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}{x}
  80 ``Byteswap'' all items of the array.  This is only supported for
  81 integer values; for other types of values, \exception{RuntimeError} is
  82 raised.  It is useful when reading data from a file written on a
  83 machine with a different byte order.
  84 \end{methoddesc}
  85
  86 \begin{methoddesc}[array]{fromfile}{f, n}
  87 Read \var{n} items (as machine values) from the file object \var{f}
  88 and append them to the end of the array.  If less than \var{n} items
  89 are available, \exception{EOFError} is raised, but the items that were
  90 available are still inserted into the array.  \var{f} must be a real
  91 built-in file object; something else with a \method{read()} method won't
  92 do.
  93 \end{methoddesc}
  94
  95 \begin{methoddesc}[array]{fromlist}{list}
  96 Append items from the list.  This is equivalent to
  97 \samp{for x in \var{list}:\ a.append(x)}
  98 except that if there is a type error, the array is unchanged.
  99 \end{methoddesc}
 100
 101 \begin{methoddesc}[array]{fromstring}{s}
 102 Appends items from the string, interpreting the string as an
 103 array of machine values (i.e. as if it had been read from a
 104 file using the \method{fromfile()} method).
 105 \end{methoddesc}
 106
 107 \begin{methoddesc}[array]{insert}{i, x}
 108 Insert a new item with value \var{x} in the array before position
 109 \var{i}.
 110 \end{methoddesc}
 111
 112 \begin{methoddesc}[array]{read}{f, n}
 113 \deprecated {1.5.1}
 114   {Use the \method{fromfile()} method.}
 115 Read \var{n} items (as machine values) from the file object \var{f}
 116 and append them to the end of the array.  If less than \var{n} items
 117 are available, \exception{EOFError} is raised, but the items that were
 118 available are still inserted into the array.  \var{f} must be a real
 119 built-in file object; something else with a \method{read()} method won't
 120 do.
 121 \end{methoddesc}
 122
 123 \begin{methoddesc}[array]{reverse}{}
 124 Reverse the order of the items in the array.
 125 \end{methoddesc}
 126
 127 \begin{methoddesc}[array]{tofile}{f}
 128 Write all items (as machine values) to the file object \var{f}.
 129 \end{methoddesc}
 130
 131 \begin{methoddesc}[array]{tolist}{}
 132 Convert the array to an ordinary list with the same items.
 133 \end{methoddesc}
 134
 135 \begin{methoddesc}[array]{tostring}{}
 136 Convert the array to an array of machine values and return the
 137 string representation (the same sequence of bytes that would
 138 be written to a file by the \method{tofile()} method.)
 139 \end{methoddesc}
 140
 141 \begin{methoddesc}[array]{write}{f}
 142 \deprecated {1.5.1}
 143   {Use the \method{tofile()} method.}
 144 Write all items (as machine values) to the file object \var{f}.
 145 \end{methoddesc}
 146
 147 When an array object is printed or converted to a string, it is
 148 represented as \code{array(\var{typecode}, \var{initializer})}.  The
 149 \var{initializer} is omitted if the array is empty, otherwise it is a
 150 string if the \var{typecode} is \code{'c'}, otherwise it is a list of
 151 numbers.  The string is guaranteed to be able to be converted back to
 152 an array with the same type and value using reverse quotes
 153 (\code{``}).  Examples:
 154
 155 \begin{verbatim}
 156 array('l')
 157 array('c', 'hello world')
 158 array('l', [1, 2, 3, 4, 5])
 159 array('d', [1.0, 2.0, 3.14])
 160 \end{verbatim}
 161
 162
 163 \begin{seealso}
 164   \seemodule{struct}{packing and unpacking of heterogeneous binary data}
 165   \seemodule{xdrlib}{packing and unpacking of XDR data}
 166 \end{seealso}