@node Character Handling, String and Array Utilities, Memory Allocation, Top
@chapter Character Handling

Programs that work with characters and strings often need to classify a
character---is it alphabetic, is it a digit, is it whitespace, and so
on---and perform case conversion operations on characters.  The
functions in the header file @file{ctype.h} are provided for this
purpose.
@pindex ctype.h

Since the choice of locale and character set can alter the
classifications of particular character codes, all of these functions
are affected by the current locale.  (More precisely, they are affected
by the locale currently selected for character classification---the
@code{LC_CTYPE} category; see @ref{Locale Categories}.)

@menu
* Classification of Characters::   Testing whether characters are
			            letters, digits, punctuation, etc.

* Case Conversion::                Case mapping, and the like.
@end menu

@node Classification of Characters, Case Conversion,  , Character Handling
@section Classification of Characters
@cindex character testing
@cindex classification of characters
@cindex predicates on characters
@cindex character predicates

This section explains the library functions for classifying characters.
For example, @code{isalpha} is the function to test for an alphabetic
character.  It takes one argument, the character to test, and returns a
nonzero integer if the character is alphabetic, and zero otherwise.  You
would use it like this:

@smallexample
if (isalpha (c))
  printf ("The character `%c' is alphabetic.\n", c);
@end smallexample

Each of the functions in this section tests for membership in a
particular class of characters; each has a name starting with @samp{is}.
Each of them takes one argument, which is a character to test, and
returns an @code{int} which is treated as a boolean value.  The
character argument is passed as an @code{int}, and it may be the
constant value @code{EOF} instead of a real character.  

The attributes of any given character can vary between locales.
@xref{Locales}, for more information on locales.@refill

These functions are declared in the header file @file{ctype.h}.
@pindex ctype.h

@cindex lower-case character
@comment ctype.h
@comment ANSI
@deftypefun int islower (int @var{c})
Returns true if @var{c} is a lower-case letter.
@end deftypefun

@cindex upper-case character
@comment ctype.h
@comment ANSI
@deftypefun int isupper (int @var{c})
Returns true if @var{c} is an upper-case letter.
@end deftypefun

@cindex alphabetic character
@comment ctype.h
@comment ANSI
@deftypefun int isalpha (int @var{c})
Returns true if @var{c} is an alphabetic character (a letter).  If
@code{islower} or @code{isupper} is true of a character, then
@code{isalpha} is also true.

In some locales, there may be additional characters for which
@code{isalpha} is true--letters which are neither upper case nor lower
case.  But in the standard @code{"C"} locale, there are no such
additional characters.
@end deftypefun

@cindex digit character
@cindex decimal digit character
@comment ctype.h
@comment ANSI
@deftypefun int isdigit (int @var{c})
Returns true if @var{c} is a decimal digit (@samp{0} through @samp{9}).
@end deftypefun

@cindex alphanumeric character
@comment ctype.h
@comment ANSI
@deftypefun int isalnum (int @var{c})
Returns true if @var{c} is an alphanumeric character (a letter or
number); in other words, if either @code{isalpha} or @code{isdigit} is
true of a character, then @code{isalnum} is also true.
@end deftypefun

@cindex hexadecimal digit character
@comment ctype.h
@comment ANSI
@deftypefun int isxdigit (int @var{c})
Returns true if @var{c} 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}.
@end deftypefun

@cindex punctuation character
@comment ctype.h
@comment ANSI
@deftypefun int ispunct (int @var{c})
Returns true if @var{c} is a punctuation character.
This means any printing character that is not alphanumeric or a space
character.
@end deftypefun

@cindex whitespace character
@comment ctype.h
@comment ANSI
@deftypefun int isspace (int @var{c})
Returns true if @var{c} is a @dfn{whitespace} character.  In the standard
@code{"C"} locale, @code{isspace} returns true for only the standard
whitespace characters:

@table @code
@item ' '
space

@item '\f'
formfeed

@item '\n'
newline

@item '\r'
carriage return

@item '\t'
horizontal tab

@item '\v'
vertical tab
@end table
@end deftypefun

@cindex blank character
@comment ctype.h
@comment GNU
@deftypefun int isblank (int @var{c})
Returns true if @var{c} is a blank character; that is, a space or a tab.
This function is a GNU extension.
@end deftypefun

@cindex graphic character
@comment ctype.h
@comment ANSI
@deftypefun int isgraph (int @var{c})
Returns true if @var{c} is a graphic character; that is, a character
that has a glyph associated with it.  The whitespace characters are not
considered graphic.
@end deftypefun

@cindex printing character
@comment ctype.h
@comment ANSI
@deftypefun int isprint (int @var{c})
Returns true if @var{c} is a printing character.  Printing characters
include all the graphic characters, plus the space (@samp{ }) character.
@end deftypefun

@cindex control character
@comment ctype.h
@comment ANSI
@deftypefun int iscntrl (int @var{c})
Returns true if @var{c} is a control character (that is, a character that
is not a printing character).
@end deftypefun

@cindex ASCII character
@comment ctype.h
@comment SVID, BSD
@deftypefun int isascii (int @var{c})
Returns true if @var{c} is a 7-bit @code{unsigned char} value that fits
into the US/UK ASCII character set.  This function is a BSD extension
and is also an SVID extension.
@end deftypefun

@node Case Conversion,  , Classification of Characters, Character Handling
@section Case Conversion
@cindex character case conversion
@cindex case conversion of characters
@cindex converting case of characters

This section explains the library functions for performing conversions
such as case mappings on characters.  For example, @code{toupper}
converts any character to upper case if possible.  If the character
can't be converted, @code{toupper} returns it unchanged.

These functions take one argument of type @code{int}, which is the
character to convert, and return the converted character as an
@code{int}.  If the conversion is not applicable to the argument given,
the argument is returned unchanged.

@strong{Compatibility Note:} In pre-ANSI C dialects, instead of
returning the argument unchanged, these functions may fail when the
argument is not suitable for the conversion.  Thus for portability, you
may need to write @code{islower(c) ? toupper(c) : c} rather than just
@code{toupper(c)}.

These functions are declared in the header file @file{ctype.h}.
@pindex ctype.h

@comment ctype.h
@comment ANSI
@deftypefun int tolower (int @var{c})
If @var{c} is an upper-case letter, @code{tolower} returns the corresponding
lower-case letter.  If @var{c} is not an upper-case letter,
@var{c} is returned unchanged.
@end deftypefun

@comment ctype.h
@comment ANSI
@deftypefun int toupper (int @var{c})
If @var{c} is a lower-case letter, @code{tolower} returns the corresponding
upper-case letter.  Otherwise @var{c} is returned unchanged.
@end deftypefun

@comment ctype.h
@comment SVID, BSD
@deftypefun int toascii (int @var{c})
This function converts @var{c} to a 7-bit @code{unsigned char} value
that fits into the US/UK ASCII character set, by clearing the high-order
bits.  This function is a BSD extension and is also an SVID extension.
@end deftypefun

@comment ctype.h
@comment SVID
@deftypefun int _tolower (int @var{c})
This is identical to @code{tolower}, and is provided for compatibility
with the SVID.  @xref{SVID}.@refill
@end deftypefun

@comment ctype.h
@comment SVID
@deftypefun int _toupper (int @var{c})
This is identical to @code{toupper}, and is provided for compatibility
with the SVID.
@end deftypefun