lang/C/the.ansi.c.programming.language/c.programming.notes/sx6d.html

   1 <!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
   2 <!-- This collection of hypertext pages is Copyright 1995-7 by Steve Summit. -->
   3 <!-- This material may be freely redistributed and used -->
   4 <!-- but may not be republished or sold without permission. -->
   5 <html>
   6 <head>
   7 <link rev="owner" href="mailto:scs@eskimo.com">
   8 <link rev="made" href="mailto:scs@eskimo.com">
   9 <title>6.4 Reading Numbers</title>
  10 <link href="sx6c.html" rev=precedes>
  11 <link href="sx7.html" rel=precedes>
  12 <link href="sx6.html" rev=subdocument>
  13 </head>
  14 <body>
  15 <H2>6.4 Reading Numbers</H2>
  16
  17 <p>The <TT>getline</TT> function of the previous section
  18 reads one line from
  19 the user,
  20 as a <em>string</em>.
  21 What if we want to read a number?
  22 One straightforward way is to read a string as before,
  23 and then immediately convert the string to a number.
  24 The standard C library contains a number of functions for doing this.
  25 The simplest to use are
  26 <TT>atoi()</TT>, which converts a string to an integer, and
  27 <TT>atof()</TT>, which converts a string to a floating-point number.
  28 (Both of these functions are declared in the header <TT>&lt;stdlib.h&gt;</TT>,
  29 so you should <TT>#include</TT> that header
  30 at the top of any file using these functions.)
  31 You could read an integer from the user like this:
  32 <pre>
  33         #include &lt;stdlib.h&gt;
  34
  35         char line[256];
  36         int n;
  37         printf("Type an integer:\n");
  38         getline(line, 256);
  39         n = atoi(line);
  40 </pre>
  41 Now the variable <TT>n</TT> contains the number typed by the user.
  42 (This assumes that the user <em>did</em> type a valid number,
  43 and that <TT>getline</TT> did not return <TT>EOF</TT>.)
  44 </p><p>Reading a floating-point number is similar:
  45 <pre>
  46         #include &lt;stdlib.h&gt;
  47
  48         char line[256];
  49         double x;
  50         printf("Type a floating-point number:\n");
  51         getline(line, 256);
  52         x = atof(line);
  53 </pre>
  54 (<TT>atof</TT> is actually declared as returning type <TT>double</TT>,
  55 but you could also use it with a variable of type <TT>float</TT>,
  56 because
  57 in general,
  58 C automatically converts between <TT>float</TT> and
  59 <TT>double</TT> as needed.)
  60 </p><p>Another way of reading in numbers,
  61 which you're likely to see in other books on C,
  62 involves the <TT>scanf</TT> function,
  63 but it has
  64 several problems,
  65 so we won't discuss it for now.
  66 (Superficially, <TT>scanf</TT> seems simple enough,
  67 which is why it's often used,
  68 especially in textbooks.
  69 The trouble is that to perform input reliably using <TT>scanf</TT>
  70 is not nearly as easy as it looks,
  71 especially when you're not sure what the user is going to type.)
  72 </p><hr>
  73 <p>
  74 Read sequentially:
  75 <a href="sx6c.html" rev=precedes>prev</a>
  76 <a href="sx7.html" rel=precedes>next</a>
  77 <a href="sx6.html" rev=subdocument>up</a>
  78 <a href="top.html">top</a>
  79 </p>
  80 <p>
  81 This page by <a href="http://www.eskimo.com/~scs/">Steve Summit</a>
  82 // <a href="copyright.html">Copyright</a> 1995-1997
  83 // <a href="mailto:scs@eskimo.com">mail feedback</a>
  84 </p>
  85 </body>
  86 </html>