isdigit() - decimal-digit character test
#include <ctype.h>
int isdigit (int c)
The isdigit(3) function tests for any decimal-digit character. In the POSIX/C locale, this is the characters:
0 1 2 3 4 5 6 7 8 9
The isdigit(3) function returns zero if the character tests false and returns non-zero if the character tests true.
isalnum(3)
isalpha(3)
isblank(3)
iscntrl(3)
isgraph(3)
islower(3)
isprint(3)
ispunct(3)
isspace(3)
isupper(3)
isxdigit(3)
tolower(3)
toupper(3)