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 an 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{'u'}{Py_UNICODE}    {Unicode character}{2}
  21   \lineiv{'h'}{signed short}  {int}              {2}
  22   \lineiv{'H'}{unsigned short}{int}              {2}
  23   \lineiv{'i'}{signed int}    {int}              {2}
  24   \lineiv{'I'}{unsigned int}  {long}             {2}
  25   \lineiv{'l'}{signed long}   {int}              {4}
  26   \lineiv{'L'}{unsigned long} {long}             {4}
  27   \lineiv{'f'}{float}         {float}            {4}
  28   \lineiv{'d'}{double}        {float}            {8}
  29 \end{tableiv}
  30
  31 The actual representation of values is determined by the machine
  32 architecture (strictly speaking, by the C implementation).  The actual
  33 size can be accessed through the \member{itemsize} attribute.  The values
  34 stored  for \code{'L'} and \code{'I'} items will be represented as
  35 Python long integers when retrieved, because Python's plain integer
  36 type cannot represent the full range of C's unsigned (long) integers.
  37
  38
  39 The module defines the following type:
  40
  41 \begin{funcdesc}{array}{typecode\optional{, initializer}}
  42 Return a new array whose items are restricted by \var{typecode},
  43 and initialized from the optional \var{initializer} value, which
  44 must be a list, string, or iterable over elements of the
  45 appropriate type.
  46 \versionchanged[Formerly, only lists or strings were accepted]{2.4}
  47 If given a list or string, the initializer is passed to the
  48 new array's \method{fromlist()}, \method{fromstring()}, or
  49 \method{fromunicode()} method (see below) to add initial items to
  50 the array.  Otherwise, the iterable initializer is passed to the
  51 \method{extend()} method.
  52 \end{funcdesc}
  53
  54 \begin{datadesc}{ArrayType}
  55 Obsolete alias for \function{array}.
  56 \end{datadesc}
  57
  58
  59 Array objects support the ordinary sequence operations of
  60 indexing, slicing, concatenation, and multiplication.  When using
  61 slice assignment, the assigned value must be an array object with the
  62 same type code; in all other cases, \exception{TypeError} is raised.
  63 Array objects also implement the buffer interface, and may be used
  64 wherever buffer objects are supported.
  65
  66 The following data items and methods are also supported:
  67
  68 \begin{memberdesc}[array]{typecode}
  69 The typecode character used to create the array.
  70 \end{memberdesc}
  71
  72 \begin{memberdesc}[array]{itemsize}
  73 The length in bytes of one array item in the internal representation.
  74 \end{memberdesc}
  75
  76
  77 \begin{methoddesc}[array]{append}{x}
  78 Append a new item with value \var{x} to the end of the array.
  79 \end{methoddesc}
  80
  81 \begin{methoddesc}[array]{buffer_info}{}
  82 Return a tuple \code{(\var{address}, \var{length})} giving the current
  83 memory address and the length in elements of the buffer used to hold
  84 array's contents.  The size of the memory buffer in bytes can be
  85 computed as \code{\var{array}.buffer_info()[1] *
  86 \var{array}.itemsize}.  This is occasionally useful when working with
  87 low-level (and inherently unsafe) I/O interfaces that require memory
  88 addresses, such as certain \cfunction{ioctl()} operations.  The
  89 returned numbers are valid as long as the array exists and no
  90 length-changing operations are applied to it.
  91
  92 \note{When using array objects from code written in C or
  93 \Cpp{} (the only way to effectively make use of this information), it
  94 makes more sense to use the buffer interface supported by array
  95 objects.  This method is maintained for backward compatibility and
  96 should be avoided in new code.  The buffer interface is documented in
  97 the \citetitle[../api/newTypes.html]{Python/C API Reference Manual}.}
  98 \end{methoddesc}
  99
 100 \begin{methoddesc}[array]{byteswap}{}
 101 ``Byteswap'' all items of the array.  This is only supported for
 102 values which are 1, 2, 4, or 8 bytes in size; for other types of
 103 values, \exception{RuntimeError} is raised.  It is useful when reading
 104 data from a file written on a machine with a different byte order.
 105 \end{methoddesc}
 106
 107 \begin{methoddesc}[array]{count}{x}
 108 Return the number of occurrences of \var{x} in the array.
 109 \end{methoddesc}
 110
 111 \begin{methoddesc}[array]{extend}{iterable}
 112 Append items from \var{iterable} to the end of the array.  If
 113 \var{iterable} is another array, it must have \emph{exactly} the same
 114 type code; if not, \exception{TypeError} will be raised.  If
 115 \var{iterable} is not an array, it must be iterable and its
 116 elements must be the right type to be appended to the array.
 117 \versionchanged[Formerly, the argument could only be another array]{2.4}
 118 \end{methoddesc}
 119
 120 \begin{methoddesc}[array]{fromfile}{f, n}
 121 Read \var{n} items (as machine values) from the file object \var{f}
 122 and append them to the end of the array.  If less than \var{n} items
 123 are available, \exception{EOFError} is raised, but the items that were
 124 available are still inserted into the array.  \var{f} must be a real
 125 built-in file object; something else with a \method{read()} method won't
 126 do.
 127 \end{methoddesc}
 128
 129 \begin{methoddesc}[array]{fromlist}{list}
 130 Append items from the list.  This is equivalent to
 131 \samp{for x in \var{list}:\ a.append(x)}
 132 except that if there is a type error, the array is unchanged.
 133 \end{methoddesc}
 134
 135 \begin{methoddesc}[array]{fromstring}{s}
 136 Appends items from the string, interpreting the string as an
 137 array of machine values (as if it had been read from a
 138 file using the \method{fromfile()} method).
 139 \end{methoddesc}
 140
 141 \begin{methoddesc}[array]{fromunicode}{s}
 142 Extends this array with data from the given unicode string.
 143 The array must be a type 'u' array; otherwise a ValueError
 144 is raised.  Use \samp{array.fromstring(ustr.decode(enc))} to
 145 append Unicode data to an array of some other type.
 146 \end{methoddesc}
 147
 148 \begin{methoddesc}[array]{index}{x}
 149 Return the smallest \var{i} such that \var{i} is the index of
 150 the first occurrence of \var{x} in the array.
 151 \end{methoddesc}
 152
 153 \begin{methoddesc}[array]{insert}{i, x}
 154 Insert a new item with value \var{x} in the array before position
 155 \var{i}. Negative values are treated as being relative to the end
 156 of the array.
 157 \end{methoddesc}
 158
 159 \begin{methoddesc}[array]{pop}{\optional{i}}
 160 Removes the item with the index \var{i} from the array and returns
 161 it. The optional argument defaults to \code{-1}, so that by default
 162 the last item is removed and returned.
 163 \end{methoddesc}
 164
 165 \begin{methoddesc}[array]{read}{f, n}
 166 \deprecated {1.5.1}
 167   {Use the \method{fromfile()} method.}
 168 Read \var{n} items (as machine values) from the file object \var{f}
 169 and append them to the end of the array.  If less than \var{n} items
 170 are available, \exception{EOFError} is raised, but the items that were
 171 available are still inserted into the array.  \var{f} must be a real
 172 built-in file object; something else with a \method{read()} method won't
 173 do.
 174 \end{methoddesc}
 175
 176 \begin{methoddesc}[array]{remove}{x}
 177 Remove the first occurrence of \var{x} from the array.
 178 \end{methoddesc}
 179
 180 \begin{methoddesc}[array]{reverse}{}
 181 Reverse the order of the items in the array.
 182 \end{methoddesc}
 183
 184 \begin{methoddesc}[array]{tofile}{f}
 185 Write all items (as machine values) to the file object \var{f}.
 186 \end{methoddesc}
 187
 188 \begin{methoddesc}[array]{tolist}{}
 189 Convert the array to an ordinary list with the same items.
 190 \end{methoddesc}
 191
 192 \begin{methoddesc}[array]{tostring}{}
 193 Convert the array to an array of machine values and return the
 194 string representation (the same sequence of bytes that would
 195 be written to a file by the \method{tofile()} method.)
 196 \end{methoddesc}
 197
 198 \begin{methoddesc}[array]{tounicode}{}
 199 Convert the array to a unicode string.  The array must be
 200 a type 'u' array; otherwise a ValueError is raised.  Use
 201 array.tostring().decode(enc) to obtain a unicode string
 202 from an array of some other type.
 203 \end{methoddesc}
 204
 205 \begin{methoddesc}[array]{write}{f}
 206 \deprecated {1.5.1}
 207   {Use the \method{tofile()} method.}
 208 Write all items (as machine values) to the file object \var{f}.
 209 \end{methoddesc}
 210
 211 When an array object is printed or converted to a string, it is
 212 represented as \code{array(\var{typecode}, \var{initializer})}.  The
 213 \var{initializer} is omitted if the array is empty, otherwise it is a
 214 string if the \var{typecode} is \code{'c'}, otherwise it is a list of
 215 numbers.  The string is guaranteed to be able to be converted back to
 216 an array with the same type and value using reverse quotes
 217 (\code{``}), so long as the \function{array()} function has been
 218 imported using \code{from array import array}.  Examples:
 219
 220 \begin{verbatim}
 221 array('l')
 222 array('c', 'hello world')
 223 array('u', u'hello \textbackslash u2641')
 224 array('l', [1, 2, 3, 4, 5])
 225 array('d', [1.0, 2.0, 3.14])
 226 \end{verbatim}
 227
 228
 229 \begin{seealso}
 230   \seemodule{struct}{Packing and unpacking of heterogeneous binary data.}
 231   \seemodule{xdrlib}{Packing and unpacking of External Data
 232                      Representation (XDR) data as used in some remote
 233                      procedure call systems.}
 234   \seetitle[http://numpy.sourceforge.net/numdoc/HTML/numdoc.htm]{The
 235            Numerical Python Manual}{The Numeric Python extension
 236            (NumPy) defines another array type; see
 237            \url{http://numpy.sourceforge.net/} for further information
 238            about Numerical Python.  (A PDF version of the NumPy manual
 239            is available at
 240            \url{http://numpy.sourceforge.net/numdoc/numdoc.pdf}).}
 241 \end{seealso}