isdigit()

Syntax:

#include <ctype.h>
int isdigit( int ch );

Description:
The function isdigit() returns non-zero if its argument is a digit between 0 and 9. Otherwise, zero is returned.

Example:

char c;
scanf( "%c", &c );
if( isdigit(c) )
  printf( "You entered the digit %c\n", c );