200 lines
7.3 KiB
TeX
200 lines
7.3 KiB
TeX
\section{\module{array} ---
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Efficient arrays of numeric values}
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\declaremodule{builtin}{array}
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\modulesynopsis{Efficient arrays of uniformly typed numeric values.}
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This module defines a new object type which can efficiently represent
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an array of basic values: characters, integers, floating point
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numbers. Arrays\index{arrays} are sequence types and behave very much
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like lists, except that the type of objects stored in them is
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constrained. The type is specified at object creation time by using a
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\dfn{type code}, which is a single character. The following type
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codes are defined:
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\begin{tableiii}{c|l|c}{code}{Type code}{C Type}{Minimum size in bytes}
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\lineiii{'c'}{character}{1}
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\lineiii{'b'}{signed int}{1}
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\lineiii{'B'}{unsigned int}{1}
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\lineiii{'h'}{signed int}{2}
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\lineiii{'H'}{unsigned int}{2}
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\lineiii{'i'}{signed int}{2}
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\lineiii{'I'}{unsigned int}{2}
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\lineiii{'l'}{signed int}{4}
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\lineiii{'L'}{unsigned int}{4}
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\lineiii{'f'}{float}{4}
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\lineiii{'d'}{double}{8}
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\end{tableiii}
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The actual representation of values is determined by the machine
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architecture (strictly speaking, by the C implementation). The actual
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size can be accessed through the \member{itemsize} attribute. The values
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stored for \code{'L'} and \code{'I'} items will be represented as
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Python long integers when retrieved, because Python's plain integer
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type cannot represent the full range of C's unsigned (long) integers.
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The module defines the following function and type object:
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\begin{funcdesc}{array}{typecode\optional{, initializer}}
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Return a new array whose items are restricted by \var{typecode}, and
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initialized from the optional \var{initializer} value, which must be a
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list or a string. The list or string is passed to the new array's
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\method{fromlist()} or \method{fromstring()} method (see below) to add
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initial items to the array.
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\end{funcdesc}
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\begin{datadesc}{ArrayType}
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Type object corresponding to the objects returned by
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\function{array()}.
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\end{datadesc}
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Array objects support the following data items and methods:
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\begin{memberdesc}[array]{typecode}
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The typecode character used to create the array.
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\end{memberdesc}
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\begin{memberdesc}[array]{itemsize}
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The length in bytes of one array item in the internal representation.
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\end{memberdesc}
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\begin{methoddesc}[array]{append}{x}
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Append a new item with value \var{x} to the end of the array.
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\end{methoddesc}
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\begin{methoddesc}[array]{buffer_info}{}
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Return a tuple \code{(\var{address}, \var{length})} giving the current
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memory address and the length in bytes of the buffer used to hold
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array's contents. This is occasionally useful when working with
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low-level (and inherently unsafe) I/O interfaces that require memory
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addresses, such as certain \cfunction{ioctl()} operations. The returned
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numbers are valid as long as the array exists and no length-changing
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operations are applied to it.
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\end{methoddesc}
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\begin{methoddesc}[array]{byteswap}{}
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``Byteswap'' all items of the array. This is only supported for
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values which are 1, 2, 4, or 8 bytes in size; for other types of
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values, \exception{RuntimeError} is raised. It is useful when reading
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data from a file written on a machine with a different byte order.
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\end{methoddesc}
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\begin{methoddesc}[array]{count}{x}
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Return the number of occurences of \var{x} in the array.
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\end{methoddesc}
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\begin{methoddesc}[array]{extend}{a}
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Append array items from \var{a} to the end of the array.
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\end{methoddesc}
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\begin{methoddesc}[array]{fromfile}{f, n}
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Read \var{n} items (as machine values) from the file object \var{f}
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and append them to the end of the array. If less than \var{n} items
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are available, \exception{EOFError} is raised, but the items that were
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available are still inserted into the array. \var{f} must be a real
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built-in file object; something else with a \method{read()} method won't
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do.
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\end{methoddesc}
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\begin{methoddesc}[array]{fromlist}{list}
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Append items from the list. This is equivalent to
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\samp{for x in \var{list}:\ a.append(x)}
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except that if there is a type error, the array is unchanged.
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\end{methoddesc}
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\begin{methoddesc}[array]{fromstring}{s}
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Appends items from the string, interpreting the string as an
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array of machine values (i.e. as if it had been read from a
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file using the \method{fromfile()} method).
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\end{methoddesc}
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\begin{methoddesc}[array]{index}{x}
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Return the smallest \var{i} such that \var{i} is the index of
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the first occurence of \var{x} in the array.
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\end{methoddesc}
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\begin{methoddesc}[array]{insert}{i, x}
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Insert a new item with value \var{x} in the array before position
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\var{i}.
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\end{methoddesc}
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\begin{methoddesc}[array]{pop}{\optional{i}}
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Removes the item with the index \var{i} from the array and returns
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it. The optional argument defaults to \code{-1}, so that by default
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the last item is removed and returned.
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\end{methoddesc}
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\begin{methoddesc}[array]{read}{f, n}
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\deprecated {1.5.1}
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{Use the \method{fromfile()} method.}
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Read \var{n} items (as machine values) from the file object \var{f}
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and append them to the end of the array. If less than \var{n} items
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are available, \exception{EOFError} is raised, but the items that were
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available are still inserted into the array. \var{f} must be a real
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built-in file object; something else with a \method{read()} method won't
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do.
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\end{methoddesc}
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\begin{methoddesc}[array]{remove}{x}
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Remove the first occurence of \var{x} from the array.
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\end{methoddesc}
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\begin{methoddesc}[array]{reverse}{}
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Reverse the order of the items in the array.
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\end{methoddesc}
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\begin{methoddesc}[array]{tofile}{f}
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Write all items (as machine values) to the file object \var{f}.
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\end{methoddesc}
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\begin{methoddesc}[array]{tolist}{}
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Convert the array to an ordinary list with the same items.
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\end{methoddesc}
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\begin{methoddesc}[array]{tostring}{}
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Convert the array to an array of machine values and return the
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string representation (the same sequence of bytes that would
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be written to a file by the \method{tofile()} method.)
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\end{methoddesc}
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\begin{methoddesc}[array]{write}{f}
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\deprecated {1.5.1}
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{Use the \method{tofile()} method.}
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Write all items (as machine values) to the file object \var{f}.
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\end{methoddesc}
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When an array object is printed or converted to a string, it is
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represented as \code{array(\var{typecode}, \var{initializer})}. The
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\var{initializer} is omitted if the array is empty, otherwise it is a
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string if the \var{typecode} is \code{'c'}, otherwise it is a list of
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numbers. The string is guaranteed to be able to be converted back to
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an array with the same type and value using reverse quotes
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(\code{``}), so long as the \function{array()} function has been
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imported using \samp{from array import array}. Examples:
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\begin{verbatim}
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array('l')
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array('c', 'hello world')
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array('l', [1, 2, 3, 4, 5])
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array('d', [1.0, 2.0, 3.14])
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\end{verbatim}
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\begin{seealso}
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\seemodule{struct}{Packing and unpacking of heterogeneous binary data.}
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\seemodule{xdrlib}{Packing and unpacking of External Data
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Representation (XDR) data as used in some remote
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procedure call systems.}
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\seetitle[http://numpy.sourceforge.net/numdoc/HTML/numdoc.html]{The
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Numerical Python Manual}{The Numeric Python extension
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(NumPy) defines another array type; see
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\url{http://numpy.sourceforge.net/} for further information
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about Numerical Python. (A PDF version of the NumPy manual
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is available at
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\url{http://numpy.sourceforge.net/numdoc/numdoc.pdf}.}
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\end{seealso}
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