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