The isdigit function tests c to determine if it is a decimal digit ('0'-'9').

The isdigit function returns a value of 1 if c is a decimal digit or a value of 0 if it is not.

isalnum, isalpha, iscntrl, isgraph, islower, isprint, ispunct, isspace, isupper, isxdigit

#include <ctype.h>
#include <stdio.h> /* for printf */

void tst_isdigit (void) {
  unsigned char i;
  char *p;

  for (i = 0; i < 128; i++) {
    p = (isdigit (i) ? "YES" : "NO");

    printf ("isdigit (%c) %s\n", i, p);
  }
}