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{tableiv}{c|l|l|c}{code}{Type code}{C Type}{Python Type}{Minimum size in bytes}
  17   \lineiv{'c'}{char}          {character}        {1}
  18   \lineiv{'b'}{signed char}   {int}              {1}
  19   \lineiv{'B'}{unsigned char} {int}              {1}
  20   \lineiv{'h'}{signed short}  {int}              {2}
  21   \lineiv{'H'}{unsigned short}{int}              {2}
  22   \lineiv{'i'}{signed int}    {int}              {2}
  23   \lineiv{'I'}{unsigned int}  {long}             {2}
  24   \lineiv{'l'}{signed long}   {int}              {4}
  25   \lineiv{'L'}{unsigned long} {long}             {4}
  26   \lineiv{'f'}{float}         {float}            {4}
  27   \lineiv{'d'}{double}        {float}            {8}
  28 \end{tableiv}
  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 ordinary sequence operations of
  55 indexing, slicing, concatenation, and multiplication.  When using
  56 slice assignment, the assigned value must be an array object with the
  57 same type code; in all other cases, \exception{TypeError} is raised.
  58 Array objects also implement the buffer interface, and may be used
  59 wherever buffer objects are supported.
  60
  61 Array objects support the following data items and methods:
  62
  63 \begin{memberdesc}[array]{typecode}
  64 The typecode character used to create the array.
  65 \end{memberdesc}
  66
  67 \begin{memberdesc}[array]{itemsize}
  68 The length in bytes of one array item in the internal representation.
  69 \end{memberdesc}
  70
  71
  72 \begin{methoddesc}[array]{append}{x}
  73 Append a new item with value \var{x} to the end of the array.
  74 \end{methoddesc}
  75
  76 \begin{methoddesc}[array]{buffer_info}{}
  77 Return a tuple \code{(\var{address}, \var{length})} giving the current
  78 memory address and the length in elements of the buffer used to hold
  79 array's contents.  The size of the memory buffer in bytes can be
  80 computed as \code{\var{array}.buffer_info()[1] *
  81 \var{array}.itemsize}.  This is occasionally useful when working with
  82 low-level (and inherently unsafe) I/O interfaces that require memory
  83 addresses, such as certain \cfunction{ioctl()} operations.  The
  84 returned numbers are valid as long as the array exists and no
  85 length-changing operations are applied to it.
  86
  87 \note{When using array objects from code written in C or
  88 \Cpp{} (the only way to effectively make use of this information), it
  89 makes more sense to use the buffer interface supported by array
  90 objects.  This method is maintained for backward compatibility and
  91 should be avoided in new code.  The buffer interface is documented in
  92 the \citetitle[../api/newTypes.html]{Python/C API Reference Manual}.}
  93 \end{methoddesc}
  94
  95 \begin{methoddesc}[array]{byteswap}{}
  96 ``Byteswap'' all items of the array.  This is only supported for
  97 values which are 1, 2, 4, or 8 bytes in size; for other types of
  98 values, \exception{RuntimeError} is raised.  It is useful when reading
  99 data from a file written on a machine with a different byte order.
 100 \end{methoddesc}
 101
 102 \begin{methoddesc}[array]{count}{x}
 103 Return the number of occurences of \var{x} in the array.
 104 \end{methoddesc}
 105
 106 \begin{methoddesc}[array]{extend}{a}
 107 Append array items from \var{a} to the end of the array.  The two
 108 arrays must have \emph{exactly} the same type code; if not,
 109 \exception{TypeError} will be raised.
 110 \end{methoddesc}
 111
 112 \begin{methoddesc}[array]{fromfile}{f, n}
 113 Read \var{n} items (as machine values) from the file object \var{f}
 114 and append them to the end of the array.  If less than \var{n} items
 115 are available, \exception{EOFError} is raised, but the items that were
 116 available are still inserted into the array.  \var{f} must be a real
 117 built-in file object; something else with a \method{read()} method won't
 118 do.
 119 \end{methoddesc}
 120
 121 \begin{methoddesc}[array]{fromlist}{list}
 122 Append items from the list.  This is equivalent to
 123 \samp{for x in \var{list}:\ a.append(x)}
 124 except that if there is a type error, the array is unchanged.
 125 \end{methoddesc}
 126
 127 \begin{methoddesc}[array]{fromstring}{s}
 128 Appends items from the string, interpreting the string as an
 129 array of machine values (as if it had been read from a
 130 file using the \method{fromfile()} method).
 131 \end{methoddesc}
 132
 133 \begin{methoddesc}[array]{index}{x}
 134 Return the smallest \var{i} such that \var{i} is the index of
 135 the first occurence of \var{x} in the array.
 136 \end{methoddesc}
 137
 138 \begin{methoddesc}[array]{insert}{i, x}
 139 Insert a new item with value \var{x} in the array before position
 140 \var{i}.
 141 \end{methoddesc}
 142
 143 \begin{methoddesc}[array]{pop}{\optional{i}}
 144 Removes the item with the index \var{i} from the array and returns
 145 it. The optional argument defaults to \code{-1}, so that by default
 146 the last item is removed and returned.
 147 \end{methoddesc}
 148
 149 \begin{methoddesc}[array]{read}{f, n}
 150 \deprecated {1.5.1}
 151   {Use the \method{fromfile()} method.}
 152 Read \var{n} items (as machine values) from the file object \var{f}
 153 and append them to the end of the array.  If less than \var{n} items
 154 are available, \exception{EOFError} is raised, but the items that were
 155 available are still inserted into the array.  \var{f} must be a real
 156 built-in file object; something else with a \method{read()} method won't
 157 do.
 158 \end{methoddesc}
 159
 160 \begin{methoddesc}[array]{remove}{x}
 161 Remove the first occurence of \var{x} from the array.
 162 \end{methoddesc}
 163
 164 \begin{methoddesc}[array]{reverse}{}
 165 Reverse the order of the items in the array.
 166 \end{methoddesc}
 167
 168 \begin{methoddesc}[array]{tofile}{f}
 169 Write all items (as machine values) to the file object \var{f}.
 170 \end{methoddesc}
 171
 172 \begin{methoddesc}[array]{tolist}{}
 173 Convert the array to an ordinary list with the same items.
 174 \end{methoddesc}
 175
 176 \begin{methoddesc}[array]{tostring}{}
 177 Convert the array to an array of machine values and return the
 178 string representation (the same sequence of bytes that would
 179 be written to a file by the \method{tofile()} method.)
 180 \end{methoddesc}
 181
 182 \begin{methoddesc}[array]{write}{f}
 183 \deprecated {1.5.1}
 184   {Use the \method{tofile()} method.}
 185 Write all items (as machine values) to the file object \var{f}.
 186 \end{methoddesc}
 187
 188 When an array object is printed or converted to a string, it is
 189 represented as \code{array(\var{typecode}, \var{initializer})}.  The
 190 \var{initializer} is omitted if the array is empty, otherwise it is a
 191 string if the \var{typecode} is \code{'c'}, otherwise it is a list of
 192 numbers.  The string is guaranteed to be able to be converted back to
 193 an array with the same type and value using reverse quotes
 194 (\code{``}), so long as the \function{array()} function has been
 195 imported using \code{from array import array}.  Examples:
 196
 197 \begin{verbatim}
 198 array('l')
 199 array('c', 'hello world')
 200 array('l', [1, 2, 3, 4, 5])
 201 array('d', [1.0, 2.0, 3.14])
 202 \end{verbatim}
 203
 204
 205 \begin{seealso}
 206   \seemodule{struct}{Packing and unpacking of heterogeneous binary data.}
 207   \seemodule{xdrlib}{Packing and unpacking of External Data
 208                      Representation (XDR) data as used in some remote
 209                      procedure call systems.}
 210   \seetitle[http://numpy.sourceforge.net/numdoc/HTML/numdoc.htm]{The
 211            Numerical Python Manual}{The Numeric Python extension
 212            (NumPy) defines another array type; see
 213            \url{http://numpy.sourceforge.net/} for further information
 214            about Numerical Python.  (A PDF version of the NumPy manual
 215            is available at
 216            \url{http://numpy.sourceforge.net/numdoc/numdoc.pdf}.}
 217 \end{seealso}