isalnum(), isalpha(), isblank(), iscntrl(), isdigit(), isgraph(), islower(), isprint(), ispunct(), isspace(), isupper(), isxdigit()

Tests for membership in subsets of the character set.

  #include <ctype.h> 
   int isalnum(int c);
   int isalpha(int c);
   int isblank(int c);
   int iscntrl(int c);
   int isdigit(int c);
   int isgraph(int c);
   int islower(int c);
   int isprint(int c);
   int ispunct(int c);
   int isspace(int c);
   int isupper(int c);
   int isxdigit(int c);

Parameter

c

A character value to test.

Remarks

These functions test ASCII characters ( 0x00 to 0x7F) and the EOF value. The results of these functions are not defined for character values in the range 0x80 to 0xFF.

The c argument is of type int so that the EOF value, which is outside the range of the char type, may also be tested.

The table below lists the behaviors of these functions.

Table 1. Character Testing Functions
Function	returns true if c is one of these values.
`isalnum(c)`	Alphanumeric characters. For the `"C"`locale, "a" to "z" , "A" to "Z" , or "0" to "9".
`isalpha(c)`	Alphabetic characters. For the `"C"`locale, "a" to "z" or "A" to "Z".
`isblank(c)`	Word-separating whitespace. For the `"C"`locale, the space and tab characters.
`iscntrl(c)`	Non-printing control characters. The delete character (0x7F) or an ordinary control character from 0x00 to 0x1F.
`isdigit(c)`	Numeric characters.
`isgraph(c)`	Non-space printing character.
`islower(c)`	A lowercase alphabetic character, from"a" to "z".
`isprint(c)`	A printable character, including the space character. The compliment of the subset that `iscntrl()`tests for.
`ispunct(c)`	A punctuation character. A punctuation character is not in the subsets for which `isalnum()`, `isblank()`, or `iscntrl()` return true.
`isspace(c)`	A whitespace character. A space, tab, return, new line, vertical tab, or form feed.
`isupper(c)`	An uppercase alphabetic character, from "A" to "Z".
`isxdigit(c)`	A hexadecimal character. From "0" to "9" , "a" to "f" , or "A" to "F" .

Listing: Character Testing Function Example


#include <stdio.h> 
#include <ctype.h> 
int main(void) 
{ 
char *test = "Fb6# 9,"; 
isalnum(test[0]) ? 
printf("%c is alphanumerical\n", test[0]) : 
printf("%c is not alphanumerical\n", test[0]); 
isalpha(test[0]) ? 
printf("%c is alphabetical\n", test[0]) : 
printf("%c is not alphabetical\n", test[0]); 
isblank(test[4]) ? 
printf("%c is a blank space\n", test[4]) : 
printf("%c is not a blank space\n", test[4]); 
iscntrl(test[0]) ? 
printf("%c is a control character\n", test[0]) : 
printf("%c is not a control character\n", test[0]); 
isdigit(test[2]) ? 
printf("%c is a digit\n", test[2]) : 
printf("%c is not a digit\n", test[2]) ; 
isgraph(test[0]) ? 
printf("%c is graphical \n", test[0]) : 
printf("%c is not graphical\n", test[0]); 
islower(test[1]) ? 
printf("%c is lowercase \n", test[1]) : 
printf("%c is not lowercase\n", test[1]); 
isprint(test[3]) ? 
printf("%c is printable\n", test[3]) : 
printf("%c is not printable\n", test[3]); 
ispunct(test[6]) ? 
printf("%c is a punctuation mark\n", test[6]) : 
printf("%c is not punctuation mark\n", test[6]); 
isspace(test[4]) ? 
printf("%c is a space\n", test[4]) : 
printf("%c is not a space\n", test[4]); 
isupper(test[0]) ? 
printf("%c is uppercase \n", test[1]) : 
printf("%c is not uppercase\n", test[1]); 
isxdigit(test[5]) ? 
printf("%c is a hexadecimal digit\n", test[5]) : 
printf("%c is not a hexadecimal digit\n", test[5]); 
return 0; 
} 
Output: 
F is alphanumerical 
F is alphabetical 
is a blank sapce 
F is not a control character 
6 is a digit 
F is graphical 
b is lower case 
# is printable 
, is a punctuation mark 
is a space 
F is uppercase 
9 is a hexadecimal digit