View source for Software:C character classification

{{C Standard Library}}

'''C character classification''' is an operation provided by a group of functions in the [[C standard library|ANSI C Standard Library]] for the [[C (programming language)|C programming language]]. These functions are used to test characters for membership in a particular class of characters, such as alphabetic characters, control characters, 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-language programmers working on the [[Software:Unix|Unix]] operating system developed [[Programming idiom|programming idiom]]s for classifying characters into different types. For example, for the [[ASCII]] character set, the following expression identifies a letter, when its value is ''true'':

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

As this may be expressed in multiple formulations, it became desirable to introduce short, standardized forms of such tests that were placed in the system-wide header file ''ctype.h''.

== Implementation ==
Unlike the above example, the character classification routines are not written as comparison tests. In most C libraries, they are written as static table lookups instead of macros or functions.

For example, an array of 256 eight-bit integers, arranged as bitfields, is created, where each bit corresponds to a particular property of the character, e.g., isdigit, isalpha. If the lowest-order bit of the integers corresponds to the isdigit property, the code could be written as

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

Early versions of [[Software:Linux|Linux]] used a potentially faulty method similar to the first code sample:

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

This can cause problems if when the macro expands, the expression substituted for ''x'' has a [[Side effect (computer science)|side effect]]. For example, if one calls ''isdigit(x++)'' or ''isdigit(run_some_program())''. It is not immediately evident that the argument to ''isdigit'' is evaluated twice. For this reason, the table-based approach is generally used.

== 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==
{{clear}}

{{CProLang|state=expanded}}

[[Category:C standard library]]

{{Sourceattribution|C character classification}}