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770 lines
23 KiB
Plaintext
770 lines
23 KiB
Plaintext
@node Character Handling, String and Array Utilities, Memory Allocation, Top
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@c %MENU% Character testing and conversion functions
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@chapter Character Handling
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Programs that work with characters and strings often need to classify a
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character---is it alphabetic, is it a digit, is it whitespace, and so
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on---and perform case conversion operations on characters. The
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functions in the header file @file{ctype.h} are provided for this
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purpose.
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@pindex ctype.h
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Since the choice of locale and character set can alter the
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classifications of particular character codes, all of these functions
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are affected by the current locale. (More precisely, they are affected
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by the locale currently selected for character classification---the
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@code{LC_CTYPE} category; see @ref{Locale Categories}.)
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The @w{ISO C} standard specifies two different sets of functions. The
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one set works on @code{char} type characters, the other one on
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@code{wchar_t} wide characters (@pxref{Extended Char Intro}).
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@menu
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* Classification of Characters:: Testing whether characters are
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letters, digits, punctuation, etc.
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* Case Conversion:: Case mapping, and the like.
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* Classification of Wide Characters:: Character class determination for
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wide characters.
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* Using Wide Char Classes:: Notes on using the wide character
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classes.
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* Wide Character Case Conversion:: Mapping of wide characters.
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@end menu
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@node Classification of Characters, Case Conversion, , Character Handling
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@section Classification of Characters
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@cindex character testing
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@cindex classification of characters
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@cindex predicates on characters
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@cindex character predicates
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This section explains the library functions for classifying characters.
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For example, @code{isalpha} is the function to test for an alphabetic
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character. It takes one argument, the character to test, and returns a
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nonzero integer if the character is alphabetic, and zero otherwise. You
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would use it like this:
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@smallexample
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if (isalpha (c))
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printf ("The character `%c' is alphabetic.\n", c);
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@end smallexample
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Each of the functions in this section tests for membership in a
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particular class of characters; each has a name starting with @samp{is}.
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Each of them takes one argument, which is a character to test, and
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returns an @code{int} which is treated as a boolean value. The
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character argument is passed as an @code{int}, and it may be the
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constant value @code{EOF} instead of a real character.
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The attributes of any given character can vary between locales.
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@xref{Locales}, for more information on locales.@refill
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These functions are declared in the header file @file{ctype.h}.
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@pindex ctype.h
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@cindex lower-case character
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@comment ctype.h
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@comment ISO
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@deftypefun int islower (int @var{c})
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Returns true if @var{c} is a lower-case letter. The letter need not be
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from the Latin alphabet, any alphabet representable is valid.
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@end deftypefun
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@cindex upper-case character
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@comment ctype.h
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@comment ISO
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@deftypefun int isupper (int @var{c})
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Returns true if @var{c} is an upper-case letter. The letter need not be
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from the Latin alphabet, any alphabet representable is valid.
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@end deftypefun
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@cindex alphabetic character
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@comment ctype.h
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@comment ISO
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@deftypefun int isalpha (int @var{c})
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Returns true if @var{c} is an alphabetic character (a letter). If
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@code{islower} or @code{isupper} is true of a character, then
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@code{isalpha} is also true.
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In some locales, there may be additional characters for which
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@code{isalpha} is true---letters which are neither upper case nor lower
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case. But in the standard @code{"C"} locale, there are no such
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additional characters.
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@end deftypefun
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@cindex digit character
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@cindex decimal digit character
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@comment ctype.h
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@comment ISO
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@deftypefun int isdigit (int @var{c})
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Returns true if @var{c} is a decimal digit (@samp{0} through @samp{9}).
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@end deftypefun
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@cindex alphanumeric character
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@comment ctype.h
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@comment ISO
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@deftypefun int isalnum (int @var{c})
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Returns true if @var{c} is an alphanumeric character (a letter or
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number); in other words, if either @code{isalpha} or @code{isdigit} is
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true of a character, then @code{isalnum} is also true.
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@end deftypefun
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@cindex hexadecimal digit character
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@comment ctype.h
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@comment ISO
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@deftypefun int isxdigit (int @var{c})
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Returns true if @var{c} is a hexadecimal digit.
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Hexadecimal digits include the normal decimal digits @samp{0} through
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@samp{9} and the letters @samp{A} through @samp{F} and
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@samp{a} through @samp{f}.
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@end deftypefun
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@cindex punctuation character
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@comment ctype.h
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@comment ISO
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@deftypefun int ispunct (int @var{c})
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Returns true if @var{c} is a punctuation character.
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This means any printing character that is not alphanumeric or a space
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character.
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@end deftypefun
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@cindex whitespace character
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@comment ctype.h
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@comment ISO
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@deftypefun int isspace (int @var{c})
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Returns true if @var{c} is a @dfn{whitespace} character. In the standard
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@code{"C"} locale, @code{isspace} returns true for only the standard
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whitespace characters:
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@table @code
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@item ' '
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space
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@item '\f'
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formfeed
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@item '\n'
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newline
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@item '\r'
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carriage return
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@item '\t'
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horizontal tab
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@item '\v'
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vertical tab
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@end table
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@end deftypefun
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@cindex blank character
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@comment ctype.h
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@comment GNU
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@deftypefun int isblank (int @var{c})
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Returns true if @var{c} is a blank character; that is, a space or a tab.
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This function is a GNU extension.
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@end deftypefun
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@cindex graphic character
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@comment ctype.h
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@comment ISO
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@deftypefun int isgraph (int @var{c})
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Returns true if @var{c} is a graphic character; that is, a character
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that has a glyph associated with it. The whitespace characters are not
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considered graphic.
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@end deftypefun
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@cindex printing character
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@comment ctype.h
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@comment ISO
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@deftypefun int isprint (int @var{c})
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Returns true if @var{c} is a printing character. Printing characters
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include all the graphic characters, plus the space (@samp{ }) character.
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@end deftypefun
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@cindex control character
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@comment ctype.h
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@comment ISO
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@deftypefun int iscntrl (int @var{c})
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Returns true if @var{c} is a control character (that is, a character that
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is not a printing character).
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@end deftypefun
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@cindex ASCII character
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@comment ctype.h
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@comment SVID, BSD
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@deftypefun int isascii (int @var{c})
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Returns true if @var{c} is a 7-bit @code{unsigned char} value that fits
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into the US/UK ASCII character set. This function is a BSD extension
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and is also an SVID extension.
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@end deftypefun
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@node Case Conversion, Classification of Wide Characters, Classification of Characters, Character Handling
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@section Case Conversion
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@cindex character case conversion
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@cindex case conversion of characters
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@cindex converting case of characters
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This section explains the library functions for performing conversions
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such as case mappings on characters. For example, @code{toupper}
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converts any character to upper case if possible. If the character
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can't be converted, @code{toupper} returns it unchanged.
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These functions take one argument of type @code{int}, which is the
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character to convert, and return the converted character as an
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@code{int}. If the conversion is not applicable to the argument given,
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the argument is returned unchanged.
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@strong{Compatibility Note:} In pre-@w{ISO C} dialects, instead of
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returning the argument unchanged, these functions may fail when the
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argument is not suitable for the conversion. Thus for portability, you
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may need to write @code{islower(c) ? toupper(c) : c} rather than just
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@code{toupper(c)}.
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These functions are declared in the header file @file{ctype.h}.
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@pindex ctype.h
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@comment ctype.h
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@comment ISO
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@deftypefun int tolower (int @var{c})
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If @var{c} is an upper-case letter, @code{tolower} returns the corresponding
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lower-case letter. If @var{c} is not an upper-case letter,
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@var{c} is returned unchanged.
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@end deftypefun
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@comment ctype.h
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@comment ISO
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@deftypefun int toupper (int @var{c})
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If @var{c} is a lower-case letter, @code{toupper} returns the corresponding
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upper-case letter. Otherwise @var{c} is returned unchanged.
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@end deftypefun
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@comment ctype.h
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@comment SVID, BSD
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@deftypefun int toascii (int @var{c})
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This function converts @var{c} to a 7-bit @code{unsigned char} value
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that fits into the US/UK ASCII character set, by clearing the high-order
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bits. This function is a BSD extension and is also an SVID extension.
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@end deftypefun
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@comment ctype.h
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@comment SVID
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@deftypefun int _tolower (int @var{c})
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This is identical to @code{tolower}, and is provided for compatibility
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with the SVID. @xref{SVID}.@refill
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@end deftypefun
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@comment ctype.h
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@comment SVID
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@deftypefun int _toupper (int @var{c})
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This is identical to @code{toupper}, and is provided for compatibility
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with the SVID.
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@end deftypefun
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@node Classification of Wide Characters, Using Wide Char Classes, Case Conversion, Character Handling
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@section Character class determination for wide characters
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The second amendment to @w{ISO C89} defines functions to classify wide
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characters. Although the original @w{ISO C89} standard already defined
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the type @code{wchar_t}, no functions operating on them were defined.
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The general design of the classification functions for wide characters
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is more general. It allows extensions to the set of available
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classifications, beyond those which are always available. The POSIX
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standard specifies how extensions can be made, and this is already
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implemented in the GNU C library implementation of the @code{localedef}
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program.
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The character class functions are normally implemented with bitsets,
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with a bitset per character. For a given character, the appropriate
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bitset is read from a table and a test is performed as to whether a
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certain bit is set. Which bit is tested for is determined by the
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class.
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For the wide character classification functions this is made visible.
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There is a type classification type defined, a function to retrieve this
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value for a given class, and a function to test whether a given
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character is in this class, using the classification value. On top of
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this the normal character classification functions as used for
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@code{char} objects can be defined.
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@comment wctype.h
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@comment ISO
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@deftp {Data type} wctype_t
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The @code{wctype_t} can hold a value which represents a character class.
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The only defined way to generate such a value is by using the
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@code{wctype} function.
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@pindex wctype.h
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This type is defined in @file{wctype.h}.
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@end deftp
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@comment wctype.h
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@comment ISO
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@deftypefun wctype_t wctype (const char *@var{property})
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The @code{wctype} returns a value representing a class of wide
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characters which is identified by the string @var{property}. Beside
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some standard properties each locale can define its own ones. In case
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no property with the given name is known for the current locale
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selected for the @code{LC_CTYPE} category, the function returns zero.
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@noindent
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The properties known in every locale are:
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@multitable @columnfractions .25 .25 .25 .25
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@item
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@code{"alnum"} @tab @code{"alpha"} @tab @code{"cntrl"} @tab @code{"digit"}
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@item
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@code{"graph"} @tab @code{"lower"} @tab @code{"print"} @tab @code{"punct"}
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@item
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@code{"space"} @tab @code{"upper"} @tab @code{"xdigit"}
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@end multitable
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@pindex wctype.h
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This function is declared in @file{wctype.h}.
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@end deftypefun
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To test the membership of a character to one of the non-standard classes
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the @w{ISO C} standard defines a completely new function.
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@comment wctype.h
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@comment ISO
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@deftypefun int iswctype (wint_t @var{wc}, wctype_t @var{desc})
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This function returns a nonzero value if @var{wc} is in the character
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class specified by @var{desc}. @var{desc} must previously be returned
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by a successful call to @code{wctype}.
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@pindex wctype.h
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This function is declared in @file{wctype.h}.
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@end deftypefun
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To make it easier to use the commonly-used classification functions,
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they are defined in the C library. There is no need to use
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@code{wctype} if the property string is one of the known character
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classes. In some situations it is desirable to construct the property
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strings, and then it is important that @code{wctype} can also handle the
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standard classes.
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@cindex alphanumeric character
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@comment wctype.h
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@comment ISO
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@deftypefun int iswalnum (wint_t @var{wc})
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This function returns a nonzero value if @var{wc} is an alphanumeric
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character (a letter or number); in other words, if either @code{iswalpha}
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or @code{iswdigit} is true of a character, then @code{iswalnum} is also
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true.
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@noindent
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This function can be implemented using
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@smallexample
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iswctype (wc, wctype ("alnum"))
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@end smallexample
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@pindex wctype.h
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It is declared in @file{wctype.h}.
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@end deftypefun
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@cindex alphabetic character
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@comment wctype.h
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@comment ISO
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@deftypefun int iswalpha (wint_t @var{wc})
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Returns true if @var{wc} is an alphabetic character (a letter). If
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@code{iswlower} or @code{iswupper} is true of a character, then
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@code{iswalpha} is also true.
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In some locales, there may be additional characters for which
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@code{iswalpha} is true---letters which are neither upper case nor lower
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case. But in the standard @code{"C"} locale, there are no such
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additional characters.
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@noindent
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This function can be implemented using
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@smallexample
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iswctype (wc, wctype ("alpha"))
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@end smallexample
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@pindex wctype.h
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It is declared in @file{wctype.h}.
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@end deftypefun
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@cindex control character
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@comment wctype.h
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@comment ISO
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@deftypefun int iswcntrl (wint_t @var{wc})
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Returns true if @var{wc} is a control character (that is, a character that
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is not a printing character).
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@noindent
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This function can be implemented using
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@smallexample
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iswctype (wc, wctype ("cntrl"))
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@end smallexample
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@pindex wctype.h
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It is declared in @file{wctype.h}.
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@end deftypefun
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@cindex digit character
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@comment wctype.h
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@comment ISO
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@deftypefun int iswdigit (wint_t @var{wc})
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Returns true if @var{wc} is a digit (e.g., @samp{0} through @samp{9}).
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Please note that this function does not only return a nonzero value for
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@emph{decimal} digits, but for all kinds of digits. A consequence is
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that code like the following will @strong{not} work unconditionally for
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wide characters:
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@smallexample
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n = 0;
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while (iswdigit (*wc))
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@{
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n *= 10;
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n += *wc++ - L'0';
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@}
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@end smallexample
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@noindent
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This function can be implemented using
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@smallexample
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iswctype (wc, wctype ("digit"))
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@end smallexample
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@pindex wctype.h
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It is declared in @file{wctype.h}.
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@end deftypefun
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@cindex graphic character
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@comment wctype.h
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@comment ISO
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@deftypefun int iswgraph (wint_t @var{wc})
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Returns true if @var{wc} is a graphic character; that is, a character
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that has a glyph associated with it. The whitespace characters are not
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considered graphic.
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@noindent
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This function can be implemented using
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@smallexample
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iswctype (wc, wctype ("graph"))
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@end smallexample
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@pindex wctype.h
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It is declared in @file{wctype.h}.
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@end deftypefun
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@cindex lower-case character
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@comment ctype.h
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@comment ISO
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@deftypefun int iswlower (wint_t @var{wc})
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Returns true if @var{wc} is a lower-case letter. The letter need not be
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from the Latin alphabet, any alphabet representable is valid.
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@noindent
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This function can be implemented using
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@smallexample
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iswctype (wc, wctype ("lower"))
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@end smallexample
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@pindex wctype.h
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It is declared in @file{wctype.h}.
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@end deftypefun
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@cindex printing character
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@comment wctype.h
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@comment ISO
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@deftypefun int iswprint (wint_t @var{wc})
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Returns true if @var{wc} is a printing character. Printing characters
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include all the graphic characters, plus the space (@samp{ }) character.
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@noindent
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This function can be implemented using
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@smallexample
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iswctype (wc, wctype ("print"))
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@end smallexample
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@pindex wctype.h
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It is declared in @file{wctype.h}.
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@end deftypefun
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@cindex punctuation character
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@comment wctype.h
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@comment ISO
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@deftypefun int iswpunct (wint_t @var{wc})
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Returns true if @var{wc} is a punctuation character.
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This means any printing character that is not alphanumeric or a space
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character.
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@noindent
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This function can be implemented using
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@smallexample
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iswctype (wc, wctype ("punct"))
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@end smallexample
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@pindex wctype.h
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It is declared in @file{wctype.h}.
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@end deftypefun
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@cindex whitespace character
|
|
@comment wctype.h
|
|
@comment ISO
|
|
@deftypefun int iswspace (wint_t @var{wc})
|
|
Returns true if @var{wc} is a @dfn{whitespace} character. In the standard
|
|
@code{"C"} locale, @code{iswspace} returns true for only the standard
|
|
whitespace characters:
|
|
|
|
@table @code
|
|
@item L' '
|
|
space
|
|
|
|
@item L'\f'
|
|
formfeed
|
|
|
|
@item L'\n'
|
|
newline
|
|
|
|
@item L'\r'
|
|
carriage return
|
|
|
|
@item L'\t'
|
|
horizontal tab
|
|
|
|
@item L'\v'
|
|
vertical tab
|
|
@end table
|
|
|
|
@noindent
|
|
This function can be implemented using
|
|
|
|
@smallexample
|
|
iswctype (wc, wctype ("space"))
|
|
@end smallexample
|
|
|
|
@pindex wctype.h
|
|
It is declared in @file{wctype.h}.
|
|
@end deftypefun
|
|
|
|
@cindex upper-case character
|
|
@comment wctype.h
|
|
@comment ISO
|
|
@deftypefun int iswupper (wint_t @var{wc})
|
|
Returns true if @var{wc} is an upper-case letter. The letter need not be
|
|
from the Latin alphabet, any alphabet representable is valid.
|
|
|
|
@noindent
|
|
This function can be implemented using
|
|
|
|
@smallexample
|
|
iswctype (wc, wctype ("upper"))
|
|
@end smallexample
|
|
|
|
@pindex wctype.h
|
|
It is declared in @file{wctype.h}.
|
|
@end deftypefun
|
|
|
|
@cindex hexadecimal digit character
|
|
@comment wctype.h
|
|
@comment ISO
|
|
@deftypefun int iswxdigit (wint_t @var{wc})
|
|
Returns true if @var{wc} is a hexadecimal digit.
|
|
Hexadecimal digits include the normal decimal digits @samp{0} through
|
|
@samp{9} and the letters @samp{A} through @samp{F} and
|
|
@samp{a} through @samp{f}.
|
|
|
|
@noindent
|
|
This function can be implemented using
|
|
|
|
@smallexample
|
|
iswctype (wc, wctype ("xdigit"))
|
|
@end smallexample
|
|
|
|
@pindex wctype.h
|
|
It is declared in @file{wctype.h}.
|
|
@end deftypefun
|
|
|
|
The GNU C library also provides a function which is not defined in the
|
|
@w{ISO C} standard but which is available as a version for single byte
|
|
characters as well.
|
|
|
|
@cindex blank character
|
|
@comment wctype.h
|
|
@comment GNU
|
|
@deftypefun int iswblank (wint_t @var{wc})
|
|
Returns true if @var{wc} is a blank character; that is, a space or a tab.
|
|
This function is a GNU extension. It is declared in @file{wchar.h}.
|
|
@end deftypefun
|
|
|
|
@node Using Wide Char Classes, Wide Character Case Conversion, Classification of Wide Characters, Character Handling
|
|
@section Notes on using the wide character classes
|
|
|
|
The first note is probably not astonishing but still occasionally a
|
|
cause of problems. The @code{isw@var{XXX}} functions can be implemented
|
|
using macros and in fact, the GNU C library does this. They are still
|
|
available as real functions but when the @file{wctype.h} header is
|
|
included the macros will be used. This is the same as the
|
|
@code{char} type versions of these functions.
|
|
|
|
The second note covers something new. It can be best illustrated by a
|
|
(real-world) example. The first piece of code is an excerpt from the
|
|
original code. It is truncated a bit but the intention should be clear.
|
|
|
|
@smallexample
|
|
int
|
|
is_in_class (int c, const char *class)
|
|
@{
|
|
if (strcmp (class, "alnum") == 0)
|
|
return isalnum (c);
|
|
if (strcmp (class, "alpha") == 0)
|
|
return isalpha (c);
|
|
if (strcmp (class, "cntrl") == 0)
|
|
return iscntrl (c);
|
|
...
|
|
return 0;
|
|
@}
|
|
@end smallexample
|
|
|
|
Now, with the @code{wctype} and @code{iswctype} you can avoid the
|
|
@code{if} cascades, but rewriting the code as follows is wrong:
|
|
|
|
@smallexample
|
|
int
|
|
is_in_class (int c, const char *class)
|
|
@{
|
|
wctype_t desc = wctype (class);
|
|
return desc ? iswctype ((wint_t) c, desc) : 0;
|
|
@}
|
|
@end smallexample
|
|
|
|
The problem is that it is not guaranteed that the wide character
|
|
representation of a single-byte character can be found using casting.
|
|
In fact, usually this fails miserably. The correct solution to this
|
|
problem is to write the code as follows:
|
|
|
|
@smallexample
|
|
int
|
|
is_in_class (int c, const char *class)
|
|
@{
|
|
wctype_t desc = wctype (class);
|
|
return desc ? iswctype (btowc (c), desc) : 0;
|
|
@}
|
|
@end smallexample
|
|
|
|
@xref{Converting a Character}, for more information on @code{btowc}.
|
|
Note that this change probably does not improve the performance
|
|
of the program a lot since the @code{wctype} function still has to make
|
|
the string comparisons. It gets really interesting if the
|
|
@code{is_in_class} function is called more than once for the
|
|
same class name. In this case the variable @var{desc} could be computed
|
|
once and reused for all the calls. Therefore the above form of the
|
|
function is probably not the final one.
|
|
|
|
|
|
@node Wide Character Case Conversion, , Using Wide Char Classes, Character Handling
|
|
@section Mapping of wide characters.
|
|
|
|
The classification functions are also generalized by the @w{ISO C}
|
|
standard. Instead of just allowing the two standard mappings, a
|
|
locale can contain others. Again, the @code{localedef} program
|
|
already supports generating such locale data files.
|
|
|
|
@comment wctype.h
|
|
@comment ISO
|
|
@deftp {Data Type} wctrans_t
|
|
This data type is defined as a scalar type which can hold a value
|
|
representing the locale-dependent character mapping. There is no way to
|
|
construct such a value apar from using the return value of the
|
|
@code{wctrans} function.
|
|
|
|
@pindex wctype.h
|
|
@noindent
|
|
This type is defined in @file{wctype.h}.
|
|
@end deftp
|
|
|
|
@comment wctype.h
|
|
@comment ISO
|
|
@deftypefun wctrans_t wctrans (const char *@var{property})
|
|
The @code{wctrans} function has to be used to find out whether a named
|
|
mapping is defined in the current locale selected for the
|
|
@code{LC_CTYPE} category. If the returned value is non-zero, you can use
|
|
it afterwards in calls to @code{towctrans}. If the return value is
|
|
zero no such mapping is known in the current locale.
|
|
|
|
Beside locale-specific mappings there are two mappings which are
|
|
guaranteed to be available in every locale:
|
|
|
|
@multitable @columnfractions .5 .5
|
|
@item
|
|
@code{"tolower"} @tab @code{"toupper"}
|
|
@end multitable
|
|
|
|
@pindex wctype.h
|
|
@noindent
|
|
These functions are declared in @file{wctype.h}.
|
|
@end deftypefun
|
|
|
|
@comment wctype.h
|
|
@comment ISO
|
|
@deftypefun wint_t towctrans (wint_t @var{wc}, wctrans_t @var{desc})
|
|
@code{towctrans} maps the input character @var{wc}
|
|
according to the rules of the mapping for which @var{desc} is a
|
|
descriptor, and returns the value it finds. @var{desc} must be
|
|
obtained by a successful call to @code{wctrans}.
|
|
|
|
@pindex wctype.h
|
|
@noindent
|
|
This function is declared in @file{wctype.h}.
|
|
@end deftypefun
|
|
|
|
For the generally available mappings, the @w{ISO C} standard defines
|
|
convenient shortcuts so that it is not necessary to call @code{wctrans}
|
|
for them.
|
|
|
|
@comment wctype.h
|
|
@comment ISO
|
|
@deftypefun wint_t towlower (wint_t @var{wc})
|
|
If @var{wc} is an upper-case letter, @code{towlower} returns the corresponding
|
|
lower-case letter. If @var{wc} is not an upper-case letter,
|
|
@var{wc} is returned unchanged.
|
|
|
|
@noindent
|
|
@code{towlower} can be implemented using
|
|
|
|
@smallexample
|
|
towctrans (wc, wctrans ("tolower"))
|
|
@end smallexample
|
|
|
|
@pindex wctype.h
|
|
@noindent
|
|
This function is declared in @file{wctype.h}.
|
|
@end deftypefun
|
|
|
|
@comment wctype.h
|
|
@comment ISO
|
|
@deftypefun wint_t towupper (wint_t @var{wc})
|
|
If @var{wc} is a lower-case letter, @code{towupper} returns the corresponding
|
|
upper-case letter. Otherwise @var{wc} is returned unchanged.
|
|
|
|
@noindent
|
|
@code{towupper} can be implemented using
|
|
|
|
@smallexample
|
|
towctrans (wc, wctrans ("toupper"))
|
|
@end smallexample
|
|
|
|
@pindex wctype.h
|
|
@noindent
|
|
This function is declared in @file{wctype.h}.
|
|
@end deftypefun
|
|
|
|
The same warnings given in the last section for the use of the wide
|
|
character classification functions apply here. It is not possible to
|
|
simply cast a @code{char} type value to a @code{wint_t} and use it as an
|
|
argument to @code{towctrans} calls.
|