C/ctype.h/isdigit

    
//Header file:     #include <ctype.h>  
//Declaration:     int isdigit(int ch); 
//Return:          returns nonzero if ch is a digit or zero is returned. 
  

  #include <ctype.h>
  #include <stdio.h>
  int main(void)
  {
    char ch;
    for(;;) {
      ch = getchar();
      if(ch == "."){
          break;
      }
      if(isdigit(ch)){
          printf("%c is a digit\n", ch);
      }
    }
    return 0;
  }
         
/*
1
1 is a digit
2
2 is a digit
3
3 is a digit
f
.
*/