\chapter{Character Array}
\label{cha:character-array}
\declaremodule{extension}{numarray.strings}
\index{character array}
\index{string array}

\section{Introduction}
\label{sec:chararray-intro}

\code{numarray}, like \code{Numeric}, has support for arrays of character data
(provided by the \code{numarray.strings} module) in addition to arrays of
numbers.  The support for character arrays in \code{Numeric} is relatively
limited, restricted to arrays of single characters.  In contrast,
\code{numarray} supports arrays of fixed length strings.  As an additional
enhancement, the \code{numarray} design supports interleaving arrays of
characters with arrays of numbers, with both occupying the same memory buffer.
This provides basic infrastructure for building the arrays of heterogenous
records as provided by \code{numarray.records} (see chapter
\ref{cha:record-array}).  Currently, neither \code{Numeric} nor \code{numarray}
provides support for unicode.

Each character array is a \index{CharArray} \code{CharArray} object in the
\code{numarray.strings} module.  The easiest way to construct a character array
is to use the \code{numarray.strings.array()} function.  For example:

\begin{verbatim}
  >>> import numarray.strings as str
  >>> s = str.array(['Smith', 'Johnson', 'Williams', 'Miller'])
  >>> print s
  ['Smith', 'Johnson', 'Williams', 'Miller']
  >>> s.itemsize()
  8
\end{verbatim}
In this example, this string array has 4 elements.  The maximum string length
is automatically determined from the data.  In this case, the created array will
support fixed length strings of 8 characters (since the longest name is 8
characters long).

The character array is just like an array in numarray, except that now each
element is conceptually a Python string rather than a number.  We can do the
usual indexing and slicing:

\begin{verbatim}
  >>> print s[0]
  'Smith'
  >>> print s[:2]
  ['Smith', 'Johnson']
  >>> s[:2] = 'changed'
  >>> print s
  ['changed', 'changed', 'Williams', 'Miller']
\end{verbatim}

\section{Character array stripping, padding, and truncation}
\label{sec:chararray-clip-pad-truncate}
CharArrays are designed to store fixed length strings of visible ASCII text.
You may have noticed that although a \code{CharArray} stores fixed length
strings, it displays variable length strings.  This is a result of the
stripping and padding policies of the CharArray class.  

When an element of a \code{CharArray} is fetched trailing whitespace is
stripped off.  The sole exception to this rule is that a single whitespace is
never stripped down to the empty string.  \code{numarray.strings} defines
whitespace as an ASCII space, formfeed, newline, carriage return, tab, or
vertical tab.

When a string is assigned to a \code{CharArray}, the string is considered
terminated by the first of any NULL characters it contains and is padded with
spaces to the full length of the \code{CharArray} itemsize.  Thus, the memory
image of a \code{CharArray} element does not include anything at or after the
first NULL in an assigned string; instead, there are spaces, and no terminating
NULL character at all.

When a string which is longer than the \code{itemsize()} is assigned to a
\code{CharArray}, it is silently truncated.

The \code{RawCharArray} baseclass of \code{CharArray} implements transparent
\code{strip()} and \code{pad()} methods, enabling the storage and retrieval of
arbitrary ASCII values within array elements.  For \code{RawCharArray}, all
array elements are identical in percieved length.  Alternate
stripping and padding policies can be implemented by subclassing
\code{CharArray} or \code{RawCharArray}.

\section{Character array functions}
\label{sec:chararray-func}
\begin{funcdesc}{array}{buffer=None, itemsize=None, shape=None, byteoffset=0,
    bytestride=None, kind=CharArray}
\label{func:str.array}
   The function \code{array} is, for most practical purposes, all a user needs 
   to know to construct a character array.

   The first argument, \code{buffer}, may be any one of the following:

   (1) \code{None} (default).  The constructor will allocate a writeable memory
   buffer which will be uninitialized.  The user must assign valid data before
   trying to read the contents or before writing the character array to a disk
   file.
   
   (2) a Python string containing binary data.  For example:
\begin{verbatim}
     >>> print str.array('abcdefg'*10, itemsize=10)
     ['abcdefgabc', 'defgabcdef', 'gabcdefgab', 'cdefgabcde', 'fgabcdefga',
      'bcdefgabcd', 'efgabcdefg']
\end{verbatim}
   
   (3) a Python file object for an open file.  The data will be copied from 
   the file, starting at the current position of the read pointer.
   
   (4) a character array.  This results in a deep copy of the input character
   array; any other arguments to \code{array()} will be silently ignored.

\begin{verbatim}
     >>> print str.array(s)
     ['abcdefgabc', 'defgabcdef', 'gabcdefgab', 'cdefgabcde', 'fgabcdefga', 
      'bcdefgabcd', 'efgabcdefg']
\end{verbatim}
   
   (5) a nested sequence of strings.  The sequence nesting implies the
   shape of the string array unless shape is specified.

\begin{verbatim}
     >>> print str.array([['Smith', 'Johnson'], ['Williams', 'Miller']])
     [['Smith', 'Johnson'],
      ['Williams', 'Miller']]
\end{verbatim}

   \code{itemsize} can be used to increase or decrease the fixed size of an
   array element relative to the natural itemsize implied by any literal data
   specified by the \code{buffer} parameter.

\begin{verbatim}
     >>> print str.array([['Smith', 'Johnson'], ['Williams', 'Miller']], 
                         itemsize=2)
     [['Sm', 'Jo'],
      ['Wi', 'Mi']])
     >>> print str.array([['Smith', 'Johnson'], ['Williams', 'Miller']], 
                         itemsize=20)
     [['Smith', 'Johnson'],
      ['Williams', 'Miller']]
\end{verbatim}
   
   \code{shape} is the shape of the character array.  It can be an integer, in
   which case it is equivalent to the number of \var{rows} in a table.  It can
   also be a tuple implying the character array is an N-D array with fixed
   length strings as its elements. \code{shape} should be consistent with
   the number of elements implied by the data buffer and itemsize.

   \code{byteoffset} indicates an offset, specified in bytes, from the start
   of the array buffer to where the array data actually begins.  
   \code{byteoffset} enables the character array to be offset from the
   beginning of a table record.  This is mainly useful for implementing
   record arrays.

   \code{bytestride} indicates the separation, specified in bytes, between
   successive elements in the last dimension of the character array.
   \code{bytestride} is used in the implementation of record arrays to space
   character array elements with the size of the total record rather than the
   size of a single string.
   
   \code{kind} is used to specify the class of the created array, and should be
   \code{RawCharArray}, \code{CharArray}, or a subclass of either.
\end{funcdesc}
   
\begin{funcdesc}{num2char}{n, format, itemsize=32}
\label{func:str.num2char}
\code{num2char} formats the numarray \code{n} using the Python string format
\code{format} and stores the result in a character array with the specified
\code{itemsize}
\begin{verbatim}
     >>> num2char(num.arange(0.0,5), '%2.2f')
     CharArray(['0.00', '1.00', '2.00', '3.00', '4.00'])
\end{verbatim}
\end{funcdesc}

\section{Character array methods}
\label{sec:recarray-methods}
CharArray object has these public methods:

\begin{methoddesc}[RawCharArray]{tolist}{}
  \code{tolist()} returns a nested list of strings corresponding to all the
  elements in the array.
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{copy}{}
  \code{copy()} returns a deep copy of the character array.
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{raw}{}
  \code{raw()} returns the corresponding \code{RawCharArray} view.
\begin{verbatim}
     >>> c=str.array(["this","that","another"])
     >>> c.raw()
     RawCharArray(['this   ', 'that   ', 'another'])
\end{verbatim}
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{resized}{n}
  \code{resized(n)} returns a copy of the array, resized so that each 
  element is of length \code{n} characters.  Caution: do not confuse this
  method with \code{resize()} which changes the number of elements
  rather than the size of each element.
\begin{verbatim}
     >>> c = str.array(["this","that","another"])
     >>> c.itemsize()
     7
     >>> d = c.resized(20)
     >>> print d
     ['this', 'that', 'another']
     >>> d.itemsize()
     20
\end{verbatim}
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{concatenate}{other}
  \code{concatenate(other)} returns a new array which corresponds to the
  element by element concatenation of \code{other} to \code{self}.  The
  addition operator is also overloaded to perform concatenation.
\begin{verbatim}
     >>> print map(str, range(3)) + array(["this","that","another one"])
     ['0this', '1that', '2another one']
     >>> print "prefix with trailing whitespace   " + array(["."])
     ['prefix with trailing whitespace   .']
\end{verbatim}
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{sort}{}
  \code{sort} modifies the \code{CharArray} inplace so that its elements are in
  sorted order. \code{sort} only works for 1D character arrays.  Like the
  \code{sort()} for the Python list, \code{CharArray.sort()} returns nothing.
\begin{verbatim}
     >>> a=str.array(["other","this","that","another"])
     >>> a.sort()
     >>> print a
     ['another', 'other', 'that', 'this']
\end{verbatim}
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{argsort}{}
   \code{argsort} returns a numarray corresponding to the permutation which will
   put the character array \code{self} into sorted order.  \code{argsort} only
   works for 1D character arrays.
\begin{verbatim}
     >>> a=str.array(["other","that","this","another"])
     >>> a.argsort()
     array([3, 0, 1, 2])
     >>> print a[ a.argsort ] 
     ['another', 'other', 'that', 'this']
\end{verbatim}
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{amap}{f}
  \code{amap} applies the function \code{f} to every element of \code{self} and
  returns the nested list of the results.  The function \code{f} should operate
  on a single string and may return any Python value.
\end{methoddesc}
\begin{verbatim}
     >>> c = str.array(['this','that','another'])
     >>> print c.amap(lambda x: x[-2:])
     ['is', 'at', 'er']
\end{verbatim}
\begin{methoddesc}[RawCharArray]{match}{pattern, flags=0}
  \code{match} uses Python regular expression matching over all elements of a
  character array and returns a tuple of numarrays corresponding to the indices
  of \code{self} where the pattern matches. \code{flags} are passed directly to
  the Python pattern matcher defined in the \code{re} module of the standard
  library.
\begin{verbatim}
     >>> a=str.array([["wo","what"],["wen","erewh"]])
     >>> print a.match("wh[aebd]")
     (array([0]), array([1]))
     >>> print a[ a.match("wh[aebd]") ]
     ['what']
\end{verbatim}
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{search}{pattern,flags=0}
  \code{search} uses Python regular expression searching over all elements of a
  character array and returns a tuple of numarrays corresponding to the indices
  of \code{self} where the pattern was found. \code{flags} are passed directly
  to the Python pattern \code{search} method defined in the \code{re} module of
  the standard library.  \code{flags} should be an or'ed combination (use the
  $\vert$ operator) of the following \code{re} variables: \code{IGNORECASE},
  \code{LOCALE}, \code{MULTILINE}, \code{DOTALL}, \code{VERBOSE}.  See the
  \code{re} module documentation for more details.
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{sub}{pattern,replacement,flags=0,count=0}
  \code{sub} performs Python regular expression pattern substitution
  to all elements of a character array. \code{flags} and \code{count} work
  as they do for \code{re.sub()}.
\begin{verbatim}
     >>> a=str.array([["who","what"],["when","where"]])
     >>> print a.sub("wh", "ph")
     [['pho', 'phat'],
      ['phen', 'phere']])
\end{verbatim}
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{grep}{pattern, flags=0}
  \code{grep} is intended to be used interactively to search a \code{CharArray}
  for the array of strings which match the given \code{pattern}.
  \code{pattern} should be a Python regular expression (see the \code{re}
  module in the Python standard library, which can be as simple as a string
  constant as shown below.
\begin{verbatim}
     >>> a=str.array([["who","what"],["when","where"]])
     >>> print a.grep("whe")
     ['when', 'where']
\end{verbatim}
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{eval}{}
  \code{eval} executes the Python eval function on each element of a character
  array and returns the resulting numarray.  \code{eval} is intended for use
  converting character arrays to the corresponding numeric arrays.  An
  exception is raised if any string element fails to evaluate.
\begin{verbatim}
     >>> print str.array([["1","2"],["3","4."]]).eval()
     [[1., 2.],
      [3., 4.]]
\end{verbatim}
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{maxLen}{}
  \code{maxLen} returns the minimum element length required to store the
  stripped elements of the array \code{self}.
\begin{verbatim}
     >>> print str.array(["this","there"], itemsize=20).maxLen()
     5
\end{verbatim}
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{truncated}{}
  \code{truncated} returns an array corresponding to \code{self} resized
  so that it uses a minimum amount of storage.
\begin{verbatim}
     >>> a = str.array(["this  ","that"])
     >>> print a.itemsize()
     6
     >>> print a.truncated().itemsize()
     4
\end{verbatim}
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{count}{s}
  \code{count} counts the occurences of string \code{s} in array \code{self}.
\begin{verbatim}
     >>> print array(["this","that","another","this"]).count("this")
     2
\end{verbatim}
\end{methoddesc}
\begin{methoddesc}[RawCharArray]{info}{}
   This will display key attributes of the character array.
\end{methoddesc}

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