Doc/lib/libarray.tex

   1 \section{\module{array} ---
   2          Efficient arrays of numeric values}
   3
   4 \declaremodule{builtin}{array}
   5 \modulesynopsis{Efficient arrays of uniformly typed numeric values.}
   6
   7
   8 This module defines a new object type which can efficiently represent
   9 an array of basic values: characters, integers, floating point
  10 numbers.  Arrays\index{arrays} are sequence types and behave very much
  11 like lists, except that the type of objects stored in them is
  12 constrained.  The type is specified at object creation time by using a
  13 \dfn{type code}, which is a single character.  The following type
  14 codes are defined:
  15
  16 \begin{tableiii}{c|l|c}{code}{Type code}{C Type}{Minimum size in bytes}
  17 \lineiii{'c'}{character}{1}
  18 \lineiii{'b'}{signed int}{1}
  19 \lineiii{'B'}{unsigned int}{1}
  20 \lineiii{'h'}{signed int}{2}
  21 \lineiii{'H'}{unsigned int}{2}
  22 \lineiii{'i'}{signed int}{2}
  23 \lineiii{'I'}{unsigned int}{2}
  24 \lineiii{'l'}{signed int}{4}
  25 \lineiii{'L'}{unsigned int}{4}
  26 \lineiii{'f'}{float}{4}
  27 \lineiii{'d'}{double}{8}
  28 \end{tableiii}
  29
  30 The actual representation of values is determined by the machine
  31 architecture (strictly speaking, by the C implementation).  The actual
  32 size can be accessed through the \member{itemsize} attribute.  The values
  33 stored  for \code{'L'} and \code{'I'} items will be represented as
  34 Python long integers when retrieved, because Python's plain integer
  35 type cannot represent the full range of C's unsigned (long) integers.
  36
  37
  38 The module defines the following function and type object:
  39
  40 \begin{funcdesc}{array}{typecode\optional{, initializer}}
  41 Return a new array whose items are restricted by \var{typecode}, and
  42 initialized from the optional \var{initializer} value, which must be a
  43 list or a string.  The list or string is passed to the new array's
  44 \method{fromlist()} or \method{fromstring()} method (see below) to add
  45 initial items to the array.
  46 \end{funcdesc}
  47
  48 \begin{datadesc}{ArrayType}
  49 Type object corresponding to the objects returned by
  50 \function{array()}.
  51 \end{datadesc}
  52
  53
  54 Array objects support the following data items and methods:
  55
  56 \begin{memberdesc}[array]{typecode}
  57 The typecode character used to create the array.
  58 \end{memberdesc}
  59
  60 \begin{memberdesc}[array]{itemsize}
  61 The length in bytes of one array item in the internal representation.
  62 \end{memberdesc}
  63
  64
  65 \begin{methoddesc}[array]{append}{x}
  66 Append a new item with value \var{x} to the end of the array.
  67 \end{methoddesc}
  68
  69 \begin{methoddesc}[array]{buffer_info}{}
  70 Return a tuple \code{(\var{address}, \var{length})} giving the current
  71 memory address and the length in bytes of the buffer used to hold
  72 array's contents.  This is occasionally useful when working with
  73 low-level (and inherently unsafe) I/O interfaces that require memory
  74 addresses, such as certain \cfunction{ioctl()} operations.  The returned
  75 numbers are valid as long as the array exists and no length-changing
  76 operations are applied to it.
  77 \end{methoddesc}
  78
  79 \begin{methoddesc}[array]{byteswap}{}
  80 ``Byteswap'' all items of the array.  This is only supported for
  81 values which are 1, 2, 4, or 8 bytes in size; for other types of
  82 values, \exception{RuntimeError} is raised.  It is useful when reading
  83 data from a file written on a machine with a different byte order.
  84 \end{methoddesc}
  85
  86 \begin{methoddesc}[array]{count}{x}
  87 Return the number of occurences of \var{x} in the array.
  88 \end{methoddesc}
  89
  90 \begin{methoddesc}[array]{extend}{a}
  91 Append array items from \var{a} to the end of the array.
  92 \end{methoddesc}
  93
  94 \begin{methoddesc}[array]{fromfile}{f, n}
  95 Read \var{n} items (as machine values) from the file object \var{f}
  96 and append them to the end of the array.  If less than \var{n} items
  97 are available, \exception{EOFError} is raised, but the items that were
  98 available are still inserted into the array.  \var{f} must be a real
  99 built-in file object; something else with a \method{read()} method won't
 100 do.
 101 \end{methoddesc}
 102
 103 \begin{methoddesc}[array]{fromlist}{list}
 104 Append items from the list.  This is equivalent to
 105 \samp{for x in \var{list}:\ a.append(x)}
 106 except that if there is a type error, the array is unchanged.
 107 \end{methoddesc}
 108
 109 \begin{methoddesc}[array]{fromstring}{s}
 110 Appends items from the string, interpreting the string as an
 111 array of machine values (i.e. as if it had been read from a
 112 file using the \method{fromfile()} method).
 113 \end{methoddesc}
 114
 115 \begin{methoddesc}[array]{index}{x}
 116 Return the smallest \var{i} such that \var{i} is the index of
 117 the first occurence of \var{x} in the array.
 118 \end{methoddesc}
 119
 120 \begin{methoddesc}[array]{insert}{i, x}
 121 Insert a new item with value \var{x} in the array before position
 122 \var{i}.
 123 \end{methoddesc}
 124
 125 \begin{methoddesc}[array]{pop}{\optional{i}}
 126 Removes the item with the index \var{i} from the array and returns
 127 it. The optional argument defaults to \code{-1}, so that by default
 128 the last item is removed and returned.
 129 \end{methoddesc}
 130
 131 \begin{methoddesc}[array]{read}{f, n}
 132 \deprecated {1.5.1}
 133   {Use the \method{fromfile()} method.}
 134 Read \var{n} items (as machine values) from the file object \var{f}
 135 and append them to the end of the array.  If less than \var{n} items
 136 are available, \exception{EOFError} is raised, but the items that were
 137 available are still inserted into the array.  \var{f} must be a real
 138 built-in file object; something else with a \method{read()} method won't
 139 do.
 140 \end{methoddesc}
 141
 142 \begin{methoddesc}[array]{remove}{x}
 143 Remove the first occurence of \var{x} from the array.
 144 \end{methoddesc}
 145
 146 \begin{methoddesc}[array]{reverse}{}
 147 Reverse the order of the items in the array.
 148 \end{methoddesc}
 149
 150 \begin{methoddesc}[array]{tofile}{f}
 151 Write all items (as machine values) to the file object \var{f}.
 152 \end{methoddesc}
 153
 154 \begin{methoddesc}[array]{tolist}{}
 155 Convert the array to an ordinary list with the same items.
 156 \end{methoddesc}
 157
 158 \begin{methoddesc}[array]{tostring}{}
 159 Convert the array to an array of machine values and return the
 160 string representation (the same sequence of bytes that would
 161 be written to a file by the \method{tofile()} method.)
 162 \end{methoddesc}
 163
 164 \begin{methoddesc}[array]{write}{f}
 165 \deprecated {1.5.1}
 166   {Use the \method{tofile()} method.}
 167 Write all items (as machine values) to the file object \var{f}.
 168 \end{methoddesc}
 169
 170 When an array object is printed or converted to a string, it is
 171 represented as \code{array(\var{typecode}, \var{initializer})}.  The
 172 \var{initializer} is omitted if the array is empty, otherwise it is a
 173 string if the \var{typecode} is \code{'c'}, otherwise it is a list of
 174 numbers.  The string is guaranteed to be able to be converted back to
 175 an array with the same type and value using reverse quotes
 176 (\code{``}), so long as the \function{array()} function has been
 177 imported using \samp{from array import array}.  Examples:
 178
 179 \begin{verbatim}
 180 array('l')
 181 array('c', 'hello world')
 182 array('l', [1, 2, 3, 4, 5])
 183 array('d', [1.0, 2.0, 3.14])
 184 \end{verbatim}
 185
 186
 187 \begin{seealso}
 188   \seemodule{struct}{packing and unpacking of heterogeneous binary data}
 189   \seemodule{xdrlib}{packing and unpacking of XDR data}
 190   \seetext{The Numeric Python extension (NumPy) defines another array
 191            type; see \emph{The Numerical Python Manual} for additional
 192            information (available online at
 193            \url{ftp://ftp-icf.llnl.gov/pub/python/numericalpython.pdf}).
 194            Further information about NumPy is available at
 195            \url{http://www.python.org/topics/scicomp/numpy.html}.}
 196 \end{seealso}