C Pocket Reference

Character Classification and Case Mapping

A number of functions for classifying and changing the case of characters with type char are defined in the header file ctype.h . These functions, whose names begin with is... or to..., accept a one-character argument whose value is between 0 and 255, or EOF.

The is... functions, listed in Table 1-39, test whether the character is a member of a specific category of characters. They return “true,” i.e., a non-zero value, if the character is in the given category. If not, the return value is 0, or “false.”

Table 1-39. Functions for character classification

Category	Function
Letter	int isalpha( int c );
Lower-case letter	int islower( int c );
Upper-case letter	int isupper( int c );
Decimal digit	int isdigit( int c );
Hexadecimal digit	int isxdigit( int c );
Letter or decimal digit	int isalnum( int c );
Printable character	int isprint( int c );
Printable character other than space ' '	int isgraph( int c );
Whitespace character	int isspace( int c );
Punctuation mark	int ispunct( int c );
Control character	int iscntrl( int c );
Space or horizontal tabulator	int isblank( int c );(*)

The following example reads a character and then tests to see whether it is a digit:

int c = getchar();     // Read a character
if ( isdigit( c ) ) ...// Is it a decimal digit?

The to... functions are used to convert characters from upper- to lower-case and vice versa, as shown in Table 1-40.

Table 1-40. Case mapping functions ...