally end in "ize" in the United States, e.g., "exercise" becomes
"exercize". Write a Perl program that "translates" such words from American to
British English, i.e., changes words ending in "ize" to end in "ise". For
extra credit, take into account variations like "exercizes" and "exercizing".

[*] Blame it on Noah Webster. The writer of the first dictionary in America,
Webster deliberately chose to spell words differently for reasons both
practical (making the language easier to learn for immigrants) and patriotic
(declaring linguistic independence).

             -----------------------------------

5) Answer to Previous Exercise

The previous exercise was to write a program to help you rhyme, where two
words are defined to "rhyme" if they have the last three letters in common.
Here is an example of such a program.

   #!/usr/bin/perl -w
   use strict;

   my $possibilities = "Terence, this is stupid stuff:
   You eat your victuals fast enough;
   There can't be much amiss, 'tis clear,
   to see the rate you drink your beer.
   But oh, good Lord, the verse you make,
   It gives a chap the belly-ache.";

   # The beginning of a poem by A. E. Housman

   while ( defined( my $line = <STDIN> ) ) {
     if ( my($last_letters) = ( $line =~ /(\w?\w?\w)\W*$/m ) ) {
       if ( my($match) = ( $possibilities =~ /^(.*$last_letters\W*)$/mi ) ) {
         print "That rhymes with: $match\n";
       }
       else {
         print "I can't think of anything that rhymes with that.\n";
       }
     }
     else {
       print "Sorry: I couldn't even begin to rhyme that one.\n";
     }
   }

The first regular expression:
     if ( my($last_letters) = ( $line =~ /(\w?\w?\w)\W*$/m ) ) {
means three "word characters", possibly followed by non-word characters,
followed by the end of the line (or the end of the string). I used \w for
brevity rather than [A-Za-z] even though \w includes digits and the underscore
character: there's no way for the computer to guess that "3" rhymes with
"see" anyway.

The question marks are there to anticipate the possibility that the last word
might be less than three letters long. (Our definition of "rhyming" didn't
actually take into account this case, but I considered it anyway.)

The last-three-letters algorithm has few false positives but a lot of false
negatives (e.g., it wouldn't recognise that any of the lines in the above poem
by Housman rhyme). To try to improve accuracy, I wrote a regular expression
that interprets the rhyming part of the word as follows:
a) if the word ends in a "y", the rhyming part is the "y" and any vowels
immediately before it: "flY", "plAY".
b) if the word ends in an "e" not directly following a vowel, the rhyming part
is defined like in (c), excecpt the silent "e" is treated as a consonant:
"possIBLE", "remAKE".
c) otherwise, the rhyming part is the part containing the last group of one or
more vowels followed by any consonants: "shACK", "sEA", "sEE".

The regular expression is as follows:

   my $vowel = '[aeiou]';
   my $consonant = '[b-df-hj-np-tv-z]';
   my $consonant_or_e_not_y = '[b-hj-np-tvwxz]';

   my $match_ending_letter = '[aiouy]';
   my $match_continuing_letter = $consonant;

   while ( defined( my $line = <STDIN> ) ) {
     if ( my($last_letters) = ( $line =~
/($vowel*$consonant*$consonant_or_e_not_y|$vowel*$match_ending_letter)\W*$/im
            ) ) {

Though impressive, in practice it's not any more accurate than the last-three-
letters algorithm, and it's probably less accurate. Oh, well.

Finding rhymes is truly not for the fainthearted programmer. For example, the
words "food" and "good" don't rhyme even though their spelling suggests that
they would, and the word "read" can be pronounced either like "reed" or "red",
depending on its function in the sentence.

But if you ever do get really good at teaching poetry to computers, perhaps
you can join Damian Conway, "the mad scientist of Perl", who wrote a Perl
module to automatically generate a haiku before displaying an error message.
As he puts it:

                      When a program dies
                   what you need is a moment
                      of serenity.

             -----------------------------------

7) Past Information

Part 1: Getting Started
         http://linuxchix.org/pipermail/courses/2003-March/001147.html

Part 2: Scalar Data
         http://linuxchix.org/pipermail/courses/2003-March/001153.html

Part 3: User Input
         http://linuxchix.org/pipermail/courses/2003-April/001170.html

Part 4: Control Structures
         http://linuxchix.org/pipermail/courses/2003-April/001184.html

Part 4.5, a review with a little new information at the end:
         http://linuxchix.org/pipermail/courses/2003-July/001297.html

Part 5: The "tr///" Operator
         http://linuxchix.org/pipermail/courses/2003-July/001302.html

Part 6: The "m//" Operator
         http://linuxchix.org/pipermail/courses/2003-August/001305.html

Part 7: More About "m//"
         http://linuxchix.org/pipermail/courses/2003-August/001322.html

             -----------------------------------

8) Credits

Works cited: "man perlop"

Thanks to Jacinta Richardson for fact checking.

             -----------------------------------

9) Licensing

This course (i.e., all parts of it) is copyright 2003 by Alice Wood and Dan
Richter, and is released under the same license as Perl itself (Artistic
License or GPL, your choice). This is the license of choice to make it easy
for other people to integrate your Perl code/documentation into their own
projects. It is not generally used in projects unrelated to Perl.


From daniel.richter@wimba.com  Fri Sep 12 22:26:49 2003
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Well, I came back from vacation a few days ago and I'm already behind. A 
new record. As one person put it, the sooner you get behind, the more time 
you have to catch up.

So I'm sorry to say that I won't get Part 9 out today. With luck it will be 
ready on Tuesday.

-- 
  [Larry] Wall [inventor of Perl] believes that people think about
  things in different ways, that natural languages accommodate many
  mindsets, and that programming languages should too.
    - Jon Udell, in his essay, "A Perl Hacker in the Land of Python"

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LinuxChix Perl Course Part 9: Simple File Access

1) Introduction
2) The "die" Command
3) Simple File Access
4) Exercise
5) Answers to Previous Exercises
6) Past Information
7) Credits
8) Licensing

             -----------------------------------

1) Introduction

As we will see shortly, Perl uses the "open" command to read from and write to
files. The "open" command is very powerful and is not limited to files, but we
won't see that until next week.

             -----------------------------------

2) The "die" Command

The "die" command does exactly that: it kills your program. Example:

   die;

Of course, you'll usually want to make the command conditional and give a
helpful error message:

   if ( $there_was_an_error ) {
     die "There was an error. Sorry. Aborting.";
   }

If you run this command (and you set $there_was_an_error to some non-zero
value), you will see:

   There was an error. Sorry. Aborting. at test.pl line 7.

Of course, if you try it yourself, "test.pl" will be replaced by the file in
which you did the test. Perl is trying to be helpful by telling you where the
program died. But if you don't want that additional information, put a newline
on the end of the message string:

   die "There was an error. Sorry. Aborting.\n";   # No file+line information.

There are two additional subtleties to the "die" command:
a) it causes the message to be sent to standard error, not standard out, so
    the user will see it even if he is piping the output to a file.
b) it sets the program's exit code to indicate that an error occurred. A
    program such as a shell script can detect this error code and react
    accordingly.

Sometimes it's helpful to include the last error message, which is stored in
the special variable "$!". We'll see more about this in a moment.

Obviously, "die" should only be used when something has prevented the program
from doing whatever it was supposed to do. To end a program without an error,
use "exit".

             -----------------------------------

3) Simple File Access

Here's a simple program to copy one file to another:

   #!/usr/bin/perl -w
   use strict;

   open MY_INPUT_FILE, "< in.txt"   or die "Can't read in.txt: $!";
   open MY_OUTPUT_FILE, "> out.txt" or die "Can't write to out.txt: $!";

   while ( defined ( my $line = <MY_INPUT_FILE> ) ) {
     print MY_OUTPUT_FILE $line;
   }

   close MY_INPUT_FILE;
   close MY_OUTPUT_FILE;

Let's explain this from the top down.

First, we can see that the "open" command in this example takes two parameters:
a file handle and a file name. You'll notice that the file handle
(MY_INPUT_FILE in this example) isn't a normal scalar because it doesn't begin
with a dollar sign. (We didn't have to declare it with "my" either.) It also
doesn't have quotes around it. Plain text like this that isn't surrounded by
explanitory punctuation (like quotes) is called a "bareword". We'll be seeing
barewords a little more in the future. We actually could have put quotes around
it (in this "open" command at least), but by convention we don't.

The "file handle" is simply a way of referring to a file once you've opened it.
In C, file handles are integers; in Perl, they are text. By convention we write
file handles in all CAPS.

The file name in this case is "in.txt". The "<" in front of it means that the
file is being opened in read mode (so we will be able to read from it but not
write to it). The white space after the "<" won't cause any problems because
"open" strips all white space from the beginning and end of the file name. As
we will see later, you can use a command called "sysopen" if you want to open
files that begin or end with whitespace characters.

After opening the file, we check whether the "open" succeeds using "or die
...". Since Perl uses "short-circuit" boolean evaluation, the "die" command
will only be executed if "open" returns false (failure). This means that if the
file doesn't exist or can't be read, we'll know it right away. It's important
to detect problems right away; otherwise we'll be scratching our heads
wondering why the read command fails.

The command to open the output file is just like the command to open the input
file, except that it uses ">" instead of "<". If we wanted to append to the
file instead of overwriting it, we would use ">>".

In the next line, we can see that we can read from the file by surrounding its
file handle with "<>". We've seen this before: remember "<STDIN>"? In that case
the file handle was "STDIN", but the same syntax can be applied to any file
handle.

The line containing "print" is a little different from any "print" statement
we've seen before because it includes a file handle. This causes "print" to
output the text to the file handle rather than standard output (STDOUT). Also
note that there's no comma between the file handle and the text to be printed.

Finally, we close both files. If we forget to close a file, Perl does a good
job of cleaning up after our sloppiness, but it's still best to close the file
explicitly.

Note that all built-in Perl functions/commands can be written with or without
parentheses, so we could have written the above program like this:

   open(MY_INPUT_FILE, "< in.txt")   or die "Can't read in.txt: $!";
   open(MY_OUTPUT_FILE, "> out.txt") or die "Can't write to out.txt: $!";

   while ( defined ( my $line = <MY_INPUT_FILE> ) ) {
     print(MY_OUTPUT_FILE $line);
   }

   close(MY_INPUT_FILE);
   close(MY_OUTPUT_FILE);

This has no effect whatsoever on the way the program functions. As usual, There
Is More Than One Way To Do It. Just make sure you don't put a space between the
function name and the parenthesis; otherwise Perl will interpret it as a list.

As we've seen, a prefix (such as "<" or ">") in front of the file name
indicates whether the file is for reading or writing. Here is a list of such
prefixes:
   Prefix  Meaning
     <     read-only
     >     write-only
     >>    write-only, appending to end of file
     +<    read+write (if you're not sure which read+write to use, it's this)
     +>    read+write, erasing the file when it's opened
     +>>   read+write, appending data to the end of the file
     |     execute command (also works as a suffix; more about this
           next week)

Additionally, a file that is just called "-" refers to standard input (if you
read from it) or standard output (if you write to it). This is a convention
used in many Unix commands.

This begs the question: what do you do if you want to open a file that begins
with one of these characters or with white space? Perl includes a more
traditional command called "sysopen" which assumes that the file name you give
it is exactly the file name you want. After opening the file, you can operate
on the file handle exactly as if you had used "open" (i.e., there is no
"sysclose"). We won't say anymore here because in practice "sysopen" is rarely
used, but it is available if you want it. To learn more about it: "perldoc -f
sysopen".

             -----------------------------------

4) Exercise

One of the previous exercises (one solution to which is found below) was to
"translate" the American "ize" into the British "ise". Modify this program to
read its input from a file and write its output to another file.

             -----------------------------------

5) Answers to Previous Exercises

a) Here is a program that replaces the word "dead" with "metabolically
different":

   #!/usr/bin/perl -w
   use strict;

   while ( defined(my $line = <STDIN>) ) {
     $line =~ s/\bdead\b/metabolically different/g;
     print $line;
   }

b) Here is a very basic program that replaces "ize" with "ise":

   #!/usr/bin/perl -w
   use strict;

   while ( defined(my $line = <STDIN>) ) {
     $line =~ s/ize\b/ise$1/g;
     print $line;
   }

Here is more advanced version that takes into account variations like
"realised" and "desensitising". It also avoids changing the spelling of the
words "size", "resize" and "downsize", which are spelled the same way in
England and America:

   #!/usr/bin/perl -w
   use strict;

   while ( defined(my $line = <STDIN>) ) {
     $line =~ s/iz(e|es|ing)\b/is$1/g;
     $line =~ s/\b(re|down)?sis(e|es|ing)\b/$1siz$2/g;
     print $line;
   }

             -----------------------------------

6) Past Information

Part 1: Getting Started
         http://linuxchix.org/pipermail/courses/2003-March/001147.html

Part 2: Scalar Data
         http://linuxchix.org/pipermail/courses/2003-March/001153.html

Part 3: User Input
         http://linuxchix.org/pipermail/courses/2003-April/001170.html

Part 4: Control Structures
         http://linuxchix.org/pipermail/courses/2003-April/001184.html

Part 4.5, a review with a little new information at the end:
         http://linuxchix.org/pipermail/courses/2003-July/001297.html

Part 5: The "tr///" Operator
         http://linuxchix.org/pipermail/courses/2003-July/001302.html

Part 6: The "m//" Operator
         http://linuxchix.org/pipermail/courses/2003-August/001305.html

Part 7: More About "m//"
         http://linuxchix.org/pipermail/courses/2003-August/001322.html

Part 8: The "s///" Operator
         http://linuxchix.org/pipermail/courses/2003-August/001330.html

             -----------------------------------

7) Credits

Works cited:
a) "man perlop"
b) "man perlopentut"
c) Kirrily Robert, Paul Fenwick and Jacinta Richardson's "Intermedia Perl",
    which you can find (along with their "Introduction to Perl") at:
    http://www.perltraining.com.au/notes.html

Thanks to Jacinta Richardson for fact checking.

             -----------------------------------

8) Licensing

This course (i.e., all parts of it) is copyright 2003 by Alice Wood and Dan
Richter, and is released under the same license as Perl itself (Artistic
License or GPL, your choice). This is the license of choice to make it easy
for other people to integrate your Perl code/documentation into their own
projects. It is not generally used in projects unrelated to Perl.

From daniel.richter@wimba.com  Tue Sep 16 18:51:29 2003
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Subject: Re: [Courses] [Perl] Part 9: Simple File Access
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Oops: there's an error.

>b) Here is a very basic program that replaces "ize" with "ise":
>
>   #!/usr/bin/perl -w
>   use strict;
>
>   while ( defined(my $line = <STDIN>) ) {
>     $line =~ s/ize\b/ise$1/g;
>     print $line;
>   }

The "$1" in this program is an error; it should simply be removed.

-- 
     He doesn't have ulcers, but he's a carrier.
                - Anonymous

From daniel.richter@wimba.com  Wed Sep 24 19:20:21 2003
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Subject: [Courses] [Perl] Part 10: Executing Commands with "open"
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LinuxChix Perl Course Part 10: Executing Commands with "open"

1) Introduction
2) Accessing Command-Line Arguments
3) Executing Programs with "open"
4) When Not to Use "open"
5) A Simple Safety Rule
6) Exercises
7) Answer to Previous Exercise
8) Past Information
9) Credits
10) Licensing

             -----------------------------------

1) Introduction

Last week we saw how to read from and write to files using the "open" command.
It turns out that "open" can also be used to execute programs and read their
output, or even send output to them. Part 10 explains how.

             -----------------------------------

2) Accessing Command-Line Arguments

All of this working with "open" is making my head hurt, so let's start out with
a minor detour into something much easier.

Command-line arguments are passed to your program on the command line (hence
the name "command-line arguments"). Suppose that you have a Perl program called
"yourscript" and you execute it like this:

   yourscript arg1 arg2 arg3

You could also execute it like this:

   perl yourscript arg1 arg2 arg3

In either case, the "arg" parameters are command-line arguments.

Your program can access its first command-line argument by calling the "shift"
function. It can access the second command-line argument by calling "shift"
again, and so on. When there are no more parameters to return, "shift" returns
undefined. (The name "shift" comes from a shell script command with the same
name and a similar purpose.)

Try this program. Don't forget to pass command-line arguments, or it won't do
anything!

   #!/usr/bin/perl -w
   use strict;

   while ( defined( my $person = shift ) ) {
     print "Hi, $person!\n";
   }

Note that we don't have to "chomp" command-line arguments because the operating
system removes the whitespace around them (unless of course the user explicitly
chooses to put whitespace around them).

Some people are surprised to learn that you CANNOT access command-line
arguments as $1, $2, etc. That's what you would do in a shell script, but in
Perl these variables are reserved for regular expression matching.
Interestingly enough, however, the variable $0 has a meaning in Perl similar to
its meaning in shell scripts.

By the way, command-line arguments can also be accessed via the array "@ARGV".
But since we haven't discussed arrays yet, we won't get into that.

             -----------------------------------

3) Executing Programs with "open"

In addition to what we saw last week, the "open" command has one more very
powerful application: it allows you to execute a command, send input and
receive output.

Try this program (it only works on Unix):

   #!/usr/bin/perl -w
   use strict;

   open DATA, "who |"   or die "Couldn't execute program: $!";
   while ( defined( my $line = <DATA> )  ) {
     chomp($line);
     print "$line\n";
   }
   close DATA;

Here's what happened: Perl saw that your "file" ended with a "pipe" (vertical
bar) character. So it interpreted the "file" as a command to be executed, and
interpreted the command's output as the "file"'s contents. The command is
"who" (which prints information on currently logged-in users). If you execute
that command, you will see that the output is exactly what the Perl program
gave you.

In this case, we "read" data from the command. To execute a command that we can
"write" (send data) to, we should place a pipe character BEFORE the command.
These options are mutually exclusive: we can read from a command or write to
it, but not both.

In the Unix world, a lot can be done by piping the output of one program into
the input of another. Perl continues this spirit.

Note that we can also send command-line parameters to the command, like this:

   open DATA, "who -H |"    or die "Couldn't execute program: $!";

In fact, Perl allows you to use "open" to do pretty much anything you would
normally do on the command-line, as this example demonstrates:

   open OUTPUT, "| grep 'foo' > result.txt"     or die "Failure: $!";

We can then write whatever we want to the "OUTPUT" filehandle. The Unix "grep"
command will filter out any text which doesn't contain the text "foo"; any text
which DOES contain "foo" will be written to "result.txt".

             -----------------------------------

4) When Not to Use "open"

Don't overuse "open". This command should only be used when you need to pipe a
lot of data to or from a program.

For programs that give no output (or if you don't mind the output to going to
the screen), use the "system" command:

   system "tar cf myArchive.tar *.c";     # Run "tar" to create an archive.

Note: "system" breaks the convention of returning true if the program succeeds.
Instead, it returns the exit code of the program plus some additional
information, or -1 if the program couldn't be executed at all. You can count on
"system" returning zero if the command succeeded, nonzero otherwise.

And for programs that only return a few lines of output, use back-ticks:

   my $username = `whoami` or die "Couldn't execute command: $!";
   chomp($username);
   print "My name is $username\n";

You can also use the "qx//" operator, which is exactly equivalent to back-ticks
except that it allows you to choose your delimiter:

   # All of the following are exactly equivalent
   # to the above command that uses back-ticks.
   my $username = qx/whoami/ or die "Couldn't execute command: $!";
   my $username = qx(whoami) or die "Couldn't execute command: $!";
   my $username = qx#whoami# or die "Couldn't execute command: $!";

             -----------------------------------

5) A Simple Safety Rule

If you use the "open" command but don't specify whether you are opening for
reading, writing or piping, Perl will assume that you are opening for reading.
But DON'T DO THAT! Here's an example of why.

One of the common uses of Perl is CGI (web) scripting. As the Perl master in
your company, suppose you write a nice little dynamic web page that allows the
user to choose a department, then generates the list of the people in that
department. You write it like this:

   open DEPARTMENT_DIRECTORY, "/usr/departments/$department" or die "...";
   # Process and output the data.
   close DEPARTMENT_DIRECTORY;

Well, one day someone will say, "hey, what happens if I enter
'../../bin/rm -fr / |'?"

Of course, what happens is that the command will do this:

   open DEPARTMENT_DIRECTORY, "/usr/departments/../../bin/rm -fr / |";

The pipe (vertical bar) at the end of the file name means that it's a command
to be executed. In fact, it's that famous command that wipes out everything on
your hard drive.

But this problem is really, really easy to avoid: just put a "<" on the front
of the file you want to read. This will cause the "open" to fail if someone
tries to trick your script into doing something other than reading.

You have been warned.

             -----------------------------------

6) Exercise

Some Unix systems include a "pidof" command, which outputs the PID(s)
associated with a currently running program:

   [me@mybox]$ pidof bash
   1032
   1059

In the above example, there are two "bash" processes running, one with PID 1032
and one with PID 1059.

Write your own "pidof" program using Perl. Use the Unix command "ps -e" to read
all of the processes that are currently running.

             -----------------------------------

7) Answer to Previous Exercise

Here is a program that reads a file (american.txt), "translates" it from
American to British spelling (at least partly), and writes the result to
another file (british.txt).

   #!/usr/bin/perl
   use strict;

   open AMERICAN, '< american.txt' or die "Couldn't open file: $!";
   open BRITISH, '> british.txt' or die "Couldn't open file: $!";

   while ( defined(my $line = <AMERICAN>) ) {
     $line =~ s/iz(e|es|ing)\b/is$1/g;
     print BRITISH $line;
   }

   close AMERICAN;
   close BRITISH;

             -----------------------------------

8) Past Information

Part 1: Getting Started
         http://linuxchix.org/pipermail/courses/2003-March/001147.html

Part 2: Scalar Data
         http://linuxchix.org/pipermail/courses/2003-March/001153.html

Part 3: User Input
         http://linuxchix.org/pipermail/courses/2003-April/001170.html

Part 4: Control Structures
         http://linuxchix.org/pipermail/courses/2003-April/001184.html

Part 4.5, a review with a little new information at the end:
         http://linuxchix.org/pipermail/courses/2003-July/001297.html

Part 5: The "tr///" Operator
         http://linuxchix.org/pipermail/courses/2003-July/001302.html

Part 6: The "m//" Operator
         http://linuxchix.org/pipermail/courses/2003-August/001305.html

Part 7: More About "m//"
         http://linuxchix.org/pipermail/courses/2003-August/001322.html

Part 8: The "s///" Operator
         http://linuxchix.org/pipermail/courses/2003-August/001330.html

Part 9: Simple File Access
         http://linuxchix.org/pipermail/courses/2003-September/001340.html

             -----------------------------------

9) Credits

Works cited:
a) "man perlop"
b) "man perlopentut"
c) Kirrily Robert, Paul Fenwick and Jacinta Richardson's "Intermediate Perl",
    which you can find (along with their "Introduction to Perl") at:
    http://www.perltraining.com.au/notes.html

Thanks to Jacinta Richardson for fact checking.

             -----------------------------------

10) Licensing

This course (i.e., all parts of it) is copyright 2003 by Alice Wood and Dan
Richter, and is released under the same license as Perl itself (Artistic
License or GPL, your choice). This is the license of choice to make it easy
for other people to integrate your Perl code/documentation into their own
projects. It is not generally used in projects unrelated to Perl.


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LinuxChix Perl Course Part 11: Perl Variables

1) Weird Names for Variables
2) Use English
3) The Special Variable "$_"
4) Exercises
5) Answer to Previous Exercise
6) Past Information
7) Credits
8) Licensing

             -----------------------------------

1) Weird Names for Variables

Perl has lots of built-in variables with odd names. We've already seen
two of them: "$/" and "$!". But there are plenty more.

For example, "$<" and "$>" are the current and effective user ID,
respectively:

   print "UID = $<\n";
   print "Effective UID = $>\n";

The variables "$(" and "$)" represent your real and effective group IDs,
but they are more complicated.

Assigning to these variables will change the user or group under which 
the process runs (within the limits of what your operating system allows 
of course: trying to set "$<" to zero won't necessarily make you root). 
But this could lead to problems with security and cross-platform 
compatibility that are too complicated to go into here, so do a little 
research before actually trying it.

The variables "$?" (result of executing a program) and "$$" (PID of 
current process) have a similar meaning to their meaning in certain 
shells, such as bash (the Bourne-Again Shell).

The variable "$," is easy to remember because its content is output 
every time you see a comma in a print statement:

   my($a,$b,$c) = ('foo', 'bar', 'baz');
   print $a, $b, $c, "\n";  # Output is: "foobarbaz"
   $, = '##';
   print $a, $b, $c, "\n";  # Output is: "foo##bar##baz##";

For the full list of weird Perl variables, see "man perlvar". Don't 
worry about memorising all of them; you'll learn the most important ones 
simply through use.

All special Perl variables are global, which means that setting them
introduces potential side effects. We will see more about that next week.

             -----------------------------------

2) Use English

No, this isn't an anglo-centric statement; it's a Perl command.

All special variables have names that are (arguably) more intuitive. For
example, "$," can also be called "$OUTPUT_FIELD_SEPARATOR" or
"$OFS". However, to use these names you must "use English;".

   #!/usr/bin/perl -w
   use strict;
   use English;

   $OUTPUT_FIELD_SEPARATOR = '|';    # Equivalent to: $, = '|';
   print 'field 1', 'field 2';
   print "\n";

However, due to an oversight in its implementation, "use English" makes
regular expression matches slower. So you are welcome to "use English" 
right now if it helps you, but it's not recommended for writing Perl 
libraries.

             -----------------------------------

3) The Special Variable "$_"

Probably the most used variable in Perl is "$_", pronounced
"dollar-underscore" and called "$ARG" if you "use English". This 
variable is passed as an understood parameter to many functions and 
operators if an expected parameter is not explicitly passed. Examples:

   chomp;       # Equivalent to chomp($_);
   print;       # Equivalent to print $_;

"$_" is the understood subject of "tr///", "m//" and "s///":

   s/foo/bar/;    # Equivalent to $_ =~ s/foo/bar/;
   if ( /baz/ ) { ... }    # Equivalent to $_ ~= m/baz/

In addition, for any file handle "HANDLE", the following are equivalent:

   while ( <HANDLE> ) { ... }
       # ... is equivalent to ...
   while ( defined( $_ = <HANDLE> ) ) { ... }

Caution: this trick only works if "<HANDLE>" is used as the sole 
criterion of a while loop. So the following do NOT set "$_":

   <HANDLE>;   # NOT equivalent to $_ = <HANDLE>;
   if ( <HANDLE> ) { ... }    # NOT $_ = <HANDLE>

The classic example use of "$_" is a program like the following:

   while ( <STDIN> ) {     # Read line of input.
     s/puppy/dog/g;        # Change "puppy" to "dog".
     s/kitten/cat/g;       # Change "kitten" to "cat".
     print;                # Output the line, with changes.
   }

Despite the "magic" nature of "$_", it can still be assigned to or from
just like any other variable. (This is handy for testing.)

   $_ = "foo";
   my $bar = $_;

We will see more uses of "$_" with time.

             -----------------------------------

4) Exercise

One previous exercise was to write a program that converted the word 
"dead" to "metabolically different". Here was my solution to this problem:

   #!/usr/bin/perl -w
   use strict;

   while ( defined(my $line = <STDIN>) ) {
     $line =~ s/\bdead\b/metabolically different/g;
     print $line;
   }

Modify this program (or your own if you prefer) so that it uses no 
variables except "STDIN" and "$_", and try to avoid explicitly naming "$_".

             -----------------------------------


5) Answer to Previous Exercise

Here is my "pidof" program.

   #!/usr/bin/perl
   use strict;

   my $program = shift;    # Program to search for.

   unless ( defined($program) ) {
     die "Please give the name of a program on the command-line.\n";
   }

   open PS, "ps -e |"    or die "Couldn't execute 'ps': $!";
   while ( my $line = <PS> ) {
     chomp($line);
                    #    PID    TTY  Time  Program
     if ( $line =~ /^\s*(\d+)\s+\S+\s+\S+\s+(\S+)\s*$/ ) {
       print "$1\n" if $2 eq $program;
     }
   }
   close PS;

             -----------------------------------

6) Past Information

Part 1:  Getting Started
          http://linuxchix.org/pipermail/courses/2003-March/001147.html

Part 2:  Scalar Data
          http://linuxchix.org/pipermail/courses/2003-March/001153.html

Part 3:  User Input
          http://linuxchix.org/pipermail/courses/2003-April/001170.html

Part 4:  Control Structures
          http://linuxchix.org/pipermail/courses/2003-April/001184.html

Part 4.5, a review with a little new information at the end:
          http://linuxchix.org/pipermail/courses/2003-July/001297.html

Part 5:  The "tr///" Operator
          http://linuxchix.org/pipermail/courses/2003-July/001302.html

Part 6:  The "m//" Operator
          http://linuxchix.org/pipermail/courses/2003-August/001305.html

Part 7:  More About "m//"
          http://linuxchix.org/pipermail/courses/2003-August/001322.html

Part 8:  The "s///" Operator
          http://linuxchix.org/pipermail/courses/2003-August/001330.html

Part 9:  Simple File Access
      http://linuxchix.org/pipermail/courses/2003-September/001340.html

Part 10:
      http://linuxchix.org/pipermail/courses/2003-September/001344.html

             -----------------------------------

7) Credits

Works cited:
a) "man perlvar"
b) Kirrily Robert, Paul Fenwick and Jacinta Richardson's "Intermediate 
Perl",
    which you can find (along with their "Introduction to Perl") at:
    http://www.perltraining.com.au/notes.html

Thanks to Jacinta Richardson for fact checking.

             -----------------------------------

8) Licensing

This course (i.e., all parts of it) is copyright 2003 by Alice Wood and 
Dan Richter, and is released under the same license as Perl itself 
(Artistic License or GPL, your choice). This is the license of choice to 
make it easy for other people to integrate your Perl code/documentation 
into their own projects. It is not generally used in projects unrelated 
to Perl.


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LinuxChix Perl Course Part 12: Side Effects with Perl Variables

1) Introduction
2) Variable Scope
3) Side Effects
4) [Non-]Exercise
5) Answer to Previous Exercise
6) Past Information
7) Credits
8) Licensing

             -----------------------------------

1) Introduction

Last week we saw how to use Perl's special variables. This week we are 
going to see an important caveat concerning these special variables. 
Developing good habits now will save us lots of debugging when we start 
writing larger programs.

             -----------------------------------

2) Variable Scope

Like most languages, Perl includes the concept of "scope". The scope of 
a variable is the part of the code in which it can be accessed.

   if ( $num != 45 ) {
     my $foo = 'bar';
     print "$foo\n";    # OK
   }
   print "$foo\n";      # D'oh! Went out of scope.

Of course, Perl will only warn us of this mistake if we "use strict". 
(Otherwise, it will just create a new variable named "$foo".)

In Perl, the scope of a variable is determined (partly) by the depth of 
the "code block" in which it was declared. A "code block" is delimited 
by opening and closing braces. Note that you can create a code block 
without using keywords such as "if":

   my $foo = 'abc';
   {       # Start inner code block.
     print "Before redefinition, foo = $foo\n";
     my $foo = 'xyz';   # New declaration in this code block.
     print "Inside code block, foo = $foo\n";
   }
   print "Outside code block, foo = $foo\n";

Try the above code. As you can see, the inner code block "inherits" the 
variables defined outside it. By contrast, variables declared inside the 
code block "vanish" when the code block is finished, even if they have 
the same names as variables declared outside the code block.

             -----------------------------------

3) Side Effects

All Perl variables are global unless explicitly declared otherwise 
(e.g., with "my"). If a global variable "$foo" appears in several places 
in your program, it's the same "$foo", even if it's mentioned in a 
separate library. This can wreak all kinds of havoc because you might 
have used the name "$foo" without knowing that there was a "$foo" in 
some library that you happened to use. This is called a "side effect", 
and it's usually not wanted. This is why you should always declare your 
variables with "my", which makes them private rather than global.

What about the special variables we talked about last week? As we have 
seen, they change the workings of Perl functions and operators. (For 
example, "$/" changes the way "<STDIN>" breaks "lines".) This is an 
expected and desired side effect.

But when we start using functions (other than those built into the 
language), we may run into situations where one part of the code doesn't 
anticipate the fact that another part of the code changed a global 
variable. Here is an example:

   # Declare function "home_dir". Takes a
   # username and returns its home directory.
   sub home_dir {
     open PASSWD, '< /etc/passwd' or die "Uh oh: $!";
     $/ = ':';               # Records separated by colons.
     # ...
     close PASSWD;
     return $home;
   }

   # Main body.
   # Read usernames (one per line).
   open FILE, '< somefile.txt' or die "NO!!!";
   while ( my $username = <FILE> ) {
     chomp($username);
     my $home = home_dir($username);
     print "$username: $home\n";
   }
   close FILE;

Because "$/" is global, the "$/" in the function is the same as the "$/" 
in the main body. So setting "$/" in the function changes the way 
"<FILE>" operates in the main body. This type of bug is difficult to 
track down.

We cannot solve the problem by declairing "$/" with "my" because we want 
it to have some side effects (besides, Perl will yell at you if you 
try). So we declare it as "local". Variables declared with "local" can 
be accessed by functions called within the code block, but after the 
code block is closed (with a "}") the change ceases to take effect. You 
should usually use "local" when you change a special Perl variable:

   # Declare function "home_dir".
   sub home_dir {
     open PASSWD, '< /etc/passwd' or die "Uh oh: $!";
     local $/ = ':';       # Declare variable and set value.
     # ...
     close PASSWD;
   }

But there's an even more subtle trap here. If we had changed the 
variable in the main body and not the function, the change would have 
been reflected in the function even if it was declared as local in the 
main body:

   # Stupid function; just for demonstration.
   sub read_line {
     return <STDIN>;
   }

   # Main body.
   local $/ = ',';
   # ...
   print read_line() . "\n";

In this example, setting "$/" has an effect on "read_line" even though 
"$/" was declared as "local". It's important that you understand the 
difference between this example and the previous one. One way to guard 
against this later problem is to enclose the changed variable in a code 
block:

   # Main body.
   {   # Introduce code block; scope is reduced to this block.
     local $/ = ',';
     # ...
   }   # End of block. Local $/ goes out of scope.
   print read_line() . "\n";

But this implies knowing that "read_line" would be affected by your 
changed variable. Are you supposed to check the implementation of every 
function you call to see if it uses any built-in Perl variables? Of 
course not! Instead, the rule is: as much as possible, you should not 
call ANY function when you have changed a special variable, unless you 
intended the change to affect that function. The correlary to that rule 
is: make the code block containing the changed variable as small as 
possible.

It's especially important to declare "$_" as local in functions, to 
avoid this problem:

   # Change employee list to keep up with people quitting.
   while ( <IN_FILE> ) {
     my $quit_person = GetPersonWhoQuit();   # Did this change "$_"?
     s/$quit_person//g;        # Uh oh: where did the line of input go?
     print OUT_FILE;           # I wonder what we just output!
   }

Of course, you should use your common sense here. For example, when you 
change "$<" (user ID), you probably want the change to be permanent and 
global. Likewise, functions should definitely declare "$_" as local, but 
declaring "$!" (last error) as local is probably less important because 
that variable is expected to change - indeed, the caller might use "$!" 
to understand why the function failed. Think carefully about the 
expected behavior, but when in doubt, avoid side effects.

You have been warned!

             -----------------------------------

4) [Non-]Exercise

There's no exercise this week.

             -----------------------------------

5) Answer to Previous Exercise

Here is a previous program which does not explicitly name any variable.

   #!/usr/bin/perl -w
   use strict;

   while ( <STDIN> ) {
     s/\bdead\b/metabolically different/g;
     print;
   }

             -----------------------------------

6) Past Information

Part 1:  Getting Started
      http://linuxchix.org/pipermail/courses/2003-March/001147.html

Part 2:  Scalar Data
      http://linuxchix.org/pipermail/courses/2003-March/001153.html

Part 3:  User Input
      http://linuxchix.org/pipermail/courses/2003-April/001170.html

Part 4:  Control Structures
      http://linuxchix.org/pipermail/courses/2003-April/001184.html

Part 4.5, a review with a little new information at the end:
      http://linuxchix.org/pipermail/courses/2003-July/001297.html

Part 5:  The "tr///" Operator
      http://linuxchix.org/pipermail/courses/2003-July/001302.html

Part 6:  The "m//" Operator
      http://linuxchix.org/pipermail/courses/2003-August/001305.html

Part 7:  More About "m//"
      http://linuxchix.org/pipermail/courses/2003-August/001322.html

Part 8:  The "s///" Operator
      http://linuxchix.org/pipermail/courses/2003-August/001330.html

Part 9:  Simple File Access
      http://linuxchix.org/pipermail/courses/2003-September/001340.html

Part 10: Executing Commands with "open"
      http://linuxchix.org/pipermail/courses/2003-September/001344.html

Part 11: Perl Variables
      http://linuxchix.org/pipermail/courses/2003-October/001345.html

             -----------------------------------

7) Credits

Works cited:
a) "man perlvar"
b) Kirrily Robert, Paul Fenwick and Jacinta Richardson's
    "Intermediate Perl", which you can find (along with their
    "Introduction to Perl") at:
    http://www.perltraining.com.au/notes.html

Thanks to Jacinta Richardson for fact checking.

             -----------------------------------

8) Licensing

This course (i.e., all parts of it) is copyright 2003 by Alice Wood and 
Dan Richter, and is released under the same license as Perl itself 
(Artistic License or GPL, your choice). This is the license of choice to 
make it easy for other people to integrate your Perl code/documentation 
into their own projects. It is not generally used in projects unrelated 
to Perl.


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LinuxChix Perl Course Part 13: Perl Style

1) Introduction
2) The Many Faces of Perl
3) The Special File Handle "ARGV"
4) Exercise
5) [Non-]Answer to Previous Exercise
6) Past Information
7) Credits
8) Licensing

             -----------------------------------

1) Introduction

An important concept in Perl is that "There Is More Than One Way To Do 
It" (TIMTOWTDY, pronounced "Tim Towtdy"). We have seen this to some 
degree already. A simple example is "tr///" and "y///", which mean 
exactly the same thing. As another example, "unless" is the same as "if 
not". Likewise, "s///" can usually do the job of "tr///".

This is one of the ways in which Perl is similar to human languages, 
which have many different words to express similar concepts (as a quick 
look through a thesaurus will demonstrate). Perl borrows other ideas 
from human languages as well. One simple example is the way "if" can be 
placed before or after the command to be conditionally executed. In 
addition, the implied use of "$_" is foreign to English speakers, but 
implied subjects are common in other languages such as Latin and Chinese.

Larry Wall, who invented Perl, is fascinated with human languages. He 
even studied them in graduate school - and for an interesting reason: 
"At the time, [my wife and I] were actually planning to be missionaries 
(more specifically, Bible translators), but we had to drop that idea for 
health reasons." But Wall doesn't regret the change of plans: he figures 
that Perl is actually more useful to the missionaries than he personally 
could have been.

Perl's similarity to a human language goes hand-in-hand with its ability 
to do many different types of jobs. As Jon Udell puts it, "[Larry] Wall 
[inventor of Perl] believes that people think about things in different 
ways, that natural languages accommodate many mindsets, and that 
programming languages should too."

We're now going to see a bit of Perl's human-language-like versatility.

             -----------------------------------

2) The Many Faces of Perl

As we have seen, Perl commands can be written as functions or statements.

   die("Error!") unless defined($foo);   # Look like functions.
   die "Error!"  unless defined $foo;    # Look like statements.

This changes the appearance only: the functionality is exactly the same.

More interestingly, the following code looks like it comes from a shell 
script, but it is actually valid Perl code:

   (-d $dir) || mkdir $dir;    # Create directory if it doesn't exist.
   $size=`wc -c $dir/*`;       # Get sizes of files.
   print <<EOF;
      Looks like
      everything's OK!
   EOF

Yes, Perl borrows "-d", "mkdir" back-ticks and "here-documents" (the 
"<<EOF" part) from your favourite shell.

Of course, the similarity is limited. Functions like "mkdir" are Perl 
built-ins; you cannot execute an arbitrary shell command directly from 
Perl. (That's why we had to use back-ticks to execute "wc".) In 
addition, you can't use "<", ">" and "|" to pipe I/O the way you would 
in a shell script (except when using "open", as we saw last week).

But I still think that it's impressive that a Perl program can be 
written like C or like a shell script.

             -----------------------------------

3) The Special File Handle "ARGV"

Okay, I admit that this section isn't too related to Perl style, but I 
had to put it somewhere!

"ARGV" is a special file handle which sequentially opens (for reading) 
all the files specified on the command-line (i.e., every command-line 
argument is interpreted as a file name and is opened). The files are 
read in the order in which they were specified on the command-line and 
as though the contents formed one big file (i.e., when you reach the end 
of a file, Perl automatically closes it and opens the next). The 
variable "$ARGV" always contains the name of the file that is currently 
being read by "<ARGV>".

In practice, you never see "<ARGV>". Instead, you see it written as 
"<>", because "ARGV" is the default file handle for reading if none is 
specified.

For example, consider the following code:

   while ( <> ) {    # while ( defined ( $_ = <ARGV> ) ) {
     print;          #  print $_;
   }

If you store this code in a file named "test.pl" and run:

   perl test.pl file1 file2 file3

the program will output the contents of "file1", then the contents of 
"file2", then the contents of "file3".

A dash ("-") given as a command-line argument refers to standard input. 
Also, if no command-line arguments are given at all, standard input is 
assumed.

             -----------------------------------

4) Exercise

The Unix program "cat" outputs data to standard out according to its 
command-line arguments, as follows:
1) If no command-line arguments are given, simply outputs standard input.
2) A command-line argument of "-" also means to output standard input.
3) Other command-line arguments refer to files, which are read and output.

The program is so called because it conCATenates data to standard output.

You will notice that this behaviour is very, very similar to the 
behaviour of "<ARGV>". (In fact, it's exactly the same, except that 
"cat" interprets arguemnts beginning with a dash as switches, whereas 
"<ARGV>" interprets them as files.) That's why "<ARGV>" is helpful: it's 
a shortcut to typical Unix behaviour.

Write a "polite cat" program that behaves like "cat", but makes harsh 
terms more polite. For example, you might replace "died" with "passed 
away". It's up to you exactly which terms you want to change. (You don't 
have to implement "cat"'s switches, of course.)

Try to use as few characters as possible without sacrificing good style.

             -----------------------------------

5) [Non-]Answer to Previous Exercise

There was no exercise last week.

             -----------------------------------

6) Past Information

Part 1: Getting Started
         http://linuxchix.org/pipermail/courses/2003-March/001147.html

Part 2: Scalar Data
         http://linuxchix.org/pipermail/courses/2003-March/001153.html

Part 3: User Input
         http://linuxchix.org/pipermail/courses/2003-April/001170.html

Part 4: Control Structures
         http://linuxchix.org/pipermail/courses/2003-April/001184.html

Part 4.5, a review with a little new information at the end:
         http://linuxchix.org/pipermail/courses/2003-July/001297.html

Part 5: The "tr///" Operator
         http://linuxchix.org/pipermail/courses/2003-July/001302.html

Part 6: The "m//" Operator
         http://linuxchix.org/pipermail/courses/2003-August/001305.html

Part 7: More About "m//"
         http://linuxchix.org/pipermail/courses/2003-August/001322.html

Part 8: The "s///" Operator
         http://linuxchix.org/pipermail/courses/2003-August/001330.html

Part 9: Simple File Access
         http://linuxchix.org/pipermail/courses/2003-September/001340.html

Part 10: Executing Commands with "open"
      http://linuxchix.org/pipermail/courses/2003-September/001344.html

Part 11: Perl Variables
      http://linuxchix.org/pipermail/courses/2003-October/001345.html

Part 12: Side Effects with Perl Variables
      http://linuxchix.org/pipermail/courses/2003-October/001347.html

             -----------------------------------

7) Credits

Works cited:
a) Jon Udell, "A Perl Hacker in the Land of Python"
    which you can view here:
    http://www.byte.com/documents/BYT20000201S0001/
    or if you don't feel like registering with BYTE.com:
    http://www.my-opensource.org/lists/myoss/2000-02/msg00090.html
b) Marjorie Richardson's interview of Larry Wall, found at:
    http://www.linuxjournal.com/article.php?sid=3394

Thanks to Jacinta Richardson for fact checking.

             -----------------------------------

8) Licensing

This course (i.e., all parts of it) is copyright 2003 by Alice Wood and 
Dan Richter, and is released under the same license as Perl itself 
(Artistic License or GPL, your choice). This is the license of choice to 
make it easy for other people to integrate your Perl code/documentation 
into their own projects. It is not generally used in projects unrelated 
to Perl.


From daniel.richter@wimba.com  Fri Oct 31 19:08:30 2003
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LinuxChix Perl Course Part 14: Arrays

1) Introduction
2) Declaring an Array
3) Accessing the Elements of an Array
4) Assigning to an Array
5) I/O with Arrays
6) Exercise
7) Answer to Previous Exercise
8) Past Information
9) Credits
10) Licensing

             -----------------------------------

1) Introduction

First of all, I want to apologise for not sending this to the list last 
week. I had to attend a funeral.   :-(

But anyway, let's talk about Perl...

Up until now we have only looked at scalar variables. It's high time we 
had a look at arrays.

Arrays in Perl are very powerful. This week we're only going to look at 
the basics, but don't get in the habit of "micro-managing" your arrays. 
Perl includes powerful macro-commands to do most of what you would 
logically want to do. We will see some of these commands in the 
following weeks.

             -----------------------------------

2) Declaring an Array

Array variables are declared just like scalar variables, except that 
they are preceded by an at-sign rather than a dollar sign:

   my @foo;

Note that "@foo" and "$foo" are totally different variables. You can use 
them both in the same code block; they won't interfere with each other 
in any way.

That wasn't so hard, was it?

             -----------------------------------

3) Accessing the Elements of an Array

Array elements are accessed using the standard "[]" operator - but with 
a twist. When a single element is accessed, the array name must be 
preceded by a dollar sign rather than an at-sign:

   my @list = ('some', 'words');
   print $list[0] . "\n";

As "man perldata" puts it: "$foo[1] is a part of @foo, not a part of 
$foo. This may seem a bit weird, but that's okay, because it is weird."

Don't let the humility of those words fool you: the system is actually 
quite clever when you fully understand it. The "[]" indicates that the 
variable is an array; the dollar sign indicates that what you want from 
it is a scalar. It may seem obvious that you would want a scalar, but 
Perl includes the ability to "slice" an array by specifying multiple 
subscripts:

   my @chars = ('A', 'B', 'C', 'D', 'E', 'F');
   my @a = @chars[1,5];     # Gets B and F.
   my @b = @chars[1 .. 4];  # Gets B, C, D and E.
   my @c = @chars[1,3..5];  # Gets B, D, E and F.

When you slice, you get back an array, so the variable name is preceded 
by an at-sign.

Note that array subscripts always start at zero. (You can change this by 
setting the special Perl variable "$[" - but that's a REALLY bad idea.)

A negative subscript is interpreted as though it had been added to the 
length of the array, so negative numbers can thought of as "counting 
backwards from the end":

   my @chars = ('a', 'b', 'c', 'd', 'e', 'f');
   print "$chars[-2]\n";    # Prints "e".

It's not an error to access a non-existant subscript; you just get back 
"undef" (the undefined value).

             -----------------------------------

4) Assigning to an Array

Perl provides many ways to assign to an array. One is a way we have 
already seen:

   my @x = ('foo', 'bar', 'baz');

Sometimes it's easier to use the "qw//" (quote words) operator:

   my @x = qw/foo bar baz/;   # Equivalent to the above.

Note that the words in "qw//" should NOT be separated by commas.

We can also assign one array to another:

   my @foo = qw{ The "qw//" operator can use any delimiter. };
   my @bar = @foo;

Note that the assignment is done by value, not by reference; subsequent 
changes to "@foo" will not affect "@bar".

Another way to assign to an array is using subscripts:

   $x[6] = 'whatever';
   @y[1,4,9] = ('one squared', 'two squared', 'three squared');

It's not an error to assign beyond the end of the array: the extra 
indices will be automatically created and assigned the value "undef":

   my @arr = qw/just three elements/;
   $arr[50] = 'xyz';      # No problem.

Note that arrays in Perl can contain any scalar:

   my @foo = ('some text', 3.14, FILE_HANDLE);

But an array CANNOT contain another array:

   $bar[3] = @foo;     # Won't work as expected.
   @bar[3] = @foo;     # Equivalent to: $bar[3] = $foo[0]

However, an array can contain a REFERENCE to an array, because a 
reference is a scalar. We'll see more about this when we learn about 
references.

             -----------------------------------

5) I/O with Arrays

As you may recall, the following reads a single line from "FOO":

   $line = <FOO>;

By assigning to an array instead of a scalar, we can read ALL the lines 
from a file handle:

   @lines = <FOO>;    # Read the whole file!

Each element of "@lines" is one line of the file (usually with newline 
characters at the end; we have to "chomp" to remove them).

Using "print" to output an array is exactly the same as calling "print" 
for each element of the array:

   print @lines;

Array subscripts are interpolated inside quotes:

   print "Line 2 is: $lines[2]";      # Works as expected.
   print "Line $i us: $lines[$i]";    # Works as expected.

A whole array can also be interpolated inside quotes. In that case, the 
elements of the array are separated by whatever '$"' is set to (it's a 
space by default):

   print "The list is: @list\n";      # Output whole list.
   print "Parts of the list are: @list[2,$i..$j]\n";   # Works!

             -----------------------------------

6) Exercise

The Unix program "tail" outputs the last few lines of a file, or 
standard input if no file is given. Write your own "tail" program in Perl.

Because we're just starting out with arrays here, we'll make three 
simplifying assumptions:
1) We assume that the input will not have fewer lines than the number of 
lines you want to display.
2) We do not allow the user to specify the number of lines to display on 
the command line (as real "tail" does).
3) We indiscriminantly read the entire file into memory regardless of 
its size.

You will notice that we haven't discussed yet how to get the length of 
an array. That's okay: we don't need the length here. Use a slice with 
negative subscripts.

             -----------------------------------

7) Answer to Previous Exercise

Here is my "polite cat" program:

   #!/usr/bin/perl -w
   use strict;

   while ( <> ) {
     s/\bzero\b/not too much/g;
     s/\bunemployed\b/at a carreer crossroads/g;
     print;
   }

             -----------------------------------

8) Past Information

Part 1: Getting Started
         http://linuxchix.org/pipermail/courses/2003-March/001147.html

Part 2: Scalar Data
         http://linuxchix.org/pipermail/courses/2003-March/001153.html

Part 3: User Input
         http://linuxchix.org/pipermail/courses/2003-April/001170.html

Part 4: Control Structures
         http://linuxchix.org/pipermail/courses/2003-April/001184.html

Part 4.5, a review with a little new information at the end:
         http://linuxchix.org/pipermail/courses/2003-July/001297.html

Part 5: The "tr///" Operator
         http://linuxchix.org/pipermail/courses/2003-July/001302.html

Part 6: The "m//" Operator
         http://linuxchix.org/pipermail/courses/2003-August/001305.html

Part 7: More About "m//"
         http://linuxchix.org/pipermail/courses/2003-August/001322.html

Part 8: The "s///" Operator
         http://linuxchix.org/pipermail/courses/2003-August/001330.html

Part 9: Simple File Access
         http://linuxchix.org/pipermail/courses/2003-September/001340.html

Part 10: Executing Commands with "open"
      http://linuxchix.org/pipermail/courses/2003-September/001344.html

Part 11: Perl Variables
      http://linuxchix.org/pipermail/courses/2003-October/001345.html

Part 12: Side Effects with Perl Variables
      http://linuxchix.org/pipermail/courses/2003-October/001347.html

Part 13: Perl Style
      http://linuxchix.org/pipermail/courses/2003-October/001349.html

             -----------------------------------

9) Credits

Works cited: man perldata

Thanks to Jacinta Richardson for fact checking.

             -----------------------------------

10) Licensing

This course (i.e., all parts of it) is copyright 2003 by Alice Wood and 
Dan Richter, and is released under the same license as Perl itself 
(Artistic License or GPL, your choice). This is the license of choice to 
make it easy for other people to integrate your Perl code/documentation 
into their own projects. It is not generally used in projects unrelated 
to Perl.


From daniel.richter@wimba.com  Thu Nov  6 22:59:05 2003
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LinuxChix Perl Course Part 15: More About Lists

1) Introduction
2) Flat Lists
3) The Length of an Array
4) The "foreach" Loop
5) Exercises
6) Answer to Previous Exercise
7) Past Information
8) Credits
9) Licensing

             -----------------------------------

1) Introduction

Last week we saw the very basics of array variables in Perl. This week 
we're still looking at the basics, but slightly higher-level basics.

By the way, last week I used the term "array", mainly because I thought 
it would be more familiar. The general term in Perl is "list". An array 
is actually a type of list: a named list. Of course, we can informally 
use the terms interchangeably.

             -----------------------------------

2) Flat Lists

As we saw last week, lists in Perl are "flat": that is, they must 
contain only scalars. This obviously makes certain complex data 
structures more difficult to represent, but it has the advantage of 
making lists easy to join and split. For example, the following will 
join three lists into one big list:

   my @everyone = (@men, @women, @children);

Remember that arrays are always assigned by value, so subsequent changes 
to, say, "@women" will have no effect on "@everyone".

Likewise, the following will insert "$new_guy" at position 5 of the 
array (pushing all the other elements back one, of course):

   @everyone = (@everyone[0..4], $new_guy, @everyone[5..-1]);

And we can even use a list of variables as an lvalue (something to 
assign to). We've already seen that in statements like this:

   my($hour,$minute,$second) = ($time =~ m/(\d+):(\d+):(\d+)/);

The "m//" operator returns a list, "$hour" is set to the first value of 
that list, "$minute" is set to the second value of the list, etc.

Note that if the number of elements on the left and right sides of the 
assignment don't match, no error message is produced:

   ($a, $b) = ('one', 'two', 'three');     # Same as: $a='one'; $b='two';
   ($a, $b, $c) = ('one', 'two');          # Ditto, but sets $c to undef!

If one of the lvalues is an array, the array sucks up all remaining values:

   ($first,$second,@rest) = @everyone;

This means that it's a mistake (not an error, but surely a mistake) to 
follow an array with another variable in an assignment:

   (@rest,$last) = @everyone;    # NO! Same as: @rest=@everyone; $last=undef
   (@a, @b) = qw/a list of words/;   # Likewise not good.
   (@a, @b) = (@c, @d);              # NOT the same as @a=@c; @b=@d;

             -----------------------------------

3) The Length of an Array

To find the length of an array, simply assign the array to a scalar 
variable:

   my @arr = ('foo', 'bar', 'baz');
   my $length = @arr;
   print "Length is: $length\n";

We'll see more about this next week.

By the way, don't try this with a literal list:

   my $length_but_doesnt_work = ('foo', 'bar');   # Sets to "bar".

I'm sorry; I should have mentioned this last week.

             -----------------------------------

4) The "foreach" Loop

To loop through the values of an array, you could use a "for" loop:

   my @foo = qw/one two three four/;
   for (my $i=0; $i<@foo; $i++) {
     print "$foo[$i]\n";
   }

However, it's usually easier to use a "foreach" loop:

   my @foo = qw/one two three four/;
   foreach my $value (@foo) {
     print "$value\n";
   }

If no scalar variable is provided in the "foreach" statement, "$_" is used:

   my @foo = qw/one two three four/;
   foreach (@foo) {
     print "$_\n";
   }

In this case (and ONLY in this case), changes to "$_" affect the array:

   my @foo = qw/one two three four/;
   foreach (@foo) {
     s/[aeiou]/X/g;   # Replace vowels with "X".
   }
   print "@foo\n";    # The array now has no vowels.

We'll see a similar way to change the values of an array when we look at 
the "map" operator.

             -----------------------------------

5) Exercises

a) Fix last week's "tail" program to take into account the possiblity 
that the user inputs a small number of lines. (See below.)

b) The Perl function "reverse" reverses the order of an array. Write 
your own implementation of "reverse", i.e.:

   my @array = qw/A B C D E F G/;

   # Your code here.

   print "Result is: @result\n";

Try to use "foreach" instead of "for".

             -----------------------------------

6) Answer to Previous Exercise

Here is an implementation of "tail" in Perl:

   #!/usr/bin/perl -w
   use strict;

   my $lines_to_output = 10;

   my @lines = <>;
   print @lines[ -$lines_to_output .. -1 ];

Again, this program is faulty because it assumes that there are at least 
ten lines of input. If there are less than ten lines, the Perl 
interpreter will complain because we're printing the undefined value 
("undef").

It really was stupid of me to suggest that you write a buggy program. 
Please excuse me.

             -----------------------------------

7) Past Information

Part 1: Getting Started
      http://linuxchix.org/pipermail/courses/2003-March/001147.html

Part 2: Scalar Data
      http://linuxchix.org/pipermail/courses/2003-March/001153.html

Part 3: User Input
      http://linuxchix.org/pipermail/courses/2003-April/001170.html

Part 4: Control Structures
      http://linuxchix.org/pipermail/courses/2003-April/001184.html

Part 4.5, a review with a little new information at the end:
      http://linuxchix.org/pipermail/courses/2003-July/001297.html

Part 5: The "tr///" Operator
      http://linuxchix.org/pipermail/courses/2003-July/001302.html

Part 6: The "m//" Operator
      http://linuxchix.org/pipermail/courses/2003-August/001305.html

Part 7: More About "m//"
      http://linuxchix.org/pipermail/courses/2003-August/001322.html

Part 8: The "s///" Operator
      http://linuxchix.org/pipermail/courses/2003-August/001330.html

Part 9: Simple File Access
      http://linuxchix.org/pipermail/courses/2003-September/001340.html

Part 10: Executing Commands with "open"
      http://linuxchix.org/pipermail/courses/2003-September/001344.html

Part 11: Perl Variables
      http://linuxchix.org/pipermail/courses/2003-October/001345.html

Part 12: Side Effects with Perl Variables
      http://linuxchix.org/pipermail/courses/2003-October/001347.html

Part 13: Perl Style
      http://linuxchix.org/pipermail/courses/2003-October/001349.html

Part 14: Arrays
      http://linuxchix.org/pipermail/courses/2003-October/001350.html

             -----------------------------------

8) Credits

Works cited:
a) man perldata
b) man perlfunc

Thanks to Jacinta Richardson for fact checking.

             -----------------------------------

9) Licensing

This course (i.e., all parts of it) is copyright 2003 by Alice Wood and 
Dan Richter, and is released under the same license as Perl itself 
(Artistic License or GPL, your choice). This is the license of choice to 
make it easy for other people to integrate your Perl code/documentation 
into their own projects. It is not generally used in projects unrelated 
to Perl.


From ruifernandes@tvtel.pt  Fri Nov  7 07:15:02 2003
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From: "Rui Fernandes" <ruifernandes@tvtel.pt>
To: <courses@linuxchix.org>
Subject: [Courses] [Perl] Part 8: The "s///" Operator
Date: Thu, 6 Nov 2003 20:14:49 -0000
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Dear Sir,

I've got a problem with a substitution in Perl. I have a string ( let's =
supose $word) that can have values like the followings - it's =
impredictable:

asd3$( .ophG

word=BA =E7htn mnfbgd=20

@word_8 klMNBVFR 12

I want to extract only the character A to Z , a to z and 0 to 1 from the =
strings so I could have:

asd3ophG

word =E7htn mnfbgd

word 8 kIMNBVFR 12


How can I do this if I don't know the number of letters in the string?
Please answer as soon as possible - It's very urgent.

Best regards,

Rui Fernandes
From tbowen@CapitalThinking.com  Sat Nov  8 09:00:30 2003
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From: "Bowen, Tricia" <tbowen@CapitalThinking.com>
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try running this on the command-line:

perl -pi.bak -e 's/\W//g' test.txt

\W is equivalent to anything other than a word character =
([^a-zA-Z0-9_]).=20

test.txt is a sample file that contains your crazy characters.

--tricia

-----Original Message-----
From: Rui Fernandes [mailto:ruifernandes@tvtel.pt]
Sent: Thursday, November 06, 2003 3:15 PM
To: courses@linuxchix.org
Subject: [Courses] [Perl] Part 8: The "s///" Operator
Importance: High


Dear Sir,

I've got a problem with a substitution in Perl. I have a string ( let's =
supose $word) that can have values like the followings - it's =
impredictable:

asd3$( .ophG

word=BA =E7htn mnfbgd=20

@word_8 klMNBVFR 12

I want to extract only the character A to Z , a to z and 0 to 1 from the =
strings so I could have:

asd3ophG

word =E7htn mnfbgd

word 8 kIMNBVFR 12


How can I do this if I don't know the number of letters in the string?
Please answer as soon as possible - It's very urgent.

Best regards,

Rui Fernandes
_______________________________________________
Courses mailing list
Courses@linuxchix.org
http://mailman.linuxchix.org/mailman/listinfo/courses
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LinuxChix Perl Course Part 16: Array Functions

1) Introduction
2) Re-Visiting Old Perl Functions
3) Six Simple Functions
4) Consider the Context
5) Exercise
6) Answers to Previous Exercises
7) Past Information
8) Credits
9) Licensing

             -----------------------------------

1) Introduction

Perl includes many functions that make handling arrays a breeze.

Some of these functions are quite versatile, i.e., you can change the 
way they work if you need a slightly different behaviour. We could never 
get through all the different customisations that can be done, but you 
can find out more about any one of these functions by typing
"perldoc -f <function name>" at the command-line.

             -----------------------------------

2) Re-Visiting Old Perl Functions

Some Perl functions that we've already seen will accept either scalars 
or lists. For example, we've seen the "chomp" function used with 
scalars, but it can also be used with a list (in which case it chomps 
every element of the list):

   chomp($scalar);
   chomp(@array);
   chomp($a, $list, $of, $scalars);

But not all Perl functions can do this. For example, the "uc" function 
capitalises all the letters in a string, but it won't accept a list. 
Always RTFM to make sure.

             -----------------------------------

3) Six Simple Functions

We're going to start with

a) split - splits a string into an array
b) join - combines an array to form a string
c) reverse - reverses the order of an array (first element becomes last)
d) sort - puts the array in alphabetical order (NOT numerical order)
e) push - adds one or more values to the end of an array
f) pop - removes and returns the last element of an array

Examples:

   my @arr = split( /[#!]+/, 'foo###bar!!!baz' );
       # Now @arr = ('foo', 'bar', 'baz')
   print join('**', @arr);      # Output is: "foo**bar**baz"
   @arr = reverse(@arr);        # Now @arr = ('baz', 'bar', 'foo')
   @arr = sort(@arr);           # Now @arr = ('bar', 'baz', 'foo')
   my $x = pop(@arr);           # Now $x='baz' and @arr=('foo', 'bar')
   push(@arr, 'Bob', 'Jane');   # Now: ('foo', 'bar', 'Bob', 'Jane')

Note that only the functions "push" and "pop" change the array variable 
passed in. This means that the other functions can take any list, not 
just an array:

   my @arr = ('foo', 'bar', 'baz');
   print join('**', @arr);      # Output is: "foo**bar**baz"
   print join('**', 'foo', 'bar', 'baz');  # Same as previous line
   print join('**', 'A', @arr, 'B');  # Output: "A**foo**bar**baz**B"

             -----------------------------------

4) Consider the Context

Many Perl functions and operators behave differently depending on the 
"context". Perl has three possible contexts: scalar context, list 
context and void context:

   my @arr = ('foo', 'bar', 'baz');
   my @a = reverse(@arr);       # Array context
   my $s = reverse(@arr);       # Scalar context
   reverse(@arr);               # Void context

   print "@a\n";       # Output is: baz bar foo
   print "$s\n";       # Output is: zabraboof (foobarbaz backwards)

Note that the context is determined by how the return value is used, NOT 
the parameters passed to the function/operator. This is quite Perl-like; 
most languages have functions that always return the same value, no 
matter how it's used.

The use of an array (variable) in scalar context returns the length of 
the array:

   my @array = ('foo', 'bar', 'baz');
   my $scalar = @array;
   print "$scalar\n";      # Output is 3.

But don't try this with a literal list:

   my $scalar = ('foo', 'bar', 'baz');       # Warnings generated here.
   print "$scalar\n";      # Output is baz.

In the above cases the context was obvious because we assigned to an 
array or scalar variable. But the context can also be indicated by an 
operator. For example, arithmetic operators are all scalar, as is string 
concatenation:

   print "The length of the array is: " . @array . "\n";

We can also force scalar context by using the "scalar" operator:

   ($length_a, $length_b) = (scalar @a, scalar @b);

"man perlfunc" tells us: "There is no equivalent operator to force an 
expression to be interpolated in list context because in practice, this 
is never needed."

             -----------------------------------

5) Exercise

Write a Perl program that reads /etc/passwd and outputs just a list of 
usernames and UIDs, ordered alphabetically by username. Remember: There 
Is More Than One Way To Do It.

             -----------------------------------

6) Answers to Previous Exercises

a) Here is a tail program that works on even a small number of lines.

   #!/usr/bin/perl -w
   use strict;

   my @lines = <>;

   my $lines_to_output = @lines;     # Get number of lines.
   $lines_to_output = 10 if $lines_to_output > 10;  # Limit number of lines.

   print @lines[ -$lines_to_output .. -1 ];

b) Here is a reversal algorithm.

   #!/usr/bin/perl -w
   use strict;

   my @array = qw/A B C D E F G/;

   my @result;
   foreach my $element (@array) {
     @result = ($element, @result);
   }

   print "Result is: @result\n";

As you might guess, it would be a little more efficient to use "push" 
and "pop", but we hadn't seen these functions last week. (Calling the 
"reverse" function would be more efficient still! :-)

             -----------------------------------

7) Past Information

Part 1: Getting Started
      http://linuxchix.org/pipermail/courses/2003-March/001147.html

Part 2: Scalar Data
      http://linuxchix.org/pipermail/courses/2003-March/001153.html

Part 3: User Input
      http://linuxchix.org/pipermail/courses/2003-April/001170.html

Part 4: Control Structures
      http://linuxchix.org/pipermail/courses/2003-April/001184.html

Part 4.5, a review with a little new information at the end:
      http://linuxchix.org/pipermail/courses/2003-July/001297.html

Part 5: The "tr///" Operator
      http://linuxchix.org/pipermail/courses/2003-July/001302.html

Part 6: The "m//" Operator
      http://linuxchix.org/pipermail/courses/2003-August/001305.html

Part 7: More About "m//"
      http://linuxchix.org/pipermail/courses/2003-August/001322.html

Part 8: The "s///" Operator
      http://linuxchix.org/pipermail/courses/2003-August/001330.html

Part 9: Simple File Access
      http://linuxchix.org/pipermail/courses/2003-September/001340.html

Part 10: Executing Commands with "open"
      http://linuxchix.org/pipermail/courses/2003-September/001344.html

Part 11: Perl Variables
      http://linuxchix.org/pipermail/courses/2003-October/001345.html

Part 12: Side Effects with Perl Variables
      http://linuxchix.org/pipermail/courses/2003-October/001347.html

Part 13: Perl Style
      http://linuxchix.org/pipermail/courses/2003-October/001349.html

Part 14: Arrays
      http://linuxchix.org/pipermail/courses/2003-October/001350.html

Part 15: More About Lists
      http://linuxchix.org/pipermail/courses/2003-November/001351.html

             -----------------------------------

8) Credits

Works cited: man perlfunc

Thanks to Jacinta Richardson for fact checking.

             -----------------------------------

9) Licensing

This course (i.e., all parts of it) is copyright 2003 by Alice Wood and 
Dan Richter, and is released under the same license as Perl itself 
(Artistic License or GPL, your choice). This is the license of choice to 
make it easy for other people to integrate your Perl code/documentation 
into their own projects. It is not generally used in projects unrelated 
to Perl.


From daniel.richter@wimba.com  Fri Nov 21 22:58:12 2003
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It's Friday morning and Part 17 of the Perl course still isn't written. 
Oh, well: it will be ready next week. Sorry for the delay, everyone.

-- 
   When a program dies / what you need is a moment / of serenity.
       - Damian Conway, whose Perl module generates haikus

From daniel.richter@wimba.com  Fri Nov 28 22:01:58 2003
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Date: Fri, 28 Nov 2003 12:01:50 +0100
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Subject: [Courses] [Perl] Part 17: grep and map
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LinuxChix Perl Course Part 17: grep and map

1) Introduction
2) grep - filters a list
3) map - transforms the values of a list
4) What "grep" and "map" have in common
5) Exercises
6) Answer to Previous Exercise
7) Past Information
8) Credits
9) Licensing

             -----------------------------------

1) Introduction

Before we finish looking at arrays in Perl, I thought we should take a 
quick look at two handy Perl functions: "grep" and "map". Both functions 
are technically operators because they allow you to do magical things 
that a function can't do, but syntactically they look like functions, so 
we refer to them as functions here.

Let me add that this will be the last e-mail before January. It's that 
busy-busy-busy time of year and I'm afraid that I have no more time to 
write about Perl than you have to read about it.

             -----------------------------------

2) grep - filters a list

The "grep" function returns only the elements of a list that meet a 
certain condition:

   @positive_numbers = grep($_ > 0, @numbers);

As you can see, each element is refered to as "$_". This (plus the fact 
that parentheses are optional) allows you write commands that look 
similar to invocations of the Unix "grep" program:

   @non_blank_lines = grep /\S/, @lines;

In addition, you can specify a code block rather than a single condition:

   @non_blank_lines = grep { /\S/ } @lines;     # Equivalent to the above.

Obviously it doesn't matter in this case, but code blocks are helpful 
when you want a complex filter with multiple lines of code. The result 
of the code block is the result of the last statement executed:

   # All positive numbers can be used as exponents,
   # but negative exponents must be integers.
   @can_be_used_as_exponent = grep {
     if ( $_ < 0 ) {
       ! /\./;          # No decimal point -> integer.
     }
     else {
       1;               # Always true.
     }
   } @array;

             -----------------------------------

3) map - transforms the values of a list

The "map" function applies a transformation to each element of a list 
and returns the result, leaving the original list unchanged (unless you 
mess it up; more on that in a moment).

   @lines_with_newlines = map( $_ . "\n", @lines_without_newlines);

As with "grep", each value in the list is refered to as "$_".

"map" can also take a block of code:

   # Replace "x@y.z" with "x at y dot z" to confuse spammers.
   @disguised_addresses = map {
       my $email = $_;
       $email =~ s/\@/ at /;
       $email =~ s/\./ dot /g;
       $email;
     } @email_addresses;

Note that it's important not to change "$_" because that would change 
the original "@email_addresses" (and you wouldn't get what you wanted in 
"@disguised_addresses").

"map" needs not be a one-to-one mapping. For example, in the following code:

   @words   =   map   m/\b(\w+)\b/g,   @lines;    # Spaces are for clarity.

the regular expression splits a string into a list of words. The "map" 
function returns the result of joining all the small lists. If a line 
contains no words, the regular expression will return an empty list, and 
that's okay.

             -----------------------------------

4) What "grep" and "map" have in common

"grep" and "map" have a lot in common. They both "magically" take a 
piece of code (either an expression or a code block) as a parameter. You 
need to put a comma after an expression but shouldn't put a comma after 
a code block.

Changing "$_" in "grep" or "map" will change the original list. This 
isn't generally a good idea because it makes the code hard to read. 
Remember that "map" builds a list of results by evaluating an 
expression, NOT by setting "$_".

A side effect of this fact is that you should not use "s///" with "map". 
The "s///" operator changes "$_" rather than returning a result, so you 
won't get what you would expect if you use it with "map" (and you 
CERTAINLY shouldn't use it with "grep").

             -----------------------------------

5) Exercises

a) Write some Perl code that, given a list of numbers, generates a list 
of square roots of those numbers. (The square root function in Perl is 
"sqrt".)

b) Modify the code to filter out any negative numbers. The result should 
be as though the negative numbers were never in the original list.

c) Write a Perl program that reads two files and outputs only the lines 
that are common to both of them.

             -----------------------------------

6) Answer to Previous Exercise

The following program reads the password file and outputs a list of 
usernames and UIDs, ordered by username:

   #!/usr/bin/perl -w
   use strict;

   open FILE, '< /etc/passwd' or die "Couldn't open file: $!";
   my @data = sort(<FILE>);
   close FILE;

   my @result;
   foreach (@data) {
     my @fields = split(/:/);    # Equivalent to split(/:/, $_)
     push @result, $fields[0] . ' -> ' . $fields[2];
   }

   print join("\n",@result) . "\n";

The above program is a nice review of Perl functions. But of course, 
There Is More Than One Way To Do It, and we could replace the bottom 
half with:

   foreach (@data) {
     s/^(.*?):.*?:(\d*):.*$/$1 -> $2/;
   }
   print join("\n",@result) . "\n";

Or to make the program really short:

   $_ = join '', @data;
   s/^(.*?):.*?:(\d*):.*$/$1 -> $2/gm;
   print;        # Prints "$_"

             -----------------------------------

7) Past Information

Part 16: Array Functions
      http://linuxchix.org/pipermail/courses/2003-November/001359.html

Part 15: More About Lists
      http://linuxchix.org/pipermail/courses/2003-November/001351.html

Part 14: Arrays
      http://linuxchix.org/pipermail/courses/2003-October/001350.html

Part 13: Perl Style
      http://linuxchix.org/pipermail/courses/2003-October/001349.html

Part 12: Side Effects with Perl Variables
      http://linuxchix.org/pipermail/courses/2003-October/001347.html

Part 11: Perl Variables
      http://linuxchix.org/pipermail/courses/2003-October/001345.html

Parts 1-10: see the end of:
      http://linuxchix.org/pipermail/courses/2003-October/001345.html

             -----------------------------------

8) Credits

Works cited:
a) man perlfunc
b) Kirrily Robert, Paul Fenwick and Jacinta Richardson's
    "Intermediate Perl", which you can find (along with their
    "Introduction to Perl") at:
    http://www.perltraining.com.au/notes.html

Thanks to Jacinta Richardson for fact checking.

             -----------------------------------

9) Licensing

This course (i.e., all parts of it) is copyright 2003 by Alice Wood and 
Dan Richter, and is released under the same license as Perl itself 
(Artistic License or GPL, your choice). This is the license of choice to 
make it easy for other people to integrate your Perl code/documentation 
into their own projects. It is not generally used in projects unrelated 
to Perl.


From dan@cellectivity.com  Fri Apr  1 20:31:50 2005
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This course is an introduction to Perl. It assumes that you have no
knowledge of Perl whatsoever, but that you have at least written
small programs in some other language. (More on that in a moment.)

The purpose of the course is not to make you a master of Perl, but
to make you proficient enough to write basic Perl programs and to
read other people's Perl code.

The lessons will go on a weekly cycle: one every Friday (including
one later today).

It's recommended that you do at least the first exercise from each
week, unless you apply what you read by writing something similar
on your own. Exercises beyond the first one are more exploratory and
so are optional: you do them if you have time. The optional exercises
will bring you a greater knowledge of Perl, but if you don't have
time for them, the first exercise is enough to get the basics.

A solution to each week's exercises will be provided the following
week.

When you complete an exercise, you can do one of three things with
it:
1) keep it to yourself
2) send it to me and I'll critique it
3) send it to the list for everyone to discuss (but give everyone
else a few days to work on it before sending your solution)

Any of these are okay, but I encourage you to send non-trivial
answers to the list so that everyone else can consider them. There
Is More Than One Way To Do It (TIMTOWTDI, pronounced Tim-Towtdy).

When discussing someone else's solution, of course please be polite
about it. Point out obscure cases that weren't taken into account
(unless they're obvious), remembering that the author may have
intentionally ignored them: it *is* just an exercise after all!

Any other discussion or questions are also welcome. I welcome private
questions/comments, but I encourage you to write to the list - even if
it's to complain that something wasn't explained well enough.  ;-)

A few people wrote to me in surprise that I discourage learning Perl
as a first language. This is because I believe that some other
language (ANY other language) would be a better tool to learn
programming. Perl has no constants, virtually no data
types, and its handling of structs and exceptions is very poor
compared to almost any other language. In addition, Perl's wide
variety of choices require the experience to choose well. I recommend
starting with some other language, such as Python, Ruby, Java or
(for the stalwart) C++. However, the choice to learn Perl is
ultimately yours; everyone is welcome in this course.

-- 
   [M]y wife and I attended grad school in linguistics at Berkeley
   and UCLA. At the time, we were actually planning to be
   missionaries (more specifically, Bible translators), but we had
   to drop that idea for health reasons. Funny thing is, now the
   missionaries probably get more good out of Perl than they'd have
   gotten out of me as a missionary. Go figure.
      - Larry Wall (inventor of Perl)

From dan@cellectivity.com  Sat Apr  2 02:14:19 2005
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LinuxChix Perl Course Part 1: Getting Started

1) The Beginning
2) Hello World
3) Warnings and Strict Compilation
4) Exercises
5) Acknowledgements
6) Licensing


           ----------------------------------------

1) The Beginning

The first step to using Perl is installing it. If you're using a
Linux or BSD system you're ahead of the game, because it's rare these
days to find an open-source installation without Perl. If you're
running Windows, you may need to download Perl, and www.perl.org is a
good place to start.

           ----------------------------------------

2) Hello World

Since we're not big on originality, we're going to start with a Hello
World program:

   print "Hello, world!\n";

To execute the program, write it in a file (say, hello.pl) and
execute the following on the command line:

   perl hello.pl

Now let's make the program a little more interesting:

   my $who = "world";
   print "Hello, $who!\n";

Finally, let's add some comments. Comments can be provided using the
"#" character, which is often called a hash or pound sign. (I believe
the correct term is "octothorp".) The comment begins at the "#" and
ends at the end of the line.

   # This is a Hello World program in Perl.
   my $who;                  # Declare variable.
   $who = "world";           # Assign variable.
   print "Hello, $who!\n";   # Print result.

           ----------------------------------------

3) Warnings and Strict Compilation

Although the above program is a valid, working progam, I want to
introduce two tools that will save you hours of debugging later on.

The first tool is strict compilation. This requires you, among other
things, to declare each variable before you use it. This will alert
you of misspelled variable names. To use strict compilation, begin
the program with "use strict;". (We'll see an example below.)

The second tool is warnings. This will cause the interpreter to
mention probable logic errors as the program executes. For example,
the statement "$a = $b + c;" is syntactically correct in Perl, but
you probably wanted a dollar sign in front of the "c". If you have
warnings turned on, Perl will flag this mistake.

To turn on warnings, you can invoke the "-w" flag on the command
line:

   perl -w hello.pl

However, this requires the user to include the flag every time she
executes the program. A better solution is to use a "shebang" line at
the beginning of the program:

   #!/usr/bin/perl -w

The perl interpreter will honor the "shebang" line even if the "-w"
flag is not invoked on the command line.

Notice that the shebang line is special in two ways:
a) It starts with a hash but is not a comment.
b) It doesn't end with a semicolon.

If you're not familiar with shebang lines, feel free to post a
question to the list (this one or the techtalk list).

An additional benefit of the "shebang" line is that it allows you to
execute the script without typing "perl" on the command line, just as
if it were a shell script or a compiled program. (Don't forget to set
execution permissions.) However, it also means that we have to know
where on the filesystem the Perl interpreter is located. We can do
this by typing "which perl", or a similar command, at the command
line.

So our Hello World program becomes:

   #!/usr/bin/perl -w
   use strict;
   my $who;                  # Declare variable.
   $who = "world";           # Assign variable.
   print "Hello, $who!\n";   # Print result.

Experienced Perl programmers may sometimes turn off these safeguards,
but it's not recommended for beginners.

           ----------------------------------------

4) Exercises

a) In the Hello World program, what does the "\n" mean? If you don't
know, try moving it around inside the string and see what result it
gives.

b) Use the man pages to find out what "Perl" stands for.

           ----------------------------------------

5) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

           ----------------------------------------

6) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.

From devdas@dvb.homelinux.org  Sat Apr  2 03:39:08 2005
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Subject: Re: [Courses] [Perl] Part 1: Getting Started
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On 01/04/05 17:10 +0100, Dan wrote:
> LinuxChix Perl Course Part 1: Getting Started
> 
> 1) The Beginning
> 2) Hello World
> 3) Warnings and Strict Compilation
> 4) Exercises
> 5) Acknowledgements
> 6) Licensing
> 
> 
>            ----------------------------------------
> 
> 1) The Beginning
> 
> The first step to using Perl is installing it. If you're using a
> Linux or BSD system you're ahead of the game, because it's rare these
> days to find an open-source installation without Perl. If you're
> running Windows, you may need to download Perl, and www.perl.org is a
> good place to start.
> 
http://www.activestate.com/ is probably a better place to point to for
Windows binary downloads.

Devdas Bhagat

From fazia@faziarizvi.net  Tue Apr  5 08:52:30 2005
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From: Fazia Begum Rizvi <fazia@faziarizvi.net>
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Hi Dan,

I've just joined the Linuxchix Courses list, specifically for your class 
on Perl. I'll be following it every week.

Since I joined a bit late, I went to the archive and read the first two 
messages. I've already done my first program. :-)

I also have my first question. Though I'll be doing later programs on a 
linux server, I put together this first one on my Mac (OSX). Everything 
worked except I could not get the program to run by simply typing 
"hello.pl" after insterting the shebang line. (It runs just fine if I type 
"perl hello.pl".

I know that some flavors of perl can be different. Is this the case here? 
Or did I perhaps write something incorrectly?

#!/usr/bin/perl -w
use strict;
# My very first perl program April 4, 2005 - Fazia
my $who;                        # Declare variable
$who = "world";                 # Assign variable
print "Hello, world!\n";        # Print result


Thanks,
ad astra,
Faz

Fazia Begum Rizvi 
Bits and Bytes: http://uweb.txstate.edu/~fr06/weblog/
Weblog: http://www.faziarizvi.net/

From devdas@dvb.homelinux.org  Tue Apr  5 09:14:58 2005
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From: Devdas Bhagat <devdas@dvb.homelinux.org>
To: courses@linuxchix.org
Subject: Re: [Courses] [Perl] Part 1: Getting Started
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On 04/04/05 17:18 -0500, Fazia Begum Rizvi wrote:
> 
> Hi Dan,
> 
> I've just joined the Linuxchix Courses list, specifically for your class 
> on Perl. I'll be following it every week.
> 
> Since I joined a bit late, I went to the archive and read the first two 
> messages. I've already done my first program. :-)
> 
> I also have my first question. Though I'll be doing later programs on a 
> linux server, I put together this first one on my Mac (OSX). Everything 
> worked except I could not get the program to run by simply typing 
> "hello.pl" after insterting the shebang line. (It runs just fine if I type 
> "perl hello.pl".
> 
On Unix systems, the OS needs to be told that the file is an executable.
You can do this by running chmod +x file. Refer to the chmod manual for
more detail on the options it takes. man 1 chmod.

Also, on most Unix systems, the current directory is not in your PATH
environment variable. The PATH is the list of directories where an
executable is looked for by the shell. To see your current PATH, 
echo $PATH in your shell. If you had marked that file as executable,
then you should try running ./hello.pl to see if it works.

> I know that some flavors of perl can be different. Is this the case here? 
> Or did I perhaps write something incorrectly?
> 
When you call the script as perl <scriptname>, the shell executes the
perl binary, which proceeds to run through the commands in <scriptname>.

When you use the shebang line, the shell invokes the interpreter
referred to by the command on the shebang line, and then that handles
the rest of the script.

> #!/usr/bin/perl -w
> use strict;
> # My very first perl program April 4, 2005 - Fazia
> my $who;                        # Declare variable
> $who = "world";                 # Assign variable
> print "Hello, world!\n";        # Print result
> 
Did you mean to use $who up there?
print "Hello, $who!\n";

Devdas Bhagat

From fazia@faziarizvi.net  Tue Apr  5 14:15:42 2005
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On Tue, 5 Apr 2005, Devdas Bhagat wrote:

> On Unix systems, the OS needs to be told that the file is an executable.
> You can do this by running chmod +x file. Refer to the chmod manual for
> more detail on the options it takes. man 1 chmod.

Yeah - that much I knew, and the chmod command works fine on OSX. (I was 
able to make it executable.) Still didn't work though, so I was stumped. 
(Not that it matters too much since I'll be doing everything else on an 
Linux server, but I was curious.)


> Also, on most Unix systems, the current directory is not in your PATH
> environment variable. The PATH is the list of directories where an
> executable is looked for by the shell. To see your current PATH,
> echo $PATH in your shell. If you had marked that file as executable,
> then you should try running ./hello.pl to see if it works.


Ah! Okay, lemme see if that works...


>> #!/usr/bin/perl -w
>> use strict;
>> # My very first perl program April 4, 2005 - Fazia
>> my $who;                        # Declare variable
>> $who = "world";                 # Assign variable
>> print "Hello, world!\n";        # Print result
>>
> Did you mean to use $who up there?
> print "Hello, $who!\n";

Yup, but I copied and pasted before I fixed that. :-)

Thanks!

-Faz


From fazia@faziarizvi.net  Tue Apr  5 14:18:41 2005
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On Tue, 5 Apr 2005, Devdas Bhagat wrote:

> then you should try running ./hello.pl to see if it works.

That worked!

Thanks,
-Faz

From suzo@spin.net.au  Tue Apr  5 15:04:18 2005
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Fazia Begum Rizvi wrote:
> 
> On Tue, 5 Apr 2005, Devdas Bhagat wrote:
> 
>> then you should try running ./hello.pl to see if it works.
> 
> 
> That worked!

I like to put "." in my path, as the fist directory to look in, to solve 
the problem of having to type "./" before everything.  Usually the path 
is set in your profile, but exactly where that is in OSX I don't know.

I usually also add a bin directory in my home directory if there isn't 
one there. I can then put in scripts that I have written.

If you find the profile there will usually be a line something like
$PATH=/path/to/somewhere:/path/to/somewhere/else

The colon in this case is a separator, and could be different.  I would 
edit this to read
$PATH=./:$HOME/sue/bin:/path/to/somewhere:/path/to/somewhere/else

sue


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On Apr 4, 2005 11:40 PM, Sue Stones <suzo@spin.net.au> wrote:
 
> I like to put "." in my path, as the fist directory to look in, to solve
> the problem of having to type "./" before everything.  

Be very careful with this.  Having . in your PATH at all is insecure;
having it first is even worse.

The reason this is a problem is that it makes it very easy to execute
malicious code unintentionally.  For example, you might be browsing
around your system and ls a directory.  If a malicious user has put a
malicious program with the name "ls" in the directory that you execute
the ls command from, you've just executed it.  If you're the only
person that ever touches your machine, it's probably not too
dangerous; otherwise it should definitely be avoided. ;-)

Your other suggestion of creating a ~/bin/ directory is much safer
(though I'd still probably put it at the end of the path rather than
the beginning, unless you write programs with the same names as your
system executable files).

> Usually the path
> is set in your profile, but exactly where that is in OSX I don't know.

OS X is happily familiar in this area: /etc/profile is the system
profile, which has the default PATH info; you can override/add on to
this in your ~/.bash_profile or ~/.bashrc.

- Colleen

From suzo@spin.net.au  Tue Apr  5 16:46:28 2005
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Colleen Hatfield wrote:
> If you're the only
> person that ever touches your machine, it's probably not too
> dangerous; otherwise it should definitely be avoided. ;-)

Personally, yes I am the only one that has ever used my machine.  (My 
only housemate for the last decade has been a dog that had absolutely no 
interest in using the computer)  But if I was in a position where 
someone else was going to use my computer even for a few hours, I would 
create an account for them.

sue


From magnio@pvv.ntnu.no  Tue Apr  5 18:02:55 2005
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From: Magni Onsoien <magnio+courses@pvv.org>
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On 2005-04-05 16:46:08 +1000, Sue Stones said:
> Colleen Hatfield wrote:
> >If you're the only
> >person that ever touches your machine, it's probably not too
> >dangerous; otherwise it should definitely be avoided. ;-)
> 
> Personally, yes I am the only one that has ever used my machine.  (My 
> only housemate for the last decade has been a dog that had absolutely no 
> interest in using the computer)  But if I was in a position where 
> someone else was going to use my computer even for a few hours, I would 
> create an account for them.

I do that before they are getting close to anythng but a locked screen
:-)

But imagine that you have created her an account and you are using the
computer together (either simultanously, like a server, or sequentally).
The other person make a script that do something pretty evil, like 'rm
-rf $HOME 2>&1>/dev/null' (which will delete your homedir and redirect
all errors to /dev/null so you won't see them). This script is called ls
and is put into /tmp (a directory writeable for anyone) and made 
executable for all.

I guess you can figure out what happens when you go to /tmp and type 'ls'
to see what's there, if you have . first in your $PATH? :-)

So making a separate account really doesn't matter if it's a multi user
system, since in reality you'll be sharing directories and stuff anyway.
And DON'T remove the world-write permissions for /tmp, it will break
your system :-) (Not breaking in the same sense as rm -rf $HOME, though...)

And for the record, the same will of course happen if the user puts her 
malicious script in her $HOME-directory and asks you to "have a look at
my homedir, something seems to be wrong there".

The best thing to do is to stick with ./script and avoid . in the $PATH.
If you make scripts, it's safer to add their directory at the end of the
$PATH (with e.g. 'export PATH=$PATH:$HOME/myscriptdir:$HOME/perlcourse').
If you WANT to have . in your $PATH, add it to the end of the $PATH
('export PATH=$PATH:.') so you won't execute scripts and binaries with
the same name as system programs when you didn't want to. If you insist
on having your own 'ls', you could either use ./ls or add an alias for
it in .bash_profile ('alias ls=$HOME/myscripts/ls').

[Ok, this isn't on perl anymore. Sorry.]


Magni :)
-- 
sash is very good for you.

From anna@cs.wits.ac.za  Tue Apr  5 22:57:48 2005
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From: "Anna Badimo" <anna@cs.wits.ac.za>
To: Magni Onsoien <magnio+courses@pvv.org>, courses@linuxchix.org
Subject: Re: [Courses] [Perl] Part 1: Getting Started
Date: Tue, 5 Apr 2005 14:51:34 +0200
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Thanks all for the course. I am so excited to get this opportunity to learn
Perl and share with experienced Linux people.

Merci,
Anna Badimo

--
WITS University


---------- Original Message -----------
From: Magni Onsoien <magnio+courses@pvv.org>
To: courses@linuxchix.org
Sent: Tue, 5 Apr 2005 09:56:04 +0200
Subject: Re: [Courses] [Perl] Part 1: Getting Started

> On 2005-04-05 16:46:08 +1000, Sue Stones said:
> > Colleen Hatfield wrote:
> > >If you're the only
> > >person that ever touches your machine, it's probably not too
> > >dangerous; otherwise it should definitely be avoided. ;-)
> > 
> > Personally, yes I am the only one that has ever used my machine.  (My 
> > only housemate for the last decade has been a dog that had absolutely no 
> > interest in using the computer)  But if I was in a position where 
> > someone else was going to use my computer even for a few hours, I would 
> > create an account for them.
> 
> I do that before they are getting close to anythng but a locked 
> screen :-)
> 
> But imagine that you have created her an account and you are using 
> the computer together (either simultanously, like a server, or 
> sequentally). The other person make a script that do something 
> pretty evil, like 'rm -rf $HOME 2>&1>/dev/null' (which will delete 
> your homedir and redirect all errors to /dev/null so you won't see 
> them). This script is called ls and is put into /tmp (a directory 
> writeable for anyone) and made executable for all.
> 
> I guess you can figure out what happens when you go to /tmp and type 
> 'ls' to see what's there, if you have . first in your $PATH? :-)
> 
> So making a separate account really doesn't matter if it's a multi user
> system, since in reality you'll be sharing directories and stuff anyway.
> And DON'T remove the world-write permissions for /tmp, it will break
> your system :-) (Not breaking in the same sense as rm -rf $HOME, 
> though...)
> 
> And for the record, the same will of course happen if the user puts 
> her malicious script in her $HOME-directory and asks you to "have a 
> look at my homedir, something seems to be wrong there".
> 
> The best thing to do is to stick with ./script and avoid . in the $PATH.
> If you make scripts, it's safer to add their directory at the end of 
> the $PATH (with e.g. 'export 
> PATH=$PATH:$HOME/myscriptdir:$HOME/perlcourse'). If you WANT to have 
> . in your $PATH, add it to the end of the $PATH
> ('export PATH=$PATH:.') so you won't execute scripts and binaries 
> with the same name as system programs when you didn't want to. If 
> you insist on having your own 'ls', you could either use ./ls or add 
> an alias for it in .bash_profile ('alias ls=$HOME/myscripts/ls').
> 
> [Ok, this isn't on perl anymore. Sorry.]
> 
> Magni :)
> -- 
> sash is very good for you.
> _______________________________________________
> Courses mailing list
> Courses@linuxchix.org
> http://mailman.linuxchix.org/mailman/listinfo/courses
------- End of Original Message -------


From avery@u.washington.edu  Wed Apr  6 02:57:01 2005
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Subject: Re: [Courses] [Perl] Part 1: Getting Started
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Is anyone else wanting to try out the perl code in the lessons from
emacs? I wanted to try out script in perl as I was reviwing it in the
lesson, without having to save to separate file. After googling around
for a bit (keywords: compile perl emacs), I found what I wanted at:
http://www.phyast.pitt.edu/~tanc/scripts/.emacs.html

If you put the following in your .emacs file, you can highlight a
portion of perl script and send it to perl. I added the last lines
(add-hook 'cperl-mode hook) so it would work in both perl-mode and
cperl-mode.

;; ------------------------------------------------------------------
;; Perl mode
;; from http://www.phyast.pitt.edu/~tanc/scripts/.emacs.html
;; ------------------------------------------------------------------
 
;; sburke@netadventure.net
(defun perl-eval ()
  "Run selected region as Perl code"
  (interactive)
  (shell-command-on-region (mark) (point) "perl ")
  )
 
(add-hook 'perl-mode-hook
                        '(lambda ()
                                (local-set-key "\M-p" 'perl-eval)
                                ))

 
(add-hook 'cperl-mode-hook
                        '(lambda ()
                                (local-set-key "\M-p" 'perl-eval)
                                ))
 
;; ------------------------------------------------------------------


To send the whole buffer to perl, I found the following at:
http://lists.gnu.org/archive/html/help-gnu-emacs/2003-08/msg00114.html

;; -------------------------------------------------------------------
;;Compilation mode for perl files
;;http://lists.gnu.org/archive/html/help-gnu-emacs/2003-08/msg00114.html
;; -------------------------------------------------------------------
(require 'compile)
(add-to-list 'compilation-error-regexp-alist '(".* \\([-a-zA-Z._/]+\\) line
\\([0-9]+\\)." 1 2))
(defun perl-compile (&optional prefix)
  "Compile perl file"
  (interactive "P")
  (let* ((include-path (if (eq window-system 'w32) "-I%SP%" "-I$SP")) 
         (compile-command (concat "perl "include-path " -cw "
(buffer-file-name)))
         (compilation-read-command prefix))
    (call-interactively 'compile)))
(require 'perl-mode)
(require 'cperl-mode)
(define-key perl-mode-map "\C-c\C-c" 'perl-compile)
(define-key cperl-mode-map "\C-c\C-c" 'perl-compile)
;; ------------------------------------------------------------------

There is also mode-compile-el at:
http://www.tls.cena.fr/~boubaker/Emacs/
I downloaded mode-compile.el, but got some error messages when I tried to
byte-compile-file in Emacs. So I'm using the work-arounds above, which
work fine for my needs right now. 

Avery


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On Apr 5, 2005 12:59 PM, Avery Ke <avery@u.washington.edu> wrote:
> Is anyone else wanting to try out the perl code in the lessons from
> emacs?

i used emacs and even after changing permissions got this error  :
Can't open perl script "hello1.pl": No such file or directory
am sure i missed something  ... ? 
thanks,
-- 
|| svaksha ||

From avery@u.washington.edu  Wed Apr  6 02:39:24 2005
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Sue Stones wrote:
>Personally, yes I am the only one that has ever used my machine.  (My 
>only housemate for the last decade has been a dog that had absolutely no 
>interest in using the computer)  But if I was in a position where 
>someone else was going to use my computer even for a few hours, I would 
>create an account for them.

Are you sure your dog has no interest? <grin> My 17 year old cat
routinely steps across my keyboard, simultaniously presses
Control-Alt-Fx, gets himself into a virtual terminal, and apparently
tries to log in. He's done this so often that I think I should set up
an account for him, and give him the password. :) Then again, he'd
probably swipe my credit card and order a couple of cases of wet
cat food and an automatic can opener. 


Avery

From dan@cellectivity.com  Fri Apr  8 23:53:49 2005
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LinuxChix Perl Course Part 2: Scalar Variables

1) Introduction
2) Scalars
3) Operators
4) Quotes
5) Exercises
6) Answers to previous exercises
7) Acknowledgements
8) Licensing


           ----------------------------------------

1) Introduction

There are four basic data types in Perl. They are:
a) scalar - A single chunk of data
b) array - A bunch of scalars, indexed by a number
c) hash (hashtable) - A bunch of scalars, indexed by another scalar
d) handle - A pointer enabling the opening of resources from the
operating system (files, directories, etc.)

For right now we're only going to look at scalars.

           ----------------------------------------

2) Scalars

A scalar may represent text or a number. A scalar variable will
always begin with a dollar sign.

To declare a scalar variable (or any other, for that matter), use the
"my" keyword:

   my $foo;                 # Initialises to the undefined value.
   my $bar = "some text";   # Initialises to a string (text).
   my $baz = -25;           # Initialises to an integer.
   my $pi  = 3.14;          # Initialises to a floating point.

Once you have declared a scalar variable, you can assign it any
scalar value you like. There are no type restrictions.

   my $a = 10;
   my $b = 20;
   $a = $a + $b;             # Now $a=30.
   $b = "a bunch of text";   # Now $b is a string. That's okay.
   my $c = "I see $a stars"; # Now $c="I see 30 stars".
   $b = undef;               # Now $b has the undefined value.

There are also no restrictions on the size of strings (text).
Standard restrictions apply on the size of numeric types (for
example, I think that integers are 64 bits in Perl, so an integer
can't exceed 2^61).

           ----------------------------------------

3) Operators

The reason that there are no type restrictions on scalars is that all
scalars are inherently text (except maybe references, but we'll get
to that later). Some operators just extract numerical information
from that text. For example:

   my $a =   2   +   5;    # Now $a=7
   my $b =  "2"  +  "5";   # Okay, same as previous.
   my $c = " 2 " + " 5 ";  # Okay, same as previous.
   my $d =  "6"  /  "3";   # Now $d=2

Because a scalar can represent so many types, the comparison
operators could lead to ambiguity. For example, when performing the
comparison "3 < 12", the result depends on whether to we want to
compare as a number or as a string (because the first character of
"12" is "1", which is less than "3").

To resolve this ambiguity, Perl has both lexical (string) and
numerical comparison operators:
   Numerical     Lexical      Meaning
     ==             eq        equal
     !=             ne        not equal
     <              lt        less than
     <=             le        less than or equal to
     >              gt        greater than
     >=             ge        greater than or equal to

In addition to these, Perl has almost all the operators found in C++:
arithmetic, binary and logical. The few exceptions include the unary
"&" (memory address, which in Perl is a backslash) and cast
operations (which don't exist in Perl).

In addition, here are two other Perl operators not found in C++:
   .    (period) string concatenation (don't use + for this)
   **   (two astrisks) exponentiation (don't use ^ for this)

           ----------------------------------------

4) Quotes

Perl treats single and double quotes similarly to a shell script:

   my $animal = 'dog';
   my $double = "Cute $animal!\n";   # Cute dog![newline]
   my $single = 'Cute $animal!\n';   # Cute [dollar]animal![slash]n

Perl supports a variety of other types of string literals, such as
"here documents":

   my $foo = <<HERE;
     This is a
     very long string.
   HERE

If you're not familiar with "here documents", don't worry about it.

           ----------------------------------------

5) Exercises

a) Write a program called 'average.pl' to calculate the average of
the following numbers: 23, 28, 31, 17, 1

(Remember an average is where you add the numbers together and divide
by the amount you have, this is learning perl not maths 101! On the
other hand, the maths is normally the hardest part.)

b) Consider the following program:

   #!/usr/bin/perl -w
   use strict;

   my $foo = 'Suzy' x 4;
   print "$foo\n";

What does the "x" operator do? Why can't we write "4 x 'Suzy'" as
well as "'Suzy' x 4"?

c) Consider the following program:

   #!/usr/bin/perl -w
   use strict;

   my $a = q/This is string A/;   # using slash
   my $b = q!This is string B!;   # using exclamation point
   my $c = q:This is string C:;   # using colon
   print "$a\n$b\n$c\n";

What does the "q" operator do? What is the advantage of using it
rather than a single quote?

           ----------------------------------------

6) Answers to previous exercises

a) "\n" is a newline.

b) Perl stands for Practical Extraction and Report Language, or
Pathologically Eclectic Rubbish Lister depending on your point of
view.

           ----------------------------------------

7) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

           ----------------------------------------

8) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.

From kproot@nerim.net  Sun Apr 10 04:18:37 2005
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Date: Sat, 9 Apr 2005 20:18:22 +0200
From: Karine Delvare <kproot@nerim.net>
To: courses@linuxchix.org
Subject: Re: [Courses] [Perl] Part 2: Scalar Variables
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Hello Dan, thanks for the Perl course!

Below is my homework, please tell me if I'm sending this too early.

> 5) Exercises
> 
> a) Write a program called 'average.pl' to calculate the average of
> the following numbers: 23, 28, 31, 17, 1

#!/usr/bin/perl -w

use strict;

my $num1 = 23;
my $num2 = 28;
my $num3 = 31;
my $num4 = 17;
my $num5 = 1;

my $total = $num1 + $num2 + $num3 + $num4 + $num5;

my $average = $total / 5;

print "Average: $average\n";

> b) Consider the following program:
> 
>    #!/usr/bin/perl -w
>    use strict;
> 
>    my $foo = 'Suzy' x 4;
>    print "$foo\n";
> 
> What does the "x" operator do? Why can't we write "4 x 'Suzy'" as
> well as "'Suzy' x 4"?

Considering that the program output was SuzySuzySuzySuzy, the "x" operator seems to concatenate a string with itself, as many times as asked for (don't know if there is a proper english term for that, but that operator is cool!)

I tried to think a bit about 4 x 'Suzy'. Perl has to know which is the string and which is the "multiplicator". In that example it could guess it, but what about 4 x 3? 4 x 3 outputs "444", but 3 x 4 outputs "3333". So operands need to be ordered.

> c) Consider the following program:
> 
>    #!/usr/bin/perl -w
>    use strict;
> 
>    my $a = q/This is string A/;   # using slash
>    my $b = q!This is string B!;   # using exclamation point
>    my $c = q:This is string C:;   # using colon
>    print "$a\n$b\n$c\n";
> 
> What does the "q" operator do? What is the advantage of using it
> rather than a single quote?

Another cool operator! The output was:
This is string A
This is string B
This is string C
So it allows to replace the quote delimiters with any character. It can be an advantage if the string to quote containes quotes and/or double-quotes (makes me think about the way you can change the regular expressions delimiters, but I'm pretty sure we will encounter these later!)

Karine

From dan@cellectivity.com  Mon Apr 11 18:41:10 2005
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> Below is my homework, please tell me if I'm sending this too early.

In fact, I think it would be better to wait 24 hours or so before 
sending your answers, just to give everyone a chance to think things 
over, regardless of time zone. Don't hurry for me, because I don't read 
my e-mail on weekends (which I know is unusual for a programmer.  :-)  )

But your answers are right on. This one is particularly insightful:

> Considering that the program output was SuzySuzySuzySuzy, the "x"
 > operator seems to concatenate a string with itself, as many
 > times as asked for (don't know if there is a proper english term
 > for that, but that operator is cool!)
> 
> I tried to think a bit about 4 x 'Suzy'. Perl has to know which is
 > the string and which is the "multiplicator". In that example it could
 > guess it, but what about 4 x 3? 4 x 3 outputs "444", but 3 x 4
 > outputs "3333". So operands need to be ordered.

-- 
  [Larry] Wall [inventor of Perl] believes that people think about
  things in different ways, that natural languages accommodate many
  mindsets, and that programming languages should too.
    - Jon Udell, in his essay, "A Perl Hacker in the Land of Python"

From fazia@faziarizvi.net  Wed Apr 13 06:36:43 2005
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On Sat, 9 Apr 2005, Karine Delvare wrote:

> Hello Dan, thanks for the Perl course!
>
> Below is my homework, please tell me if I'm sending this too early.
>
>> 5) Exercises
>>
>> a) Write a program called 'average.pl' to calculate the average of
>> the following numbers: 23, 28, 31, 17, 1
>
> #!/usr/bin/perl -w
>
> use strict;
>
> my $num1 = 23;
> my $num2 = 28;
> my $num3 = 31;
> my $num4 = 17;
> my $num5 = 1;
>
> my $total = $num1 + $num2 + $num3 + $num4 + $num5;
>
> my $average = $total / 5;
>
> print "Average: $average\n";

Nifty. I eneded up doing this:


#!/bin/perl -w
# Perl Course Program
# Part 2: Scalar Variables
# Exercise "a" : Write a program called 'average.pl' to calculate the average of
# the following numbers: 23, 28, 31, 17, 1

use strict;
my $sum;
$sum = 23 + 28 + 31 + 17 + 1;
my $average;
$average = $sum / 5;

print "The average of 23, 28, 31, 17 and 1 is $average\n";



ad astra,
Faz

   (    Fazia Begum Rizvi
    )   www.faziarizvi.net
  ___
  \_/o

From swalk@ya.com  Sat Apr  9 22:24:50 2005
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Just letting you know that I am here and following the course as a newbie 
to programming. So far it all seems OK and there is not much to say. I am 
sure I will be squealing with pain before long :)

My only comment is on the homework average problem. I did this
#!/usr/bin/perl -w
use strict;

my $sum = 23 + 28 + 31 + 17 + 1;
print "The total is $sum \n";

my  $mean= $sum / 5;
print "The average is $mean \n";

Which works fine but is a bit of a cop out as I counted the number of items 
summed and  gave the result to the program. I expect that later in the 
course there will be a way of counting the number of entries in a variable.

Steve


From mrei0999@rz.uni-hildesheim.de  Sun Apr 10 01:00:48 2005
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Hi Steve, hi all

> I expect that later in
> the course there will be a way of counting the number of entries in a
> variable.

That won't be a problem once we get to arrays. He haven't done them yet,
but usually you would simply put all the numbers in an array, sum them up
and then divide the result by the length of the array.

Regards,
Meike

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> Just letting you know that I am here and following the course as a 
> newbie to programming.
> 
[snip]
> 
> Which works fine but is a bit of a cop out as I counted the number of 
> items summed and  gave the result to the program. I expect that later in 
> the course there will be a way of counting the number of entries in a 
> variable.

You may be a newby, but you certainly have the mind of a programmer! 
Rest assured that within two weeks we'll be able to ask the user to 
enter as many numbers as he likes, and then compute the average.

-- 
   When a program dies / what you need is a moment / of serenity.
       - Damian Conway, whose Perl module generates haikus

From dan@cellectivity.com  Sat Apr 16 00:04:12 2005
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        Fri, 15 Apr 2005 15:04:54 +0100
Subject: [Courses] [Perl] Part 3: User Input and "chomp"
From: Dan <dan@cellectivity.com>
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LinuxChix Perl Course Part 3: User Input and "chomp"

1) Introduction
2) The Line Input Operator
3) The chomp Function
4) Exercises
5) Answers to Previous Exercises
6) Acknowledgements
7) Licensing


          ----------------------------------------

1) Introduction

Try executing the following program:


  #!/usr/bin/perl -w
  use strict;
  
  my $line = <STDIN>;
  print "You wrote: $line";


You'll notice that the program seems to "hang" rather that printing
and then terminating. This means it is waiting for some user input.
Type something and then press enter and see what happens.

  you@localhost$  ./the-program.pl
  Let me add some input
  You wrote: Let me add some input
  you@localhost$

          ----------------------------------------

2) The Line Input Operator

The line input operator <>, (also known as the diamond operator)
allows your program to read data from file handles.  In particular
the special filehandle STDIN can be used to read input from the
keyboard.  For example:

  $line = <STDIN>;

          ----------------------------------------

3) The chomp Function

Let's change the introductory program a little bit:

  #!/usr/bin/perl -w
  use strict;
  
  my $line = <STDIN>;
  print "You wrote: {$line}\n";

Now when we execute it, we see this:

  you@localhost$  ./the-program.pl
  Let me add some input
  You wrote: {Let me add some input
  }
  you@localhost$

As you can see, the line input operator includes the newline at the
end of the input. There are lots of ways to remove that newline, but
the best way is to use the "chomp" function:

  #!/usr/bin/perl -w
  use strict;
  
  my $line = <STDIN>;
  chomp($line);
  print "You wrote: {$line}\n";

Note that "chomp" doesn't return the result; it changes whatever is
passed in. That means we could have combined two lines in the above
program:

  chomp(my $line = <STDIN>);

But never do this:

  $line = chomp($line);    # No!

One final note: "chomp" was introduced as a replacement for "chop",
which removes the last character from a string no matter what it is.
"chomp" only removes a line input separator (the newline).

          ----------------------------------------

4) Exercises

a) Write a currency conversion program to convert between dollars and
euros.

b) To make the previous exercise more interesting, have the program
ask for the exchange rate.

c) Try running the following program:

  #!/usr/bin/perl -w
  use strict;
  
  local $/ = ",";
  my $line = <STDIN>;
  print "$line\n";

What happens when you run it?  Try entering a line without a comma
followed by one with a comma. Can you explain what is happening? What
effect does this have on the "chomp" function?

If you need help, try reading "perldoc perlvar":

  $ perldoc perlvar

and looking under "INPUT_RECORD_SEPARATOR"

          ----------------------------------------

5) Answers to Previous Exercises

a) This program computes the average of five numbers.

  #!/usr/bin/perl -w
  use strict;
  
  my $sum = 23 + 28 + 31 + 17 + 1;
  my $average = $sum/5;
  
  print "The average is: $average\n";

b) The "x" operator repeats a string.

The number of repeats must always be the second parameter. As Karine
put it so well, "Perl has to know which is the string and which is
the "multiplicator". In that example it could guess it, but what
about 4 x 3? 4 x 3 outputs '444', but 3 x 4 outputs '3333'." So
operands need to be ordered.

As a contrast, the Python language allows you to "multiply" strings
by integers using the "*" operator. But because Python has a clear
separation between text and numbers, it accepts operands in either
order.

c) The "q" operator is simply an alias for the single quote. The only
advantage to using it is that it makes it easier to write strings
which include single quotes, because we can choose another character
to delimit the string. The "qq" operator is an alias for the double
quote.

          ----------------------------------------

6) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

          ----------------------------------------

7) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.



From kproot@nerim.net  Thu Apr 21 04:28:17 2005
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Date: Wed, 20 Apr 2005 17:57:47 +0200
From: Karine Delvare <kproot@nerim.net>
To: courses@linuxchix.org
Subject: Re: [Courses] [Perl] Part 3: User Input and "chomp"
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Oops... saturday I told myself I should wait this time, and then I completely forgot about the course! sorry


On Fri, 15 Apr 2005 15:04:01 +0100
Dan <dan@cellectivity.com> wrote:

> LinuxChix Perl Course Part 3: User Input and "chomp"
> 
> 4) Exercises
> 
> a) Write a currency conversion program to convert between dollars and
> euros.

#!/usr/bin/perl -w
use strict;

print "Conversion from dollars to euros
Please enter an amount:\n";
my $dollars = <STDIN>;
chomp $dollars;

my $euros = $dollars * 0.76;
print "$dollars dollars are $euros euros\n";

> b) To make the previous exercise more interesting, have the program
> ask for the exchange rate.

For that exercise I tried to write the line: my $rate = chomp(<STDIN>);
because it was more "natural" for me than a function (chomp) being applied to a assignment. Which is very bad as you just told chomp does not return any value. Moreover, it cannot modify <STDIN>, I got that error: Can't modify <HANDLE> in chomp. Bad, bad me :)

#!/usr/bin/perl -w
use strict;

print "Conversion from dollars to euros
Please enter exchange rate:\n";
chomp(my $rate = <STDIN>);
print "Please enter an amount:\n";
chomp(my $dollars = <STDIN>);

my $euros = $dollars * $rate;
print "$dollars dollars are $euros euros\n";

> c) Try running the following program:
> 
>   #!/usr/bin/perl -w
>   use strict;
>   
>   local $/ = ",";
>   my $line = <STDIN>;
>   print "$line\n";
> 
> What happens when you run it?  Try entering a line without a comma
> followed by one with a comma. Can you explain what is happening?

When I enter a line without a comma, a new blank line seems to be waiting for more input. I then entered a line with a comma (like "hello, somebody here?"), and I got my first line printed, and the second one printed without anything after the comma.

Thoughts:
1- there is not any loop in this perl file, so it seems that $line contains my two lines.
2- the program stops asking from input as soon as it gets a coma. It does not even go any further than the coma.
3- that behaviour was observed before with the newline character

So it seems the comma has now replaced the newline character, but only for the "stop taking input" part (that is, I didn't see my first newline turn into a comma at the end of my first line).
By "default", that strange $/ variable must equal "\n", and must be what you called : line input separator.

> What effect does this have on the "chomp" function?

Adding chomp to the input line, I saw I got the same result, but cut just before the comma instead of just after. So chomp behaves well and removes the line input separator, which is not the newline in that case, but the comma.

> If you need help, try reading "perldoc perlvar":
> 
>   $ perldoc perlvar
> 
> and looking under "INPUT_RECORD_SEPARATOR"

Interesting reading :)

Karine

From dan@cellectivity.com  Wed Apr 20 23:17:57 2005
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Subject: [Courses] [Perl] My bad ideas
From: Dan <dan@cellectivity.com>
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Hi everyone.

Last week I asked everyone to wait about a day before sending your
solutions to the list. I now realise that this was a mistake, so please
disregard what I said.

I wanted to make sure people didn't feel left out because they happened
to be asleep when our discussions took place. However, I seem to have
spoilt some of the fun and spontenaity(sp?) of the group. If you're
going to program, it's got to be fun!!!

So please resume sending your ideas and solutions to the list whenever
you feel like it.

--
   Distributed computing is like driving a wagon pulled
   by a thousand chickens.
                - Paul Lindner



From dan@cellectivity.com  Fri Apr 22 23:41:49 2005
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Subject: [Courses] [Perl] Part 4: Simple File Access
From: Dan <dan@cellectivity.com>
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LinuxChix Perl Course Part 4: Simple File Access

1) Introduction
2) The "die" Command
3) An Example Program
4) Exercises
5) Answers to Previous Exercises
6) Acknowledgements
7) Licensing

          ----------------------------------------

1) Introduction

Last week we saw how to read input from the keyboard. This week we
are going to expand on that topic to read from and write to files.

          ----------------------------------------

2) The "die" Command

Before getting into file access, we'll explain a handy command:
"die". This command does exactly what its name suggests: it kills
your program with an error message. Example:

  die;

Of course, you'll usually want to make the command conditional and
give a helpful error message. You could use it inside an "if"
statement, but we're going to use it with "or" instead:

  chomp($line) or die "Expected newline at end of $line";

The "or" operator works similarly to "||" in shell scripts. Without
going into detail, it causes the "die" command to be executed only if
the "chomp" command returns zero (failure).

By the way, if you want to terminate a program without an error, use
"exit".

          ----------------------------------------

3) An Example Program

The following example program creates a duplicate of a file, but with
line numbers.

  #!/usr/bin/perl -w
  use strict;
  
  # Open file for input (file name preceded by "<").
  open MY_INPUT, "< file1.txt" or die "Couldn't open input file: $!";
  
  # Open file for output (file name preceded by ">").
  open MY_OUTPUT, "> file2.txt" or die "Couldn't open output file:
$!";
  
  # Read from input file and write to output file.
  my $counter = 1;
  while ( defined( my $line = <MY_INPUT> ) ) {
    print MY_OUTPUT "$counter: $line";
    $counter++;
  }
  
  # Close the files.
  close MY_INPUT;
  close MY_OUTPUT;

Four lines of this program are particularly interesting: the two
"open" statements, the "while" loop, and the "print" statement. Let's
look at each one in turn.

The open statement takes two arguments. The first is a file handle,
which by convention is written in all CAPS. Note that, also by
convention, we don't put quotes around it; it's called a "bareword".
STDIN is a predefined file handle, but in this case we're declaring a
new one.

The second parameter to "open" is the name of the file. It should be
preceded by "<" if it's we're opening it for reading and by ">" if
we're opening it for writing (or ">>" if we're appending). Any spaces
before or after the filename are trimmed off. If you want to open a
file with strange characters like spaces, use "sysopen" instead.

If the "open" statement fails, the "die" statement is executed.
Notice the "$!": this special variable contains an explanation of the
last error encountered.

The "while" loop continues until "<MY_INPUT>" returns the undefined
value, which is the signal that we've reached the end of the file.
(Note: in Perl a "while" loop must be contained in braces, even if it
only has one statement. The same is true with "if".)

Finally, the "print" statement outputs its string to MY_OUTPUT. Note
that there is no comma after MY_OUTPUT. Also note that we didn't have
to add newlines because we never removed the newlines from the input.

          ----------------------------------------

4) Exercises

a) Write a Perl program that counts the number of lines in a file.
(Just hard-code the name of the file.)

b) Write a Perl program that reads a file, strips blank lines and
outputs the result to another file. To test if a line is (not) blank,
use:

  if ( $line ne "\n" ) {
    # ...
  }

Of course, if you chomp() the newline, compare with the empty string
instead.

c) If you don't specify "<" or ">" in the open statement, the file
will be opened for reading. Why is it still important to include a
"<"?

d) Upon reaching the end of a file, the line input operator "<>"
returns the undefined value, which evaluates to false. So why is it
necessary to include "defined" in the while loop? Why can't we
instead it write it like this:

  while ( my $line = <INPUT> )     # Don't do this! Use "defined"!

e) Try running the following program:

  #!/usr/bin/perl -w
  use strict;
  
  open MY_OUTPUT, "| grep 'foo' " or die "Couldn't run grep: $!";
  
  print MY_OUTPUT "A line with 'foo' passes.\n";
  print MY_OUTPUT "A line without it doesn't pass.\n";
  
  close MY_OUTPUT;

What does the vertical bar mean to the "open" statement? What happens
if you put the vertical bar after the command rather than before?

f) Try running the following program:

  #!/usr/bin/perl -w
  use strict;
  
  open MY_INPUT, "< file1.txt" or die "Couldn't open input file: $!";
  
  local $/ = undef;
  my $content = <MY_INPUT>;
  print "Content is: [$content]\n";
  
  close MY_INPUT;

What effect does it have to set "$/" to the undefined value?

          ----------------------------------------

5) Answers to Previous Exercises

(a) and (b) The following program takes converts dollars to euros:

  !/usr/bin/perl -w
  use strict;
  
  print "How many euros in a dollar? ";
  my $conversion_factor = <STDIN>;
  
  print "Give me a number of dollars (no dollar sign, please). ";
  chomp(my $amount = <STDIN>);
  
  my $answer = $conversion_factor * $amount;
  print "$amount dollars equals $answer euros.\n";

Notice that I chomped $amount (removed the newline) because it's
output on the last line. I could have chomped $conversion_factor as
well, but that wasn't necessary in this case.

c) The special variable $/ controls where the <> operator breaks
"lines", and what "chomp" removes. If you set it to a comma, then
<STDIN> will ignore newlines and break at a comma.

          ----------------------------------------

6) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

          ----------------------------------------

7) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.


From devdas@dvb.homelinux.org  Sat Apr 23 00:24:15 2005
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From: Devdas Bhagat <devdas@dvb.homelinux.org>
To: courses@linuxchix.org
Subject: Re: [Courses] [Perl] Part 4: Simple File Access
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On 22/04/05 14:41 +0100, Dan wrote:
<snip>
> The open statement takes two arguments. The first is a file handle,

Nit: The open function takes one, two or three arguments. See perldoc -f
open for the differences.

<snip>

> The "while" loop continues until "<MY_INPUT>" returns the undefined
> value, which is the signal that we've reached the end of the file.
> (Note: in Perl a "while" loop must be contained in braces, even if it
> only has one statement. The same is true with "if".)

Nit: while, for, foreach and if take a block as a parameter.
while (EXPR) { BLOCK }

{} are block delimiters.

Devdas Bhagat

From kproot@nerim.net  Sun Apr 24 21:06:54 2005
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From: Karine Delvare <kproot@nerim.net>
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Subject: Re: [Courses] [Perl] Part 4: Simple File Access
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On Fri, 22 Apr 2005 14:41:42 +0100
Dan <dan@cellectivity.com> wrote:

> LinuxChix Perl Course Part 4: Simple File Access
>
> 4) Exercises
>=20
> a) Write a Perl program that counts the number of lines in a file.
> (Just hard-code the name of the file.)

That one was easily done by removing code from your example :

#!/usr/bin/perl -w
use strict;

# Open file for input (file name preceded by "<").
open MY_INPUT, "< file1.txt" or die "Couldn't open input file: $!";

# Read from input file and count lines.
my $counter =3D 0;
while ( defined( my $line =3D <MY_INPUT> ) ) {
  $counter++;
}

# Close the files.
close MY_INPUT;

# Print result
print "The file contains $counter lines.\n";

> b) Write a Perl program that reads a file, strips blank lines and
> outputs the result to another file. To test if a line is (not) blank,
> use:
>=20
>   if ( $line ne "\n" ) {
>     # ...
>   }
>=20
> Of course, if you chomp() the newline, compare with the empty string
> instead.

I thought the second solution was nicest and I chomped the lines, but of =
course I forgot to add a newline when writing to the second file... so he=
re is the second version.

#!/usr/bin/perl -w
use strict;

# Open file for input (file name preceded by "<").
open MY_INPUT, "< file1.txt" or die "Couldn't open input file: $!";

# Open file for output (file name preceded by ">").
open MY_OUTPUT, "> file3.txt" or die "Couldn't open output file: $!";

# Read from input file and write to output file.
while ( defined( my $line =3D <MY_INPUT> ) ) {
  chomp $line;
  if ($line ne "") {
    print MY_OUTPUT "$line\n";
  }
}

# Close the files.
close MY_INPUT;
close MY_OUTPUT;

> c) If you don't specify "<" or ">" in the open statement, the file
> will be opened for reading. Why is it still important to include a
> "<"?

I'd say, for clarity ?

Ok, I tried to go a bit further and called a file "<". But, I don't know =
how to escape it correctly in the perl script and I couldn't go much furt=
her in my test.

> d) Upon reaching the end of a file, the line input operator "<>"
> returns the undefined value, which evaluates to false. So why is it
> necessary to include "defined" in the while loop? Why can't we
> instead it write it like this:
>=20
>   while ( my $line =3D <INPUT> )     # Don't do this! Use "defined"!

Looks like the good ol' error everyone makes in every language. On the la=
st line, <INPUT> will return undefined, so my $line =3D <INPUT> will succ=
esfully assign the undefined value to $line, and the code in the while lo=
op will be executed one more time, using an undefined value.

> e) Try running the following program:
>=20
>   #!/usr/bin/perl -w
>   use strict;
>  =20
>   open MY_OUTPUT, "| grep 'foo' " or die "Couldn't run grep: $!";
>  =20
>   print MY_OUTPUT "A line with 'foo' passes.\n";
>   print MY_OUTPUT "A line without it doesn't pass.\n";
>  =20
>   close MY_OUTPUT;
>=20
> What does the vertical bar mean to the "open" statement? What=20
> happens if you put the vertical bar after the command rather than=20
> before?

I had to run that one to guess what it was doing, and I'm still not sure =
I fully understand it - I'm not a shell genius. Happily, I use "|grep 'fo=
o' " enough for my own use to know that it will only output the lines tha=
t contain 'foo' from what the command before the vertical bar (the pipe i=
f I'm not mistaking) should output.
But here, I have difficulties to find what is the command before.

Running the script, I see that only the first line, with 'foo' in it, is =
printed, which is not so surprising, but I'm still very confused at the w=
ay it is done.

So, MY_OUTPUT is a file handle, but not a file on disk (as I don't see an=
y filename). And, the second argument to open, which should be the name o=
f a file, preceded by "<" or ">" or ">>". That's where I'm lost.

I "intuitively" understand that the two lines written to MY_OUTPUT are im=
mediately processed with "|grep 'foo' ", but I wouldn't be able to explai=
n why.

Reading perldoc -f open doesn't help either as it also states that the se=
cond argument gives the filename.

Hmmm... readin further in perldoc, I get :
               If the filename begins with =E2=80=99=E2=8E=AA=E2=80=99, t=
he filename is interpreted as
               a command to which output is to be piped, and if the filen=
ame
               ends with a =E2=80=99=E2=8E=AA=E2=80=99, the filename is i=
nterpreted as a command which
               pipes output to us.

So there it is. I wonder if lack of shell knowledge was the problem, but =
that one was really not easy.

Even though the second question seems to be answered too, I get errors wh=
en I try to put the pipe at the end :
Filehandle MY_OUTPUT opened only for input

> f) Try running the following program:
>=20
>   #!/usr/bin/perl -w
>   use strict;
>  =20
>   open MY_INPUT, "< file1.txt" or die "Couldn't open input file: $!";
>  =20
>   local $/ =3D undef;
>   my $content =3D <MY_INPUT>;
>   print "Content is: [$content]\n";
>  =20
>   close MY_INPUT;
>=20
> What effect does it have to set "$/" to the undefined value?

I did not see any effet, the script outputs my file as is... I was expect=
ing getting the file with no newlines.

Karine

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Hrm. I didn't get the original lesson/exercise email on this part. Could 
someone forward that to me?

-Faz


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> Hrm. I didn't get the original lesson/exercise email on this part. Could 
> someone forward that to me?

All messages to this list are stored in the LinuxChix archives. The one
you want is:
http://mailman.linuxchix.org/pipermail/courses/2005-April/001892.html

-- 
  [Mozilla] Firefox is better than Explorer by leaps and bounds. I
  don't miss Explorer one iota. Give Firefox a day's worth of Web
  surfing, and you won't either.
      - Forbes (short link: http://tinyurl.com/56j8m )



From fazia@faziarizvi.net  Tue Apr 26 04:12:44 2005
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On Mon, 25 Apr 2005, Dan wrote:

>> Hrm. I didn't get the original lesson/exercise email on this part. Could
>> someone forward that to me?
>
> All messages to this list are stored in the LinuxChix archives. The one
> you want is:
> http://mailman.linuxchix.org/pipermail/courses/2005-April/001892.html


Thanks!
ad astra,
Faz

Fazia Begum Rizvi 
Bits and Bytes: http://uweb.txstate.edu/~fr06/weblog/

From dan@cellectivity.com  Sat Apr 30 00:19:15 2005
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Subject: [Courses] [Perl] Part 5: The m// Operator
From: Dan <dan@cellectivity.com>
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LinuxChix Perl Course Part 5: The m// Operator

1) Introduction
2) An Example Program
3) Perl Regular Expressions
4) The Many Forms of m//
5) Exercises
6) Answers to Previous Exercises
7) Acknowledgements
8) Licensing


          ----------------------------------------

1) Introduction

Now we're getting into the truly interesting parts of Perl. In my
opinion the m// operator is one of the two most fun things about the
language. (The other is the s/// operator, which we'll get to in two
weeks.) These two operators alone make the language worth learning.
If you remember nothing else, remember these two operators.

          ----------------------------------------

2) An Example Program

Try executing the following program, then typing a few lines of
input. (Hint: try entering a line containing "foo".)

  #!/usr/bin/perl -w
  use strict;
  
  while ( defined( my $line = <STDIN> ) ) {
    if ( $line =~ m/foo/ ) {
      print "You entered a line containing 'foo'.\n";
    }
    else {
      print "You entered a line that doesn't contain 'foo'.\n";
    }
  }

Notice that the line with m// is does not use a single or double
equals sign, but rather an operator that we haven't seen before: an
equals sign followed by a tilde.

This example doesn't do justice to the power of m//, because m//
actually does regular expression matching. So let's look at Perl
regular expressions in a little more detail.

          ----------------------------------------

3) Perl Regular Expressions

I'm going to assume that you have some familiarity with regular
expressions. If you don't, feel free to ask the list.

Perl adds additional wildcards and capabilities to the regular
expression syntax provided by some other tools. This makes Perl
regular expressions somewhat proprietary, though people like them so
much that they've been built into several other languages (such as
PHP) as extensions. But remember that what works in Perl isn't
necessarily going to work in grep or some other tool.

In addition to the standard wildcards ("." for any character, etc.),
Perl adds these:
\s    white space
\S    anything but white space
\w    word character
\W    non-word character
\d    digit: equivalent to [0-9]
\D    non-digit

Perl also supports all the repetition flags you've probably seen in
your life:
*     zero or more
+     one or more
?     zero or one
{n}   exactly "n" instances
{m,n} between "m" and "n" instances

Here are the characters that have special meaning in Perl regular
expressions (I hope I got them all):
  . * + ? | ^ $ @ ( ) [ ] { } \
  and the delimiter (usually slash)
You escape a character (make it lose its special meaning) by
preceding it with a backslash.

Here are some quick examples of regular expressions in Perl:

  if ( $x =~ m/[a-z]{4}/ ) { ... }    # 4-letter word (lowercase)
  if ( $x =~ m/[A-Za-z]{4}/ ) { ... } # 4-letter word (any case)
  if ( $x =~ m/\@/ ) { ... }          # e-mail address (see below)

That last example reminds us of an important point: if part of the
string matches, the whole string matches. If the regular expression
must match the entire string, use ^ and $, like this:

  if ( $x =~ m/abc/ ) { ... }    # matches "abc" and "xabcx"
  if ( $x =~ m/^abc$/ ) { ... }  # matches only "abc"

          ----------------------------------------

4) The Many Forms of m//

In our examples we have been using a slash as a delimiter, but the
delimiter can be almost any punctuation. In other words, all of the
following are equivalent (they match one or more capital letters):

  if ( $x =~ m/[A-Z]+/ ) { ... }
  if ( $x =~ m:[A-Z]+: ) { ... }
  if ( $x =~ m![A-Z]+! ) { ... }
  if ( $x =~ m#[A-Z]+# ) { ... }
  if ( $x =~ m<[A-Z]+> ) { ... }

This avoids the need to escape slashes. However, if you do use a
slash as the delimiter, you can leave out the "m":

  if ( $x =~ /[A-Z]+/ ) { ... }

          ----------------------------------------

5) Exercises

a) Write a program that reads /etc/passwd and outputs the line
corresponding to your account.

b) Write a program that reads a C++ file and outputs all the #include
statements. Assume that no include statement spans multiple lines,
but consider that it might be indented.

          ----------------------------------------

6) Answers to Previous Exercises

a) The following program counts the number of lines in a file.

  #!/usr/bin/perl -w
  use strict;
  
  open MY_INPUT, "< file1.txt" or die "Couldn't open input file: $!";
  
  my $count = 0;
  while ( defined( <MY_INPUT> ) ) {
    $count++;
  }
  
  close MY_INPUT;
  
  print "The file has $count lines.\n";

b) The following program copies only non-blank lines from one file to
another.

  #!/usr/bin/perl -w
  use strict;
  
  open MY_INPUT,  "< file1.txt" or die "Couldn't open input file:
$!";
  open MY_OUTPUT, "> file2.txt" or die "Couldn't open output file:
$!";
  
  while ( defined( my $line = <MY_INPUT> ) ) {
    if ( $line ne "\n" ) {
      print MY_OUTPUT $line;
    }
  }
  
  close MY_INPUT;
  close MY_OUTPUT;

c) If you don't specify "<" or ">" in the open statement, the file
will be opened for reading. But you should get in the habit of
specifying a "<" anyway, because you'll often be reading a file that
the user passed on the command line. You don't want to erase a
valuable file just because the user accidentally (or maliciously)
preceded the filename with a ">".

d) The reason we must use "defined" to determine if we have reached
the end of the file is that some strings evaluate numerically to
zero, which evaluates to false. For example, if the very last line of
a file is the digit zero without a newline, we might skip over it if
we don't test it using "defined". That's the kind of bug that will
take you forever to find!

e) A vertical bar causes "open" to execute a program instead of
opening a file. If the vertical bar comes before the program name,
you can use "print" to write to the program's standard input. If the
vertical bar comes after the program name, you can use the <>
operator to read the program's output.

f) If you set $/ to undef, <> will continue reading until it reaches
the end of the file (which doesn't make sense with STDIN, unless you
pipe the input from a file). This is called "slurp mode".

          ----------------------------------------

7) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

          ----------------------------------------

8) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.



From kproot@nerim.net  Mon May  2 02:52:30 2005
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Date: Sun, 1 May 2005 18:52:10 +0200
From: Karine Delvare <kproot@nerim.net>
To: courses@linuxchix.org
Subject: Re: [Courses] [Perl] Part 5: The m// Operator
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On Fri, 29 Apr 2005 15:19:05 +0100
Dan <dan@cellectivity.com> wrote:

> LinuxChix Perl Course Part 5: The m// Operator

> 5) Exercises
> 
> a) Write a program that reads /etc/passwd and outputs the line
> corresponding to your account.

#!/usr/bin/perl -w
use strict;

open MY_INPUT, "< /etc/passwd" or die "Couldn't open input file: $!";

while (defined(my $line = <MY_INPUT>))
{
        if ($line =~ /^edhel:/)
        {
                print $line;
        }
}

close MY_INPUT;

I wonder if there is a way to know when your regular expression is "enough". I started to search for /edhel/, then thought that would match zedhel or edhel2 if such users existed, so I tried /^edhel:/, but then I thought, what if it catches more than I want (like, if I had not thought about zedhel2). Do I have to be careful enough, or are there tips / techniques to get as accurate as possible with regexps ?

> b) Write a program that reads a C++ file and outputs all the #include
> statements. Assume that no include statement spans multiple lines,
> but consider that it might be indented.

#!/usr/bin/perl -w
use strict;

print "Please enter the path to a C++ file:\n";
my $file = <STDIN>;

open MY_INPUT, "< $file" or die "Couldn't open input file: $!";

while (defined(my $line = <MY_INPUT>))
{
        if ($line =~ /\s*#include /)
        {
                print $line;
        }
}

close MY_INPUT;

-- 
Karine Delvare
http://edhel.nerim.net/

From dan@cellectivity.com  Sat May  7 01:11:11 2005
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Subject: [Courses] [Perl] Part 6: Data Extraction with m//
From: Dan <dan@cellectivity.com>
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LinuxChix Perl Course Part 6: Data Extraction with m//

1) Introduction
2) Match Extraction
3) Greedy Matching
4) Exercise
5) Answers to Previous Exercises
6) Acknowledgements
7) Licensing


          ----------------------------------------

1) Introduction

The m// operator is actually much more powerful than we saw in the
last lesson. In this lesson we're going to see how to use it to
extract information.

          ----------------------------------------

2) Match Extraction

The m// operator allows you to extract text from a string. Simply
enclose the relevant part of the pattern in parentheses. The content
of the first set of parentheses will be stored in the variable $1,
the second in $2, etc.

Example:
  # Match digits-colon-digits-colon-digits
    if ( $time =~ /(\d+):(\d+):(\d+)/ ) {
      $hour = $1;     # First set of parentheses
      $minute = $2;   # Second set of parentheses
      $second = $3;   # Third set of parentheses
    }
    else {
      print "Time wasn't valid.\n";
    }

Alternatively, m// returns a list of the matches, so we can compress
the above example into one line:

  if ( ($hour, $minute, $second) = ( $time =~ /(\d+):(\d+):(\d+)/ ) )

When extracting text, we don't technically need to put the m// in an
"if" statement, but it's highly recommended. You should always take
into account the possibility that the pattern doesn't match.

          ----------------------------------------

3) Greedy Matching

Examine the following program. What will it output?

  #!/usr/bin/perl -w
  use strict;
  
  my $html = '<p>foo</p> <p>bar</p> <p>baz</p>';
  
  if ( $html =~ m:<p>(.*)</p>: ) {
    print "$1\n";
  }
  
It doesn't output "foo", "bar" or "baz". Instead, it outputs "foo</p>
<p>bar</p> <p>baz" This is because the asterisk wildcard is "greedy":
it always includes the maximum amount of text possible.

If you don't want a greedy match, add a question mark after the
asterisk:

  if ( $html =~ m:<p>(.*?)</p>: )        # Output is "foo".

Ditto for the plus sign:

  if ( $html =~ m:<p>(.+?)</p>: )

Greediness never affects WHETHER there is a match; it only affects
exactly what text is matched.

          ----------------------------------------

4) Exercise

Modify your program from last week's exercise, the one that reads
/etc/passwd, so that it outputs only your home directory and your
shell (not the rest of the line).

          ----------------------------------------

5) Answers to Previous Exercises

a) The following program reads /etc/passwd and outputs the line
corresponding to my account name:

  #!/usr/bin/perl -w
  use strict;
  
  my $username = 'dan';
  
  open PASSWD, "< /etc/passwd" or die "Couldn't open file.";
  
  while ( defined( my $line = <PASSWD> ) ) {
    if ( $line =~ m/^$username:/ ) {
      print $line;
    }
  }
  
  close PASSWD;

To avoid hard-coding the account name, we could use the UID instead:

  #!/usr/bin/perl -w
  use strict;
  
  my $uid = $<;     # $< is system account number (UID).
  
  open PASSWD, "< /etc/passwd" or die "Couldn't open file.";
  
  while ( defined( my $line = <PASSWD> ) ) {
    if ( $line =~ m/^[^:]+:[^:]*:$uid:/ ) {
      print $line;
    }
  }
  
  close PASSWD;

b) This program reads a C++ file and outputs all #include statements:

  #!/usr/bin/perl -w
  use strict;
  
  open IN, "< foo.c" or die "Couldn't open file.";
  
  while ( defined( my $line = <IN> ) ) {
    if ( $line =~ m/^\s*#include/ ) {
      print $line;
    }
  }
  
  close IN;

          ----------------------------------------

6) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

          ----------------------------------------

7) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.


From kproot@nerim.net  Mon May  9 00:45:00 2005
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Date: Sun, 8 May 2005 16:44:54 +0200
From: Karine Delvare <kproot@nerim.net>
To: courses@linuxchix.org
Subject: Re: [Courses] [Perl] Part 6: Data Extraction with m//
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On Fri, 06 May 2005 16:11:00 +0100
Dan <dan@cellectivity.com> wrote:

> LinuxChix Perl Course Part 6: Data Extraction with m//
> 
> 4) Exercise
> 
> Modify your program from last week's exercise, the one that reads
> /etc/passwd, so that it outputs only your home directory and your
> shell (not the rest of the line).

#!/usr/bin/perl -w
use strict;

open MY_INPUT, "< /etc/passwd" or die "Couldn't open input file: $!";

print "Please enter a username:\n";
chomp(my $user = <STDIN>);

my $found = 0;
my $home;
my $shell;

while (defined(my $line = <MY_INPUT>))
{
        if (($home, $shell) = ($line =~ /^$user:.*:(.*):(.*)$/))
        {
                print "Home is: $home\nShell is: $shell\n";
                $found = 1;
        }
}

if (!$found)
{
        print "This user does not exist.\n";
}

close MY_INPUT;



Provided I'm sure shell is the last info and home the one before that, I decided to use a greedy match for all the info between the user and the home directory.
Does greedy / non-greedy affect performances?

-- 
Karine Delvare
http://edhel.nerim.net/

From dan@cellectivity.com  Sat May  7 01:48:21 2005
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Subject: [Courses] [Perl] Re: exercises
From: Dan <dan@cellectivity.com>
To: Karine Delvare <kproot@nerim.net>
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Hi Karine.

I'm sorry: for some reason I didn't receive your post. I only saw it as
I browsed the LinuxChix archives.

Your first exercise is excellent. Good for you for remembering to begin
with "^" and end with ":".

> I wonder if there is a way to know when your regular expression
> is "enough".

That's a classic problem with regular expressions: it really is hard to
know that you won't get any false matches. Caveat hacker.

Now on to your second example:

> my $file = <STDIN>;
> 
> open MY_INPUT, "< $file" or die "Couldn't open input file: $!";

Notice that you didn't chomp the input, but the "open" statement takes it anyway.

> if ($line =~ /\s*#include /)

This is good, but you forgot to start the pattern with "^" this time. Caveat hackor!

-- 
 [Larry] Wall [inventor of Perl] believes that people think about
 things in different ways, that natural languages accommodate many
 mindsets, and that programming languages should too.
   - Jon Udell, in his essay, "A Perl Hacker in the Land of Python"



From dan@cellectivity.com  Fri May 13 18:58:34 2005
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Subject: [Courses] [Perl] Part 7: Changing Text with the s/// Operator
From: Dan <dan@cellectivity.com>
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LinuxChix Perl Course Part 7: Changing Text with the s/// Operator

1) Introduction
2) An Example
3) Data Extraction with s///
4) Pitfalls
5) Exercises
6) Answer to Previous Exercise
7) Acknowledgements
8) Licensing


          ----------------------------------------

1) Introduction

The s/// operator is my (personal) favorite operator in Perl. It
provides a very powerful means of changing text based on regular
expressions.

          ----------------------------------------

2) An Example

Try running this program:

  #!/usr/bin/perl -w
  use strict;
  
  my $message = 'You need to RTFM.';
  
  $message =~ s/RTFM/Read The Fine Manual/;
  print "$message\n";

As you can see, the s/// operator substitutes one string for another.

Although the above example doesn't demonstrate it very well, s///
takes a regular expression. So the following substitution would also
work:

  $message =~ s/[A-Z]{4}/Read The Fine Manual/;

By the way, s/// can take any delimiter, just like its cousin m//:

  $x =~ s/foo/bar/;
  $x =~ s!foo!bar!;
  $x =~ s#foo#bar#;
  $x =~ s<foo><bar>;

          ----------------------------------------

3) Data Extraction with s///

Like m//, s/// can use parentheses to extract data. The data can then
be inserted into the substitution expression, like this:

  # Change reference from GIF to PNG.
  $html =~ s/<img src="(.*?)\.gif"/<img src="$1.png"/;

The "$1" on the right refers to the "(.*?)" on the left.

          ----------------------------------------

4) Pitfalls

There are three common sources of errors when using s///. Let's
illustrate them with an example.

  #!/usr/bin/perl -w
  use strict;
  
  my $html = '<p>Just <!-- xxx --> some <!-- xxx --> text</p>';
  $html =~ s/<!--.*-->//;   # Strip HTML comments - maybe.
  print "$html\n";

The first problem is greediness, just as we saw with m//. The s///
will match from the beginning of the first comment to the end of the
second comment, and strip out the word "some" in the middle. Instead,
let's try this:

  $html =~ s/<!--.*?-->//;      # Notice the "?".

That turns off greediness: the match will be as small as possible.

But there's another problem: only one comment will be removed. If we
want to remove both comments, we could execute the s/// line twice,
but a better way is to use the "g" option ("g" for "global
substitution"):

  $html =~ s/<!--.*?-->//g;     # Notice the "g" on the end.

Now every match will be substituted, no matter how many there are.

As for the final pitfall, we'll have to change the example a little:

  #!/usr/bin/perl -w
  use strict;
  
  my $html = "<p>A <!-- \n multi-line \n --> string in Perl</p>";
  $html =~ s/<!--.*?-->//g;   # Strip HTML comments - maybe.
  print "$html\n";

Each "\n" is a newline. It's silly to hard-code them like that, but
if we had read the HTML code from a file there would probably be a
lot of newlines scattered around and we'd have to deal with them.

If you try executing this program, you'll see that the substitution
doesn't occur. That's because by default the "." pattern matches any
character EXCEPT a newline. To get "." to match newlines as well, use
the "s" option (for "treat as Single line"):

  $html =~ s/<!--.*?-->//gs;   # Note the "s" after the "g".

In this case, we combined the "g" and "s" options. That's fine, and
the order doesn't matter.

So to conclude, there are three common pitfalls with s///:
a) greediness: use "?" to turn it off
b) number of replacements: use "g" to replace all occurrences
c) newlines: use "s" to match across newlines

          ----------------------------------------

5) Exercises

a) Harry Potter fans know that Professor Lockhart succeeded Professor
Quirrel as Dark Arts teacher. What they don't know is that Headmaster
Dumbledore got a Perl hacker to write a quick script to change the
professor's name on Hogwarts' web site. It read from standard input
and sent the result to standard output. What did the script look
like?

b) Harvard cardiologist Thomas Michel writes in his "Guide to
Politically Correct Cardiology" about the importance of using
inoffensive medical terms. For example, he suggests saying
"metabolically different" instead of the highly offensive "dead"[*].
Write a Perl program that reads a medical diagnosis (or any other
input) and uses "s///" to change the word "dead" to "metabolically
different". How are you going to avoid false matches such as
"deaden"?

[*] Yes, the paper is real and the scientist is real. Of course, he
wasn't taking himself too seriously when he wrote the paper.

c) American English is slightly different from UK English in several
respects, one of which is spelling[*]. For example, words ending in
"ise" in England generally end in "ize" in the United States, e.g.,
"exercise" becomes "exercize". Write a Perl program that "translates"
such words from American to UK English, i.e., changes words ending in
"ize" to end in "ise". For extra credit, take into account variations
like "exercizes" and "exercizing".

[*] Blame it on Noah Webster. The writer of the first dictionary in
America, Webster deliberately chose to spell words differently for
reasons both practical (making the language easier to learn for
immigrants) and patriotic (declaring linguistic independence).

d) Consider the following program:

  #!/usr/bin/perl -w
  use strict;
  
  my $verse = "
  99 bottles of beer on the wall.
  99 bottles of beer.
  Take one down, pass it around,
  98 bottles of beer on the wall.\n\n";
  
  print $verse;
  $verse =~ s/(\d+)/ $1 - 1 /ge;
  print $verse;

Notice the "e" option in the "s///" statement. What does it do?

(In case you're wondering, I don't drink beer. The words come from an
old and boring song which counts from 99 to 1.)

          ----------------------------------------

6) Answer to Previous Exercise

Here is a program that outputs your home directory and shell.

  #!/usr/bin/perl -w
  use strict;
  
  my $uid = $<;     # $< is system UID.
  
  open PASSWD, "< /etc/passwd" or die "Couldn't open file.";
  
  while ( defined( my $line = <PASSWD> ) ) {
    if ( $line =~ m/^[^:]+:[^:]*:$uid:.*:(.*?):(.*?)$/ ) {
      print "Home dir = $1\n";
      print "Shell = $2\n";
    }
  }
  
  close PASSWD;

Note that the regular expression demonstrates two ways to prevent
greediness. On the left side of the regular expression, I use /[^:]/
to ensure that the match can't include a colon, which separates the
fields. On the right side I use /.*?/, which causes the match to be
as short as possible.

          ----------------------------------------

7) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

          ----------------------------------------

8) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.


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Subject: [Courses] [Perl] Part 8: The Special Variable $_
From: Dan <dan@cellectivity.com>
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LinuxChix Perl Course Part 8: The Special Variable $_

1) Introduction
2) An Implied Variable
3) $_ in While Loops
4) Back to the Original Example
5) Exercises
6) Answers to Previous Exercises
7) Acknowledgements
8) Licensing


          ----------------------------------------

1) Introduction

If you've read real-life Perl code before, you may remember being
puzzled by parts that read like this:

  while ( <STDIN> ) {
    s/foo/bar/;
    print;
  }

Most of this already looks familiar from the previous few weeks. But
this code is confusing because it doesn't look like any variables are
assigned or changed. We're now going to unravel this mystery.

          ----------------------------------------

2) An Implied Variable

We've seen that Perl has some funny variables, like "$<" for your
UID. But by far the most useful of these is "$_" (pronounced
"dollar-underscore").

This variable is passed as an understood parameter to many functions
and operators if an expected parameter is not explicitly passed.
Examples:

  chomp;       # Equivalent to chomp($_);
  print;       # Equivalent to print $_;

"$_" is also the understood subject of "m//" and "s///":

  s/foo/bar/;             # Equivalent to $_ =~ s/foo/bar/
  if ( /baz/ ) { ... }    # Equivalent to $_ =~ m/baz/

By the way, despite the "magic" nature of "$_", it can still be
assigned to or from just like any other variable.

          ----------------------------------------

3) $_ in While Loops

For any file handle "HANDLE", the following are equivalent:

  while ( <HANDLE> ) { ... }
      # ... is equivalent to ...
  while ( defined( $_ = <HANDLE> ) ) { ... }

Note that this trick only works if "<HANDLE>" is used as the sole
criterion of a while loop. So the following do NOT set "$_":

  <HANDLE>;                  # NOT equivalent to $_ = <HANDLE>
  if ( <HANDLE> ) { ... }    # NOT equivalent to $_ = <HANDLE>

          ----------------------------------------

4) Back to the Original Example

After this explanation, the example we started with looks a lot
clearer.

  while ( <STDIN> ) {   # while ( defined( $_ = <STDIN> ) ) {
    s/foo/bar/;         #   $_ =~ s/foo/bar/;
    print;              #   print $_;
  }                     # }

          ----------------------------------------

5) Exercises

a) Modify a recent program that we've written to use "$_" instead of
a variable. Try to avoid explicitly naming "$_".

b) Another "implied" variable is "ARGV", a which is the default
argument to the diamond operator "<>". Try running the following
program to find out what it does.

  #!/usr/bin/perl -w
  use strict;
  
  while ( <> ) {
    print;
  }

Hint: try specifying several file names on the command line.

          ----------------------------------------

6) Answers to Previous Exercises

a) The following program changes "Quirrel" to "Lockhart":

  #!/usr/bin/perl -w
  use strict;
  
  while ( defined(my $line = <STDIN>) ) {
    $line =~ s/Quirrel/Lockhart/g;
    print $line;
  }

To simply things we don't worry about false matches. However, there
is a small chance that we'll run into trouble if there's a discussion
of squirrels. The answer to the next exercise explains how to avoid
false matches.

b) Rather than just explaining the solution to this one, I'm going to
go through the reasoning to find the solution.

When we try to convert "dead" to "metabolically different", we want
to match the whole word "dead", but not words that contain it. So we
do something like this:

  $line =~ s/ dead / metabolically different /g;

The spaces around the word avoid false matches like "deaden", but
they also mean that the word won't be replaced if it's followed by a
comma or some other punctuation. So we want a line like this:

  $line =~ s/[^A-Za-z]dead[^A-Za-z]/metabolically different/g;

But that causes the characters before and after the word to be
replaced as well, so we have to include them on the right side of the
substitution:

  $line =~
    s/([^A-Za-z])dead([^A-Za-z])/${1}metabolically different$2/g;

Oh dear: now we won't match a line that begins or ends with "dead"
because our pattern requires a character before and after the word.
So we'll have to code some special cases:

  $line =~
    s/([^A-Za-z])dead([^A-Za-z])/${1}metabolically different$2/g;
  $line =~ s/^dead([^A-Za-z])/metabolically different$1/g;
  $line =~ s/([^A-Za-z])dead$/${1}metabolically different/g;
  $line =~ s/^dead$/metabolically different/g;

Relax: it gets better now. The point of this whole mess was to bring
up some useful Perl functionality. We can avoid having to write
"[^A-Za-z]" all the time by using \W instead. \w (lowercase) matches
a "word character" (letter, digit or underscore). \W (uppercase)
matches anything that's not \w. So we can write this instead:

  $line =~ s/(\W)dead(\W)/${1}metabolically different$2/g;
  # ... and so on

But an even better solution is to use the word boundary character:
\b. This handy little guy matches the space between two characters:
between a \w and a \W (in either order) or between a \w and the
beginning or end of the string. It's actually zero-length, so we
don't have to worry about replacing it.

So we can do everything in one substitution:

  $line =~ s/\bdead\b/metabolically different/g;

And the whole program looks like this:

  #!/usr/bin/perl -w
  use strict;
  
  while ( defined(my $line = <STDIN>) ) {
    $line =~ s/\bdead\b/metabolically different/g;
    print $line;
  }

c) To change U.S. spelling to U.K. spelling, we use \b as in the
previous exercise.

  #!/usr/bin/perl -w
  use strict;
  
  while ( defined(my $line = <STDIN>) ) {
    $line =~ s/ize\b/ise/g;
    print $line;
  }

Here is a more advanced version that takes into account variations
like "realised" and "desensitising". It also avoids changing the
spelling of the words "size", "resize" and "downsize", which are
spelt the same way in the U.K. as in America:

  #!/usr/bin/perl -w
  use strict;
  
  while ( defined(my $line = <STDIN>) ) {
    $line =~ s/iz(e|es|ed|ing)\b/is$1/g;
    $line =~ s/\b(re|down)?sis(e|es|ed|ing)\b/$1siz$2/g;
    print $line;
  }

d) The "e" option causes the substitution to be evaluated as Perl
code. You can use this to cause "s///" to call an arbitrary function
on every match.

          ----------------------------------------

7) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

          ----------------------------------------

8) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.


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Subject: [Courses] [Perl] Part 9: Intro to Arrays
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LinuxChix Perl Course Part 9: Intro to Arrays

1) Introduction
2) A Simple Example
3) Using foreach
4) Filling an Array
5) Exercises
6) Answers to Previous Exercises
7) Acknowledgements
8) Licensing

          ----------------------------------------

1) Introduction

As you may recall, there are four basic data types in Perl:
a) scalar - A single chunk of data
b) array - A bunch of scalars, indexed by a number
c) hash (hashtable) - A bunch of scalars, indexed by another scalar
known as a key
d) handle - A pointer enabling the opening of resources from the
operating system (files, directories, etc.)

Up until now we've only looked at scalars and handles, but now we're
going to look at arrays.

Arrays in Perl are both handy and powerful, but it takes time to
learn to use them properly. For this reason, we're going to spend
three weeks looking at arrays:
a) This week we'll see array basics.
b) Next week we'll look at some helpful array functions.
c) The following week we'll see the painful details of arrays.

          ----------------------------------------

2) A Simple Example

Here's a simple program that outputs what I'll eat for lunch today.

  #!/usr/bin/perl -w
  use strict;
  
  my @lunch = ('sandwich', 'apple', 'cookie');
  print "My lunch today: @lunch\n";

As you can see, an array variable is preceded by "@" rather than "$".

          ----------------------------------------

3) Using foreach

The above example wasn't very helpful because we didn't access the
array elements separately. Let's try a slightly more complicated
example.

  #!/usr/bin/perl -w
  use strict;
  
  my @lunch = ('sandwich', 'apple', 'cookie');
  foreach my $item (@lunch) {
    print "I'll eat a $item.\n";
  }

In addition to accessing each member of an array, "foreach" can be
used to change members of an array. This program is a demonstration.

  #!/usr/bin/perl -w
  use strict;
  
  my @arr = ('metadata', 'meta-information', 'metaphysics');
  foreach my $item (@arr) {
    $item =~ s/meta-?/useful /;
  }
  print "@arr\n";

          ----------------------------------------

4) Filling an Array

There are many ways to fill an array. Here are a few.

a) From a file handle:

  my @lines = <HANDLE>;        # Read whole file.

b) From another array:

  @b = @a;

In the above case, "@b" contains a copy of "@a", not a pointer to it.
Changes to either will not affect the other.

c) Using m//g:

  # The following finds everything that comes after the letter "Q".
  my $text = "There's an Iraqi squid on my qwerty keyboard!";
  my @letters = ( $text =~ m/q([a-z])/g );
  print "@letters\n";        # Output is: i u w

The "g" option means a global match: return everything at once.

          ----------------------------------------

5) Exercises

a) Write a program that reads an HTML file and removes all the HTML
tags, leaving just the text. (Yes, I know we could do this without
arrays, but read the file as an array of lines.)

Note: if spaces magically appear at the beginning of the lines in
your output file, it's because you wrote the "print" statement like
this:
  print OUTPUT "@lines";      # Quotes around @lines
Instead, try writing it like this:
  print OUTPUT @lines;        # No quotes

b) Consider this program:

  #!/usr/bin/perl -w
  use strict;
  
  my @arr = ('sing', 'ring', 'fling');
  foreach (@arr) {
    s/ing/ung/;
  }
  print "@arr\n";

If no scalar variable is specified in the "foreach" statement, what
variable is used?

c) Consider this program:

  #!/usr/bin/perl -w
  use strict;
  
  my @a = ('aardvark', 'beaver', 'ant', 'squirrel');
  my @b = grep /^a/, @a;     # Match words starting with A.
  print "@b\n";

What does "grep" do? "grep" is a very powerful tool, and well worth
reading the manual for:

  perldoc -f grep

          ----------------------------------------

6) Answers to Previous Exercises

a) This is a variation of the program that converts U.S. spelling to
U.K. spelling. This one changes the spelling of e.g. "color" to
"colour".

  #!/usr/bin/perl -w
  use strict;
  
  while ( <STDIN> ) {
    s/(\w\w[^aeiou])or\b/$1our/g;
    print;
  }

(Actually, this example isn't as good as the previous one because
there are more exceptions to the rule. For example, the word
"monitor" is spelt the same way in the U.S. and the U.K.)

b) The "ARGV" file handle, which is implied by an empty diamond
operator "<>", treats each command-line parameter as a file name and
reads from each of these files in succession. If no file is specified
on the command-line, it reads from standard input. This is the
standard behaviour for many Unix commands.

          ----------------------------------------

7) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

          ----------------------------------------

8) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.


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Subject: [Courses] [Perl] Part 10: Array Functions
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LinuxChix Perl Course Part 10: Array Functions

1) Introduction
2) push, pop, shift, unshift
3) split and join
4) sort and reverse
5) Consider the Context
6) Exercises
7) Answers to Previous Exercises
8) Acknowledgements
9) Licensing


          ----------------------------------------

1) Introduction

Last week we saw some basic uses of arrays. This week we're going to
look at the powerful functions that Perl provides for manipulating
arrays.

Next week we're going to see how to index arrays. The reason we save
that for last is that you don't need to index arrays very often: Perl
does most of the low-level array manipulation for you.

          ----------------------------------------

2) push, pop, shift, unshift

"push" and "pop" add and remove items from the end of an array.

  my @arr = ('A', 'B', 'C', 'D');
  my $popped = pop(@arr);      # now $popped = 'D'
  push(@arr, 'X', 'Y');        # now @arr = A B C X Y

"shift" and "unshift" are similar, but they add or remove items from
the beginning. If you don't pass an array to "shift", the list of
command-line parameters ("@ARGV") is used instead. This is the most
common use if "shift".

          ----------------------------------------

3) split and join

"split" splits a string:

  my $password_file_entry = 'root:x:0:0:root:/root:/bin/bash';
  my @fields = split(/:/, $password_file_entry);
  print "@fields\n";    # Output is: root x 0 0 root /root /bin/bash

The first parameter to "split" is a regular expression, so it can
split strings in very sophisticated ways.

The opposite of "split" is "join":

  my @path = ('/bin', '/usr/sbin', '/usr/bin' );
  print join(':', @path);      # Output is: /bin:/usr/sbin:/usr/bin

          ----------------------------------------

4) sort and reverse

"sort" returns an array in alphabetical order (NOT numerical order).

  my @animals = ('gopher', 'bat', 'yak');
  my @sorted = sort(@animals);
  print "@sorted\n";      # Output is: bat gopher yak

"sort" allows you to give a code block or a reference to a comparison
function as the first argument. We haven't gotten to function
references yet, but the following convenient trick allows you to sort
in numerical order:

  @arr = sort( {$a<=>$b} @array_of_numbers );

"reverse" reverses the order of an array. This is NOT the same as
sorting it in reverse order, unless the array was already sorted.

          ----------------------------------------

5) Consider the Context

Many Perl functions and operators behave differently depending on the
"context". Perl has three possible contexts: scalar context, list
context and void context:

  my @arr = ('foo', 'bar', 'baz');
  my @a = reverse(@arr);       # Array context
  my $s = reverse(@arr);       # Scalar context
  reverse(@arr);               # Void context
  
  print "@a\n";       # Output is: baz bar foo
  print "$s\n";       # Output is: zabraboof (foobarbaz backwards)

Note that the context is determined by how the return value is used,
NOT the parameters passed to the function/operator. Many languages
allow a function to change according to its input, but Perl is the
only language that I know that allows a function to change its
behaviour depending on how its output is used!

The use of an array in scalar context returns the length of the
array.

In the above cases the context was obvious because we assigned to an
array or scalar variable. But the context can also be indicated by an
operator. For example, arithmetic operators are all scalar, as is
string concatenation:

  print "The length of the array is: " . @array . "\n";

          ----------------------------------------

6) Exercises

a) Write a program that mimics the Unix command "sort". (No, you
don't have to implement all the command-line options; just read from
standard in and write to standard out.)

b) Write a summation program that accepts input composed of positive
integers separated by plus signs with any amount of space in between.
For example, use it to compute this line:

  15   +  5+100+  22+   1

Remember, There Is More Than One Way To Do It (TIMTOWTDI).

          ----------------------------------------

7) Answers to Previous Exercises

a) The following program strips HTML tags from a file:

  #/usr/bin/perl -w
  use strict;
  
  open MY_INPUT,  "< file1.html" or die "Couldn't open input file:
$!";
  my @lines = <MY_INPUT>;        # Read whole file.
  close MY_INPUT;
  
  foreach my $line (@lines) {
    $line =~ s/<.*?>//g;
  }
  
  open MY_OUTPUT, "> file2.html" or die "Couldn't open output file:
$!";
  print MY_OUTPUT @lines;
  close MY_OUTPUT;

b) If no scalar variable is specified in a "foreach" loop, the
special variable "$_" is used.

c) The "grep" command in Perl compares returns all elements of an
array that meet a certain condition. Each element is stored
sequentially in "$_" an the condition is tested. Examples of "grep"
statements:

  @a = grep /foo/, @b;        # Containing "foo"
  @a = grep( $_ > 5, @b );    # Numbers greater than 5
  @a = grep defined, @b;      # Filter undef.

As with all Perl functions, parentheses are optional with "grep".

          ----------------------------------------

8) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

          ----------------------------------------

9) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.


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Subject: [Courses] [Perl] Part 11: More About Arrays
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LinuxChix Perl Course Part 11: More About Arrays

1) Introduction
2) Indexing Arrays
3) Flat Lists
4) Exercise
5) Answers to Previous Exercises
6) Acknowledgements
7) Licensing

          ----------------------------------------

1) Introduction

In the previous two weeks we've had a lot of fun with arrays. This
week is going to be a little "heavier", focusing on the details.

          ----------------------------------------

2) Indexing Arrays

As you might have guessed, arrays are indexed using good old "[]" -
but with a twist. The array name is (usually) preceded by a dollar
sign rather than an at-sign:

  my @list = ('some', 'words');
  print "$list[0]\n";       # Indexes start at zero.

As "man perldata" puts it: "$foo[1] is a part of @foo, not a part of
$foo. This may seem a bit weird, but that's okay, because it is
weird."

Don't let the humility of those words fool you: the system is
actually quite clever when you fully understand it. The "[]"
indicates that the variable is an array; the dollar sign indicates
that what you want from it is a scalar. It may seem obvious that you
would want a scalar, but Perl allows you to "slice" an array by
specifying multiple indices:

  my @chars = ('A', 'B', 'C', 'D', 'E', 'F');
  my @a = @chars[1,5];     # Gets B and F.
  my @b = @chars[1 .. 4];  # Gets B, C, D and E.
  my @c = @chars[1,3..5];  # Gets B, D, E and F.
  
  my @n = (2, 5);
  my @d = @chars[@n];      # Gets C and F.
  @chars[@n] = @n;         # Now @chars = A B 2 D E 5.

When you slice, you get back an array, so the variable name is
preceded by an "@".

A negative subscript is interpreted as though it had been added to
the length of the array, so negative numbers can be thought of as
"counting backwards from the end":

  my @chars = ('A', 'B', 'C', 'D', 'E', 'F');
  print "$chars[-2]\n";    # Prints "E".

It's not an error to access a non-existant subscript; you just get
back "undef" (the undefined value). If you assign to a non-existant
subscript, the array will be automatically extended.

          ----------------------------------------

3) Flat Lists

An array is a type of list. All lists in Perl are "flat": they can
only contain scalars, not other lists. Example:

  my @men = ('Bob', 'Joe');
  my @women = ('Alice', 'Ronda', 'Patricia');
  my @everyone = (@men, @women);     # Copy, not reference.
  my $length = $#everyone + 1;       # $length is 5.
  print "$length names: @everyone\n";

Trying to assign a whole array to a single array element won't work.
It would seem that you can't create an array of arrays, but Perl
provides a rather quirky way of getting around this limitation. We'll
see it when we get to references, but if you're really curious (and
bold), read "perldoc perllol". Note: in that document, brackets are
used not only for array indexing, but also for array references.

          ----------------------------------------

4) Exercise

a) It's often convenient to communicate with spreadsheets and
databases by writing files in Comma Separated Values (CSV) format.
Under Windows, if you double-click a file ending in ".CSV", it will
automatically be opened by Excel.

A CSV file might look like this:

  This is A1,This is B1,This is C1,
  This is A2,This is B2,This is C2,This is D2
  This row contains numbers,51,187,,

Suppose that this spreadsheet holds your old bank account records:

  Deposit,1000,1988-04-25,Payday
  Withdrawal,50,1988-04-28,
  Cheque,150,1988-04-30,Roof repair

You realise that it would make more sense for everything except
deposits to be negative, but the data are used in other calculations,
so you decide to insert a column with the changed data rather than
altering the original data.

Write a Perl program that inserts a new column between the amount and
the date (column C). The new column C should be exactly the same as
column B, except that it should be positive for deposits and negative
for everything else.

For extra credit, sort by date. (Don't do any complex date
conversions: the date is represented conveniently for this!)

          ----------------------------------------

5) Answers to Previous Exercises

a) The following program sorts its input:

  #!/usr/bin/perl -w
  use strict;
  
  my @input = <>;   # or use <STDIN> if you want.
  print sort(@input);

Note that we use "<>" instead of "<STDIN>". This is for convenience:
any file names provided on the command line will automatically be
opened and read.

b) The following program computes a sum:

  #!/usr/bin/perl -w
  use strict;
  
  my $line = <STDIN>;
  my @numbers = split( /\s*\+\s*/, $line );
  my $sum = 0;
  foreach my $n (@numbers) {
    $sum += $n;
  }
  print $sum;

The point of the example was to use "split". But it would make just
as much sense to use "m//g":

  my @numbers = ( $line =~ /(\d+)/g );

In fact, using "m//g" would have the advantage that we could more
easily extend the program to work with subtraction:

  #!/usr/bin/perl -w
  use strict;
  
  my $line = <STDIN>;
  my @numbers = ( $line =~ /(-|\+|\d+)/g );
  my $sum = 0;
  my $sign = '+';
  foreach my $x (@numbers) {
    if ( $x =~ /\d/ ) {
      if ( $sign eq '-' ) {
        $sum -= $x;
      }
      else {
        $sum += $x;
      }
    }
    else {
      $sign = $x;
    }
  }
  print $sum;

          ----------------------------------------

6) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

          ----------------------------------------

7) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.


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Subject: [Courses] [Perl] Part 12: Hashes
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LinuxChix Perl Course Part 12: Hashes

1) Introduction
2) An Example
3) Hashes and Lists
4) Keys and Values
5) Exercises
6) Answer to Previous Exercise
7) Acknowledgements
8) Licensing


          ----------------------------------------

1) Introduction

Recall that there are four basic data types in Perl:
a) scalar - A single chunk of data
b) array - A bunch of scalars, indexed by a number
c) hash (hashtable) - A bunch of scalars, indexed by another scalar
d) handle - A pointer enabling the opening of resources from the
operating system (files, directories, etc.)

We've seen three of these four, so there's only one to go: hashes. I
assume that you've already been introduced to hashtables, but if you
haven't, you should still be able to follow. Don't be afraid to ask
the list for examples of when hashtables are useful.

          ----------------------------------------

2) An Example

The following is an example program.

  #!/usr/bin/perl -w
  use strict;
  
  my %capitals;   # percent sign = hash
  $capitals{'England'} = 'London';
  $capitals{China}    = 'Beijing';   # No quotes around China. OK.
  
  print "Enter a country. ";
  chomp(my $country = <STDIN>);
  
  if ( defined( $capitals{$country} ) ) {
    print "The capital of $country is $capitals{$country}\n";
  }
  else {
    print "I don't know that country.\n";
  }

          ----------------------------------------

3) Hashes and Lists

If a hash is assigned to or from a list, the items alternate key1,
value1, key2, value2...

  my %h = ('a', 1, 'b', 2, 'c', 3);
  print $h{b};    # Output is: 2

To make the association more apparent, we can use the "=>" operator,
also called "fat comma":

  my %h = ('a' => 1, 'b' => 2, 'c' => 3);
  print $h{b};    # Output is: 2

However, apart from the key-value pairing, a hash has no order:

  my %h = ('a' => 1, 'b' => 2, 'c' => 3);
  my @a = %h;    # No problem, but order is lost.
  print $a[0];   # It will be one of a,b,c - but we don't know which!

          ----------------------------------------

4) Keys and Values

The "keys" function returns the list of keys in a hashtable:

  foreach my $k ( keys(%hash) ) {
    print "$k : $hash{$k}\n";
  }

The less used "values" function returns the list of values associated
with the keys.

          ----------------------------------------

5) Exercises

a) Write a program that reads /etc/passwd and creates a mapping of
usernames to UIDs. Test by inviting the user to enter a username and
outputting the UID.

b) Write a Perl program that reads standard input and removes
duplicate lines (so if you enter "foo", then "bar", then "foo" again,
the second "foo" is not output). Remember: There Is More Than One Way
To Do It (TIMTOWTDI).

c) The "reverse" function reverses the order of an array:

  @arr = reverse(@arr);

What happens if you use "reverse" on a hash?

d) In our examples we haven't used numbers as hash keys, even though
Perl allows it. If you are indexing by number, you would normally use
an array rather than a hash. However, there are cases when a hash
with integer keys is more appropriate than an array. Can you name
such a case?

e) The original BASIC interpreter might be considered a primitive
IDE, because it was both an editor and an interpreter. You might use
it something like this (with "user" and "comp" added for clarity):

  user> 10 PRINT "What is your name?";
  comp> Ok
  
  user> 20 INPUT NAME$
  comp> Ok
  
  user> 30 PRINT "Hello "; NAME$
  comp> Ok
  
  user> LIST
  comp> 10 PRINT "What is your name?"
  comp> 20 INPUT NAME$
  comp> 30 PRINT "Hello "; NAME$
  
  user> RUN
  comp> What is your name?
  user> Patricia
  comp> Hello Patricia
  comp> Ok
  
  user> SAVE "hello.bas"
  comp> Ok

User commands beginning with a number mean lines in the program.
Programs are always executed in order by line number, regardless of
the order in which the lines were entered. Each line entered
overwrites any line with the same number that may already exist. This
was one way of editing programs before we had visual editors.

Write your own subset of the BASIC interpreter. The choice of
commands is up to you, but I recommend PRINT, INPUT, LIST and (of
course) RUN.

Hint 1: There are several good uses for hashes here, not the least of
which is mapping line numbers to lines.

Hint 2: Though you can require the user to enter uppercase if you
want, a better solution is to use the "uc" function and/or "m//i".

Hint 3: Remember that to sort a list in numerical order, you use
"sort( {$a<=>$b} @array )"

          ----------------------------------------

6) Answer to Previous Exercise

a) This program processes a CSV file, including sorting rows by date.

  #/usr/bin/perl -w
  use strict;
  
  open MY_INPUT,  "< input.csv" or die "Couldn't open input file:
$!";
  my @lines = <MY_INPUT>;        # Read whole file.
  close MY_INPUT;
  
  foreach my $line (@lines) {
    # Split line into fields.
    my @fields = split /,/, $line;
  
    # Decide whether amount should be negative.
    my $amount = $fields[1];
    $amount = -$amount if $fields[0] !~ /deposit/i;
  
    # Reconstruct fields, with date at the beginning
    # (for sorting) and new amount inserted in the middle.
    @fields = ( $fields[2], @fields[0,1], $amount,
                @fields[2..$#fields] );
    $line = join(',', @fields);
  }
  
  # Sort by date (which is currently at the beginning).
  @lines = sort @lines;
  
  # Fast way of removing date from the beginning.
  s/^.*?,// for (@lines);
  
  open MY_OUTPUT, "> output.csv" or die "Couldn't open output file:
$!";
  print MY_OUTPUT @lines;        # Write whole file (with newlines).
  close MY_OUTPUT;

          ----------------------------------------

7) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

          ----------------------------------------

8) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.


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Subject: [Courses] [Perl] Part 13: Functions
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LinuxChix Perl Course Part 13: Functions

1) Introduction
2) A Simple Example
3) A More Complicated Example
4) Exercises
5) Answers to Previous Exercises
6) Acknowledgements
7) Licensing


          ----------------------------------------

1) Introduction

As we write larger and larger programs, the need for functions
becomes apparent. This week we're (finally) going to see how to write
functions in Perl.

          ----------------------------------------

2) A Simple Example

We're going to start with a simple example that returns whatever is
passed in to it.

  sub identity_function {
    return @_;     # @_ is array of parameters.
  }
  
  my @result = identity_function('foo', 'bar', 'baz');
  print "@result\n";

Notice that the function accepts any number of parameters (in the
form of an array, namely "@_") and returns any number of parameters
(also in the form of an array). Try calling it like this:

  my @arr = ('b', 'c', 'd');
  my @result = identity_function('a', @arr, 'e');
  print "@result\n";    # Output is: a b c d e

As you can see, the array is expanded when passed in. There is no way
to pass in or return two distinct arrays; they are always merged.
However, when we learn about references, we'll see that it's possible
to pass in or return references to two distinct arrays.

Perl provides wonderful flexibility in allowing any number of
parameters to be passed in or returned. USE IT! For example, if you
write a "max" function to return the largest number, don't limit the
user to two: accept any number of parameters.

          ----------------------------------------

3) A More Complicated Example

Now let's try an example that's a little more interesting. In Perl
there is no command to perform an s/// operation on an array, so
here's a function to do just that.

  #!/usr/bin/perl -w
  use strict;
  
  sub array_s {
    my $find = shift;          # Get first parameter.
    my $replacement = shift;   # Get second parameter.
  
    foreach ( @_ ) {           # Now do the work.
      s/$find/$replacement/g;  # s///g = Global replacement.
    }
    
    # No return statement: change was made to input params.
  }
  
  my $x = 'the';
  my $y = 'good';
  my $z = 'samaritan';
  array_s('[aeiou]', '_', $x, $y, $z);
  
  print "$x $y $z\n";

A few things to note:
a) "shift" is applied to "@_" if you provide it with no arguments.
This has an effect similar to the "shift" statement in shell scripts.
b) A change to "@_" also changes one of the variables passed in. (In
other words, non-literals are passed by reference.)

I don't actually recommend changing "@_"; it would probably be better
to return the result instead. But Perl gives you lots of options, and
one of those options is changing what's passed in.

By the way, in Perl 4 we would have called the function with a
preceeding ampersand, like this:

  &array_s('[aeiou]', '_', $x, $y, $z);

This is no longer necessary in Perl 5, but you may see it in legacy
code, and the ampersand is associated with functions in other ways
which we'll see later.

          ----------------------------------------

4) Exercises

a) The "reverse" function, when used in scalar context, reverses the
order of the characters in a string. It also acts on "$_", but we
still can't do this:

  @arr = map(reverse, @arr);   # Doesn't work.

Write a "strictly_scalar_reverse" function that returns all the
elements of an array written backwards. So
"strictly_scalar_reverse('foo','bar');" will return "'oof','rab'".

Bonus credit to anyone who can determine why using "map" doesn't
work, and double bonus if you find a work-around.

b) Consider the following code:

  sub foo($$$) {
    print "You called foo with arguments @_\n";
  }

What happens if you call "foo" with exactly three scalar parameters?
What happens if you call it with more or fewer? What happens if you
call it with an array of exactly three elements?  

c) Consider the following code:

  sub bar {
    if ( wantarray ) {
      return ('a', 'lot', 'of', 'scalars');
    }
    else {
      return 'one';
    }
  }
  
  my @x = bar;
  my $y = bar;
  print "@x\n";
  print "$y\n";

What does the "wantarray" function do?

          ----------------------------------------

5) Answers to Previous Exercises

a) The following program uses /etc/passwd to map usernames to UIDs.
Note that it takes advantage of "$_", which will no doubt delight
some and disgust others.

  #!/usr/bin/perl -w
  use strict;
  
  my %map;
  
  open PASSWD, "< /etc/passwd" or die "Couldn't open file.";
  while ( <PASSWD> ) {
    if ( /^(.*?):.*?:(\d+):/ ) {
      $map{$1} = $2;
    }
    else {
      die "Encountered weird data in password file.";
    }
  }
  close PASSWD;
  
  print "Enter a username and I'll give you a UID.\n";
  
  while ( my $username = <STDIN> ) {
    chomp($username);
    if ( defined($map{$username}) ) {
      print "User $username has UID $map{$username}\n";
    }
    else {
      print "Sorry: I don't know that user.\n";
    }
  }

b) The right way to write a program that removes duplicate lines is
to store previous lines as keys in a hash:

  #/usr/bin/perl -w
  use strict;
  
  my %previous;
  while ( my $line = <STDIN> ) {
    if ( ! defined( $previous{$line} ) ) {
      $previous{$line} = 1;    # 1 is a dummy value.
      print $line;             # No newline: wasn't chomped.
    }
  }

Of course, There Is More Than One Way To Do It (TIMTOWTDI). A less
elegant way is to store previous lines in an array and "grep" for
them. This is much less efficient because it doesn't take advantage
of the fast lookup abilities of a hashtable. But here's an example of
how to do it.

  #/usr/bin/perl -w
  use strict;
  
  #
  # A much less efficient way to do the same thing.
  #
  
  my @previous;
  while ( my $line = <STDIN> ) {
    if ( ! grep {$_ eq $line} @previous ) {
      push(@previous, $line);
      print $line;         # No need for \n : wasn't chomped.
    }
  }

c) Calling "reverse" on a hash causes keys to become values and
values to become keys.

d) A hash is more appropriate than an array for indexing by number if
the indeces are sparsely populated. For example, the first exercise
was to map usernames to UIDs, but mapping UIDs to usernames would
also be an appropriate use of a hash, because a system with only a
few users will usually have UIDs over 500 (sometimes over 1000). You
could store these in an array, but a lot of space (memory) would be
wasted, so a hash is probably a better choice. The following exercise
uses a hash to map line numbers to commands in BASIC for the same
reason.

e) Here's the BASIC interpreter I came up with. It supports the
PRINT, INPUT, LIST, RUN and SAVE commands, though it's very light on
error checking.

  #!/usr/bin/perl -w
  use strict;
  
  my %code;    # Mapping of lines to commands.
  my %vars;    # Mapping of variables to values.
  
  while ( my $line = <STDIN> ) {
  
    # Convert to uppercase, except stuff in quotes.
    # Adding a space prevents split from dropping last quote.
    my @chunks = split /"/, $line . ' ';
    for (my $i=0; $i<@chunks; $i+=2) {
      $chunks[$i] = uc($chunks[$i]);
    }
    $line = join('"', @chunks);
  
    $line =~ s/^\s*|\s*$//g;    # Remove white space.
  
    # User entered a line of code (starts with number).
    if ( $line =~ /^(\d+)\s*(.*)$/ ) {
      $code{$1} = $2;
      delete $code{$1} if $2 eq '';    # Blank line = delete.
      print "Ok\n";
    }
  
    # User asked for a LIST of his code.
    elsif ( $line =~ /^LIST\s*(\d*)$/ ) {
      my @line_nums = ($1 ne '' ? $1 : keys(%code));
      @line_nums = sort( {$a <=> $b} @line_nums);
      foreach my $num (@line_nums) {
        print "$num $code{$num}\n";
      }
      print "Ok\n";
    }
  
    # User asked to RUN his code.
    elsif ( $line eq 'RUN' ) {
      my @line_nums = sort( {$a <=> $b} keys(%code));
      foreach my $num (@line_nums) {
        my($command,$rest) = split( /\s+/, $code{$num}, 2 );
        #print "Debug: $command|$rest\n";
        if ( $command eq 'INPUT' ) {
          print '? ';
          chomp($vars{$rest} = <STDIN>);
        }
        elsif ( $command eq 'PRINT' ) {
          # A PRINT takes args separated by semicolons.
          my $ends_in_semicolon = ($rest =~ /;\s*$/);
          while ( defined($rest) && $rest ne '' ) {
            if ($rest =~ /^"/) {    # If it's a string.
              (my $string, $rest) = ($rest =~
/^"(.*?)"\s*;?\s*(.*)$/);
              print $string;
            }
            else {
              (my $var, undef, $rest) = ($rest =~
/^(.*?)\s*(;\s*(.*)|$)/);
              print $vars{$var} if defined($vars{$var});
            }
          }
          # A PRINT command ending in a semicolon gets no newline.
          print "\n" unless $ends_in_semicolon;
        }
        else {
          print "Unrecognised command at line $num: $command\n";
        }
      }
    }
  
    # User asked to SAVE the program.
    elsif ( $line =~ /^SAVE\s+"(.*)"$/ ) {
      my $ok = open OUTPUT, "> $1";
      if ( $ok ) {
        foreach my $number (sort keys %code) {
          print OUTPUT "$number $code{$number}\n";
        }
        close OUTPUT;
        print "Ok\n";
      }
      else { warn "Couldn't write $1: $!\n"; }
    }
  
    elsif ( $line eq 'QUIT' ) {
      print "Ok\n";
      exit;
    }
  
    elsif ( $line eq '' ) {
      # Do nothing.
    }
  
    else {
      print "Unrecognised command\n";
    }
  }

As you can see, there's a lot of functionality in a little code. We
might be tempted to think that we could easily add more features and
write a real BASIC interpreter, but it turns out that we're already
approaching the limits of what Perl can do easily. (For example, we
haven't considered considered assignments to variables or
mathematical expressions.) Perl's power comes from manipulating text
without worrying about the details, but in a programming language you
MUST worry about the details. You could write a
parser/lexer/interpreter in Perl, but it turns out that it wouldn't
be any easier than implementing it in any other language.

          ----------------------------------------

6) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

          ----------------------------------------

7) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.


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Subject: [Courses] [Perl] Part 14: References
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LinuxChix Perl Course Part 14: References

1) Introduction
2) Learning About Scalars - Again!
3) A List of Lists
4) An Array of Hashes
5) Where to Find Out More
6) Exercises
7) Answers to Previous Exercises
8) Acknowledgements
9) Licensing


          ----------------------------------------

1) Introduction

The programs that we've been writing are fairly simple, but you may
have already run up against the limitations imposed by flat lists.
For example, we might want to read the password file and convert each
line into a list of fields, but we have no good way to join the lists
into a list of lists representing the whole file.

With references we're going to solve that problem. There are lots of
uses for references, including:
a) creating an array of arrays, or a hashtable of arrays, or a
hashtable of hashtables.
b) passing several distinct arrays to a function, or returning
several distinct arrays.
c) creating a "function pointer", allowing us to chain
tranformations.
d) writing our own function that can take code blocks, allowing us to
extend the language (sort of).

This lesson is rather long and counterintuitive, but try to stick
with it to the end. References aren't as much fun as some other
concepts (like s///) but they're essential to writing non-trivial
programs, and in particular for object-oriented programming in Perl.

          ----------------------------------------

2) Learning About Scalars - Again!

Recall that there are four basic data types in Perl. They are:
a) scalar - A single chunk of data
b) array - A bunch of scalars, indexed by a number
c) hash (hashtable) - A bunch of scalars, indexed by another scalar
d) handle - A pointer enabling the opening of resources from the
operating system (files, directories, etc.)

We have seen all four types. One would think that the simplest type,
scalars, would hold no more secrets from us. But there is one type of
scalar that we haven't seen yet: a reference.

Try running this program:

  my $x = 'foo';
  my $r = \$x;
  ${$r} = 'bar';
  print '$x = ' . "$x\n";         # Output is: bar
  print '${$r} = ' . "${$r}\n";   # Output is: bar

As you can see, "$r" is a pointer to "$x". But in Perl it's called a
reference, and it doesn't have any of the painful properties
associated with pointers in C++. You can't generate a memory leak in
Perl even if you try. In that sense, Perl's references are much more
like Java's pointers than C++'s pointers.

Because a reference is a scalar, it can be stored in an array. Which
is where most of its utility comes from, as we'll see next.

          ----------------------------------------

3) A List of Lists

The following code creates a reference to an array:

  my @a = ('foo', 'bar', 'baz');
  my $r = \@a;
  ${$r}[1] = 'x';    # Replace "bar" with "x".
  $r->[2] = 'y';     # Replace "baz" with "y" (nicer syntax).
  print "@a\n";

Knowing this, we can create an array of arrays.

  my @main_list;
  for (my $i=0; $i<5; $i++) {
    my @sub_list = ('foo', 'bar');
    $main_list[$i] = \@sub_list;
  }
  print "${$main_list[1]}[0]\n";   # Output is "foo"
  print "$main_list[1]->[0]\n";    # Shorter form of above.
  print "$main_list[1][0]\n";      # Shortest form of above.

NOTE: "@sub_list" is declared inside the loop, so each iteration
creates a new variable. If it was declared outside the loop, every
array index would point to the same variable, and that probably isn't
what you want.

The final optimisation we can perform on the syntax is replacing the
"\@sub_list" with a list in square brakets:

  for (my $i=0; $i<5; $i++) {
    $main_list[$i] = ['foo', 'bar'];
  }

The square brackets generate a reference to a list without using an
intermediate temporary variable.

          ----------------------------------------

4) An Array of Hashes

As you might have guessed, we can create a reference to a hash as
follows:

  $r = \%h;

Just as square brackets create a reference to a list, braces create a
reference to a hash. So we can create a list of hashes like this:

  my @list_of_hashes;
  for (my $i=0; $i<5; $i++) {
    $list_of_hashes[$i] = { 'tea'=>'coffee', 'pear'=>'lemon' };
  }
  print "$list_of_hashes[1]{pear}\n";   # Output is "lemon"

          ----------------------------------------

5) Where to Find Out More

Hashes aren't as intuitive as some other aspects of Perl, but we've
gotten through the basics. You'll find more uses for references as
you read other people's code and documentation.

If you want to dig in further, here are some good places:

  perldoc perlreftut
  perldoc perlref
  perldoc perldsc
  perldoc perllol
  perldoc -f ref

          ----------------------------------------

6) Exercises

a) Write a Perl program that reads "/etc/passwd" and creates an array
of arrays of fields such that "$passwd[$i][$j]" represents line "$i",
field "$j". (The first field is the username, the second field is the
password, etc.) How would you search this array for all the users who
use a given shell?

b) Change the previous program so that it uses an array of hashes
instead, with each user account a hash mapping "username" to
username, "uid" to UID, etc. Now how will you implement a search?

c) Consider the following program:

  #!/usr/bin/perl -w
  use strict;
  
  my $s = 'a scalar';
  my $r1 = \$s;
  my $r2 = ['a', 'reference', 'to', 'an', 'array'];
  print ref($r1);
  print ref($r2);
  print ref($s);

what does the "ref" function do? Where do you think it's usually
used?

d) Perl has another mechanism for references, illustrated as follows:

  $foo = 'xyz';
  $bar = 'foo';
  print $$bar;      # output is: xyz

(By the way, Perl won't let you do this if you "use strict".)

This is the only reference mechanism that existed before Perl 5. Why
is Perl 5's mechanism such an improvement?

e) Consider the following code:

  #!/usr/bin/perl -w
  use strict;
  
  sub make_uppercase {
    my @result = @_;
    foreach my $item ( @result ) {
      $item = uc($item);    # uc = upper-case
    }
    return @result;
  }
  
  my $transformation = \&make_uppercase;
  my @transformed = &$transformation('foo', 'bar');
  print "@transformed\n";

What does the "\&" operator do?

f) Consider this code:

  #!/usr/bin/perl -w
  use strict;
  
  sub DEBUG() {
    return 1;
  }
  
  sub do_if_in_debug_mode (&) {
    my $func = shift;
    &$func if DEBUG;
  }
  
  do_if_in_debug_mode {
    print "Just testing.\n";
  };

Now write your own "extention" to the Perl language.

          ----------------------------------------

7) Answers to Previous Exercises

a) The first exercise was to write a function to do what one might
expect this to do:

  @arr = map(reverse, @arr);   # Doesn't work.

and then maybe to explain why the above code doesn't work as
expected.

Here's a function that will do what we want:

  sub scalar_reverse {
    my @result = @_;
    foreach my $element (@result) {
      $element = reverse($element);
    }
    return @result;
  }

As for why the code with "map" doesn't work, the reason is the
difference between scalar context and list context. "reverse" in
scalar context reverses the order of the characters in its input, but
in list context it does something different. "map" always calls a
function in list context. To override this, use the "scalar" keyword:

  my @arr = ('foo', 'bar');
  @arr = map {scalar reverse} @arr;
  print "@arr\n";             # Output is: oof rab

b) In this declaration:

  sub foo($$$)

The part in parentheses is called the prototype. In this case, it
specifies that it is expecting three scalar parameters. Perl will
complain if you provide more or fewer, or even if you provide an
array with exactly three elements.

Incidentally, some people prefer to write a "normal" (unlimited
arguments) Perl function like this:

  sub foo(@)

c) The "wantarray" function returns true if its containing function
was called in list context. It returns false if the function was
called in scalar context, and it returns the undefined value if the
function was called in void context (i.e., the return value is not
used).

          ----------------------------------------

8) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

          ----------------------------------------

9) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.


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Subject: [Courses] [Perl] Part 15
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LinuxChix Perl Course Part 15: Using Modules and OOP

1) Introduction
2) A Simple Module
3) Using Objects
4) Where to go from here
5) Exercises
6) Answers to Previous Exercises
7) Acknowledgements
8) Licensing


          ----------------------------------------

1) Introduction

Even though it seems quite long, this course just scratches the
surface of Perl. Perl really does have a whole universe of
functionalities and tricks, and this course is already drawing to a
close.

But before we end things, we're going to see how to use Perl modules,
which (as it turns out) is closely related to object-oriented
programming in Perl. We're not going to see how to WRITE a module or
class; just how to USE one, because we're not yet at a level where it
would be useful to write a module. (But see the further reading
section if you want to write your own.)

          ----------------------------------------

2) A Simple Module

Here's an example of a Perl program that uses a module:

  #!/usr/bin/perl -w
  use strict;
  use HTML::Entities;
  
  my $line = <STDIN>;      # Read line of text.
  print encode_entities($line);

As you might have noticed, we start by declaring that we use the
"HTML::Entities" module. This handy module encodes and decodes HTML
entities (e.g., replacing "<" with "&lt;". After we declare that we
use the module, we can call the functions that it contains, in this
case "encode_entities".

By the way, we could also have written the function call like this:

  HTML::Entities::encode_entities($line)

As you might have noticed, the double-colon separates namespaces in
Perl (just as it does in C++).

All Perl modules include documentation, so you can read more about
"HTML::Entities" like this:

  perldoc HTML::Entities

          ----------------------------------------

3) Using Objects

Now that we've seen basic module use, we're going to look at a module
that contains a class.

  #!/usr/bin/perl -w
  use strict;
  use LWP::UserAgent;
  
  my $browser = LWP::UserAgent->new();
  
  my $response = $browser->get('http://www.w3.org/');
  
  if ($response->is_success) {
    print $response->content;
  }
  else {
    die $response->status_line;
  }

This program retrieves the W3C's home page. Let's go through it
line-by-line.

The first thing we do is declare that we're going to use the
"LWP::UserAgent" module. This module contains a user-agent (often
called a web browser) which retrieves web pages.

Next we create a new object with "LWP::UserAgent->new()". A few
things to note here:
a) The class name is "LWP::UserAgent", just like the package name.
This is a byproduct of the way OOP is built into Perl.
b) The object is created with "new". This is NOT a requirement (the
constructor could be called anything), but it is common.
c) The object is in fact a reference. This means that if you write
"$object1=$object2", both variables will refer to the same object.

When we call "$browser->get()", the user-agent contacts the web
server and returns its response. The response is actually a
"HTTP::Response", but notice that we don't have to import that module
or even use its name.

Finally, we check whether the request was successful and output the
result. Note that "$response->is_success", "$response->content" and
"$response->status_line" are method (function) calls just like
"$browser->get", even though there are no parentheses.

          ----------------------------------------

4) Where to go from here

Whatever you want to do, there's probably a Perl module that does a
good portion of it. Search the Comprehensive Perl Archive Network
(CPAN: http://www.cpan.org/ ).

Many of the modules on CPAN are actually already installed on your
system, so check before going to the trouble of installing a module.
The easiest way to see if a module is installed is to try to "use"
it, or try to read its documentation (as explained next).

To read the documentation of an (installed) module, use "perldoc":

  perldoc LWP::UserAgent

(You can also use "man"; both work. There Is More Than One Way To Do
It!)

To see all the modules installed on your system, you might want to
see the fourth item under:

  perldoc perlfaq3

However, that doesn't work for me :-/ so I do this:

  locate '*.pm'

One last thing: while you could download and install modules from
CPAN, the most popular Perl modules are probably available via your
Linux distro's package management system (such as "rpm" or "apt"). Do
a quick search there first.

          ----------------------------------------

5) Exercises

a) Consider this program:

  use strict;
  use LWP::UserAgent;
  
  my $ua = LWP::UserAgent->new();
  print ref($ua) . "\n";

What does "ref" return when passed an object?

b) Consider this code:

  #!/usr/bin/perl -w
  use strict;
  use English;
  
  $INPUT_RECORD_SEPARATOR = ':';
  print $/;

What does "use English" accomplish?

For more information, you can consult:

  perldoc perlvar

          ----------------------------------------

6) Answers to Previous Exercises

a) This Perl program stores the password file in a list of lists:

  #!/usr/bin/perl -w
  use strict;
  
  my @list;
  
  open PASSWD, "< /etc/passwd" or die "Couldn't open file.";
  while ( <PASSWD> ) {
    push @list, [ split /:/ ];
  }
  close PASSWD;

Here is one way to search such a list:

  print "Enter a username and I'll give you a UID.\n";
  
  while ( defined( my $username = <STDIN> ) ) {
    chomp($username);
    my @matches = grep { $_->[0] eq $username } @list;
    if ( @matches ) {
      print "User $username has UID $matches[0][2]\n";
    }
    else {
      print "Sorry: I don't know that user.\n";
    }
  }

b) Here's a way of storing the password file as an array of hashes:

  #!/usr/bin/perl -w
  use strict;
  
  my @list;
  
  open PASSWD, "< /etc/passwd" or die "Couldn't open file.";
  while ( <PASSWD> ) {
    my @fields = split /:/;
    push @list, { username=>$fields[0], password=>$fields[1],
                  uid=>$fields[2],      gid=>$fields[3],
                  name=>$fields[4],     home=>$fields[5],
                  shell=>$fields[6] };
  }
  close PASSWD;

It can be searched like this:

  print "Enter a username and I'll give you a UID.\n";
  
  while ( defined( my $username = <STDIN> ) ) {
    chomp($username);
    my @matches = grep { $_->{username} eq $username } @list;
    if ( @matches ) {
      print "User $username has UID $matches[0]{uid}\n";
    }
    else {
      print "Sorry: I don't know that user.\n";
    }
  }

c) The "ref" function allows you to determine whether a scalar is a
reference, and even the data type to which it refers.

d) Perl 4 included a mechanism called "symbolic" references, which
allowed one variable to access another using its name. This method is
not recommended because it can make your life even more miserable
than normal pointers can.

e) The "\&" operator creates a reference to a function.

f) Larry Wall provides us with some very interesting syntax:

  sub try (&@) {
      my($try,$catch) = @_;
      eval { &$try };
      if ($@) {
          local $_ = $@;
          &$catch;
      }
  }
  sub catch (&) { $_[0] }
  
  try {
      die "phooey";
  } catch {
      /phooey/ and print "unphooey\n";
  };

Briefly, this example takes advantage of Perl's "$@" variable, which
stores the latest "exception" (a "die" inside an "eval").

          ----------------------------------------

7) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

          ----------------------------------------

8) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.


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Subject: [Courses] [Perl] Part 16: The End
From: Dan <dan@cellectivity.com>
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LinuxChix Perl Course Part 16: The End

1) Parting Words
2) RTFM
3) Use Strict and Warnings
4) Maintainability
5) Platform Independent Shell Scripts
6) TIMTOWDTI
7) Don't Use Perl for Everything
8) Answers to Previous Exercises
9) Acknowledgements
10) Licensing


          ----------------------------------------

1) Parting Words

Congratulations: you have finished the Perl course. You are now ready
to write your own small Perl scripts or, better yet, dig into someone
else's Perl code to learn more.

In this "epilog" I give my final words of advice.

          ----------------------------------------

2) RTFM

Perl truly has almost endless tools and tricks, and even the most
seemingly banal parts of the language have surprises. Browse through
the avalanche of documentation a little at a time. The complete list
is here:

  [you@localhost]$ man perl

As Larry Wall said in an interview
<http://www.linuxjournal.com/article/3394>, "I've certainly tried to
put a universe of ideas into Perl, with some amount of success."

Don't reinvent the wheel! RTFM, and then search CPAN.

          ----------------------------------------

3) Use Strict and Warnings

I said this at the beginning of the course, and I'm repeating it now:
begin all of your Perl programs like this:

  #!/usr/bin/perl -w
  use strict;

Those two lines will save you hours of debugging.

          ----------------------------------------

4) Maintainability

The standard rules on making maintainable code apply all the more to
Perl, because Perl doesn't force you to write readable code. In
particular, think about how you'll scratch your head a year from now
wondering what that big regular expression means. You'd better put in
lots of comments!

          ----------------------------------------

5) Platform Independent Shell Scripts

Consider using Perl as a platform independent shell scripting
language. Most shell scripts depend on a host of other programs (like
"grep" and "sed") whose exact implementations vary from one machine
to another - not to mention the fact that they're not available on
Windows. Perl is platform independent, with certain obvious
exceptions such as the "chmod" function.

Perl even allows you to think in shell script mode, as this mini Perl
script demonstrates:

  if ( -d $dir ) {
    chdir $dir;
  }
  unlink "some_unwanted_file";

          ----------------------------------------

6) TIMTOWDTI

Perl hackers insist that There Is More Than One Way To Do It
(TIMTOWDTI, pronounced Tim-Towdty). Look at all the possibilities.

But also remember to use the best tool for every job. For example,
don't do heavy XML processing with m// and s///. Instead, use Perl's
own XML libraries, which are introduced at
http://www.xml.com/pub/a/2000/04/05/feature/

          ----------------------------------------

7) Don't Use Perl for Everything

Finally, extend TIMTOWDTI by considering possibilities beyond Perl.
Perl is the best tool for a lot of things, but it's not the best tool
for everything. If you don't need Perl's text processing power, then
consider other languages like Python and Ruby. If you're mostly
manipulating XML, consider using XSLT with a small Perl front-end.

          ----------------------------------------

8) Answers to Previous Exercises

a) The "ref" function, when called on an object, returns the object's
class.

In Perl a class is identified by the package (namespace) in which the
it is defined. Every module must also have its own package, so a
class is package and a module is a package. To find out more about
packages:

  perldoc -f package

b) The "English" module provides more intuitive names for weird Perl
variables like "$(" (group ID) or "$|" (automatic output flushing).
For more information on the "English" module:

  perldoc English

For more information on those funny Perl variables:

  perldoc perlvar

          ----------------------------------------

9) Acknowledgements

A big thank you to Jacinta Richardson for suggestions and
proofreading. More advanced Perl users might want to check out the
free material from Perl Training Australia
<http://www.perltraining.com.au/>, which she is a part of.

Other contributors include Meryll Larkin.

          ----------------------------------------

10) Licensing

This course (i.e., all parts of it) is copyright 2003-2005 by Dan
Richter and Alice Wood, and is released under the same license as
Perl itself (Artistic License or GPL, your choice). This is the
license of choice to make it easy for other people to integrate your
Perl code/documentation into their own projects. It is not generally
used in projects unrelated to Perl.


From basic_meo@hotmail.com  Fri Dec 17 23:22:58 2004
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Hi every one,
do anybody have an idea how to control parralell port with perl command?

the basic thing I want to do is send a command to device in parallel por and 
be able to read the respond...



From lawgon@thenilgiris.com  Fri Dec 17 23:46:49 2004
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On Friday 17 December 2004 05:52 pm, nameBasic One wrote:
> Hi every one,
> do anybody have an idea how to control parralell port with perl command=
?
>
> the basic thing I want to do is send a command to device in parallel po=
r
> and be able to read the respond...

try this:
http://search.cpan.org/~scott/Device-ParallelPort-1.00/lib/Device/Paralle=
lPort.pm

regards
kg

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  wrote:
> 
> Hi every one,
> do anybody have an idea how to control parralell port with perl command?
> 
> the basic thing I want to do is send a command to device in parallel por 
> and be able to read the respond...
> 

G'day nameBasic One,

I personally can't help you, and in fact would like to point out that 
this is not the appropriate Linuxchix list for this kind of question. 
There are a number of others that would most likely be able to help you.

You could repost the question on any of the following lists and I think 
you ar likely to get some help on any of them.  They are

Newchix for people who are new to linux
Techtalk for technical questions
Programming for programming specific discussions

Are you a memeber of any of these lists?  If not, I would highly 
recomend them.  I suggest you repost your question to your pick of these 
lists, where I think that you will get some help.  (I think my 
prefeerenc would be programming since it concerns perl commands).

sue