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 ctype.h are provided for this purpose. 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 LC_CTYPE category; see the section called “Categories of Activities that Locales Affect”.)
The ISO C standard specifies two different sets of functions. The one set works on char type characters, the other one on wchar_t wide characters (the section called “Introduction to Extended Characters”).
This section explains the library functions for classifying characters. For example, 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:
if (isalpha (c)) printf ("The character `%c' is alphabetic.\n", c);
Each of the functions in this section tests for membership in a particular class of characters; each has a name starting with is. Each of them takes one argument, which is a character to test, and returns an int which is treated as a boolean value. The character argument is passed as an int, and it may be the constant value EOF instead of a real character.
The attributes of any given character can vary between locales. Chapter 8, for more information on locales.
These functions are declared in the header file ctype.h. int function>islower/function> (int c) Returns true if c is a lower-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.
int function>isupper/function> (int c) Returns true if c is an upper-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.
int function>isalpha/function> (int c) Returns true if c is an alphabetic character (a letter). If islower or isupper is true of a character, then isalpha is also true.
In some locales, there may be additional characters for which isalpha is true--letters which are neither upper case nor lower case. But in the standard "C" locale, there are no such additional characters.
int function>isdigit/function> (int c) Returns true if c is a decimal digit (0 through 9).
int function>isalnum/function> (int c) Returns true if c is an alphanumeric character (a letter or number); in other words, if either isalpha or isdigit is true of a character, then isalnum is also true.
int function>isxdigit/function> (int c) Returns true if c is a hexadecimal digit. Hexadecimal digits include the normal decimal digits 0 through 9 and the letters A through F and a through f.
int function>ispunct/function> (int c) Returns true if c is a punctuation character. This means any printing character that is not alphanumeric or a space character.
int function>isspace/function> (int c) Returns true if c is a whitespace character. In the standard "C" locale, isspace returns true for only the standard whitespace characters:
space
formfeed
newline
carriage return
horizontal tab
vertical tab
int function>isblank/function> (int c) Returns true if c is a blank character; that is, a space or a tab. This function is a GNU extension.
int function>isgraph/function> (int c) Returns true if c is a graphic character; that is, a character that has a glyph associated with it. The whitespace characters are not considered graphic.
int function>isprint/function> (int c) Returns true if c is a printing character. Printing characters include all the graphic characters, plus the space () character.
int function>iscntrl/function> (int c) Returns true if c is a control character (that is, a character that is not a printing character).
int function>isascii/function> (int c) Returns true if c is a 7-bit unsigned char value that fits into the US/UK ASCII character set. This function is a BSD extension and is also an SVID extension.