Editing C character classification

{{Short description|Operations in the C standard library that classify characters}}
{{refimprove|date=October 2011}}
{{C Standard Library}}

'''C character classification''' is a group of operations in the [[C standard library]] that test a [[Character (computing)|character]] for membership in a particular class of characters; such as alphabetic, control, etc. Both single-byte, and wide characters are supported.<ref>{{cite book | url=http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf  | title=ISO/IEC 9899:1999 specification | at=p. 193, § 7.4}}</ref>

== History ==
Early [[c (programming language)|C]] [[programmer]]s working on the [[Unix]] operating system developed [[programming idiom]]s for classifying characters. For example, the following [[source code|code]] evaluates as ''true'' for an [[ASCII]] letter character <code>c</code>:

<syntaxhighlight lang="c">
('A' <= c && c <= 'Z') || ('a' <= c && c <= 'z')
</syntaxhighlight>

Eventually, the interface to common character classification functionality was codified in the C standard library file ''ctype.h''.

== Implementation ==
For performance, the standard character classification functions are usually implemented as macros instead of [[function (programming)|functions]]. But, due to limitations of macro evaluation, they are generally not implemented today as they were in early versions of [[Linux]] like:

<syntaxhighlight lang="c">
#define isdigit(c) ((c) >= '0' && (c) <= '9')
</syntaxhighlight>

This can lead to an error when the macro parameter <code>x</code> is expanded to an expression with a [[Side effect (computer science)|side effect]]; for example: <code>isdigit(x++)</code>. If the implementation was a function, then x would be incremented only once. But for this macro definition it is incremented twice.

To eliminate this problem, a common implementation is for the macro to use table lookup. For example, the standard library provides an array of 256 integers {{endash}} one for each character value {{endash}} that each contain a bit-field for each supported classification. A macro references an integer by character value index and accesses the associated bit-field. For example, if the low bit indicates whether the character is a digit, then the <code>isdigit</code> macro could be written as:

<syntaxhighlight lang="c">
#define isdigit(c) (TABLE[c] & 1)
</syntaxhighlight>

The macro argument, <code>c</code>, is referenced only once, so is evaluated only once.

== Overview of functions ==
The functions that operate on single-byte characters are defined in ''ctype.h'' header file (''cctype'' in C++). 
The functions that operate on wide characters are defined in ''wctype.h'' header file (''cwctype'' in C++).

The classification is evaluated according to the effective locale.

{| class="wikitable" style="font-size:0.85em;"
! Byte<br>character
! Wide<br>character
! Description
|-
| {{anchor|isalnum}}<code>[http://en.cppreference.com/w/c/string/byte/isalnum isalnum]</code>
| {{anchor|iswalnum}}<code>[http://en.cppreference.com/w/c/string/wide/iswalnum iswalnum]</code>
| checks whether the operand is alphanumeric
|-
| {{anchor|isalpha}}<code>[http://en.cppreference.com/w/c/string/byte/isalpha isalpha]</code>
| {{anchor|iswalpha}}<code>[http://en.cppreference.com/w/c/string/wide/iswalpha iswalpha]</code>
| checks whether the operand is alphabetic
|-
| {{anchor|islower}}<code>[http://en.cppreference.com/w/c/string/byte/islower islower]</code>
| {{anchor|iswlower}}<code>[http://en.cppreference.com/w/c/string/wide/iswlower iswlower]</code>
| checks whether the operand is lowercase
|-
| {{anchor|isupper}}<code>[http://en.cppreference.com/w/c/string/byte/isupper isupper]</code>
| {{anchor|iswupper}}<code>[http://en.cppreference.com/w/c/string/wide/iswupper iswupper]</code>
| checks whether the operand is an uppercase
|-
| {{anchor|isdigit}}<code>[http://en.cppreference.com/w/c/string/byte/isdigit isdigit]</code>
| {{anchor|iswdigit}}<code>[http://en.cppreference.com/w/c/string/wide/iswdigit iswdigit]</code>
| checks whether the operand is a digit
|-
| {{anchor|isxdigit}}<code>[http://en.cppreference.com/w/c/string/byte/isxdigit isxdigit]</code>
| {{anchor|iswxdigit}}<code>[http://en.cppreference.com/w/c/string/wide/iswxdigit iswxdigit]</code>
| checks whether the operand is hexadecimal
|-
| {{anchor|iscntrl}}<code>[http://en.cppreference.com/w/c/string/byte/iscntrl iscntrl]</code>
| {{anchor|iswcntrl}}<code>[http://en.cppreference.com/w/c/string/wide/iswcntrl iswcntrl]</code>
| checks whether the operand is a control character
|-
| {{anchor|isgraph}}<code>[http://en.cppreference.com/w/c/string/byte/isgraph isgraph]</code>
| {{anchor|iswgraph}}<code>[http://en.cppreference.com/w/c/string/wide/iswgraph iswgraph]</code>
| checks whether the operand is a graphical character
|-
| {{anchor|isspace}}<code>[http://en.cppreference.com/w/c/string/byte/isspace isspace]</code>
| {{anchor|iswspace}}<code>[http://en.cppreference.com/w/c/string/wide/iswspace iswspace]</code>
| checks whether the operand is [[Whitespace character|space]]
|-
| {{anchor|isblank}}<code>[http://en.cppreference.com/w/c/string/byte/isblank isblank]</code>
| {{anchor|iswblank}}<code>[http://en.cppreference.com/w/c/string/wide/iswblank iswblank]</code>
| checks whether the operand is a blank space character
|-
| {{anchor|isprint}}<code>[http://en.cppreference.com/w/c/string/byte/isprint isprint]</code>
| {{anchor|iswprint}}<code>[http://en.cppreference.com/w/c/string/wide/iswprint iswprint]</code>
| checks whether the operand is a printable character
|-
| {{anchor|ispunct}}<code>[http://en.cppreference.com/w/c/string/byte/ispunct ispunct]</code>
| {{anchor|iswpunct}}<code>[http://en.cppreference.com/w/c/string/wide/iswpunct iswpunct]</code>
| checks whether the operand is punctuation
|-
| {{anchor|tolower}}<code>[http://en.cppreference.com/w/c/string/byte/tolower tolower]</code>
| {{anchor|towlower}}<code>[http://en.cppreference.com/w/c/string/wide/towlower towlower]</code>
| converts the operand to lowercase
|-
| {{anchor|toupper}}<code>[http://en.cppreference.com/w/c/string/byte/toupper toupper]</code>
| {{anchor|towupper}}<code>[http://en.cppreference.com/w/c/string/wide/towupper towupper]</code>
| converts the operand to uppercase
|-
| {{n/a}}
| {{anchor|iswctype}}<code>[http://en.cppreference.com/w/c/string/wide/iswctype iswctype]</code>
| checks whether the operand falls into specific class
|-
| {{n/a}}
| {{anchor|towctrans}}<code>[http://en.cppreference.com/w/c/string/wide/towctrans towctrans]</code>
| converts the operand using a specific mapping
|-
| {{n/a}}
| {{anchor|wctype}}<code>[http://en.cppreference.com/w/c/string/wide/wctype wctype]</code>
| returns a wide character class to be used with <code>iswctype</code>
|-
| {{n/a}}
| {{anchor|wctrans}}<code>[http://en.cppreference.com/w/c/string/wide/wctrans wctrans]</code>
| returns a transformation mapping to be used with <code>towctrans</code>
|}

==References==
{{reflist}}

==External links==
{{wikibooks|A Little C Primer|C Character Class Test Library}}
{{wikibooks|C Programming|C character classification|C Programming/C Reference}}
{{clear}}

{{CProLang|state=expanded}}

[[Category:C standard library]]
{{Use dmy dates|date=October 2017}}