#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <assert.h>

#ifndef LENGTH
#define LENGTH 30
#endif

#define ZERO '0'

void read_string(char *str);
char read_char(char *prompt);
int check_bounds(char lower, char upper);
int count_digits(char *str, char lower, char upper);

int main(void)
{
   char str[LENGTH];
   char lower;
   char upper;

   read_string(str);
   lower = read_char("Lower Bound (0-9): ");
   upper = read_char("Upper Bound (0-9): ");

   assert(check_bounds(lower, upper));
 
   int count = count_digits(str, lower, upper);
   printf("The String contains %d digits between %c and %c, inclusive.\n", count, lower, upper);
   return 0;

}

void read_string(char *str)
{
   printf("String? ");
   char *rtn = fgets(str, LENGTH, stdin);
   if (rtn == NULL) {
      printf("Error\n");
   }
}

char read_char(char *prompt)
{
   printf("%s", prompt);
   char rtn = getchar();
   char ch;
   while ((ch = getchar()) != '\n') {
      ; //do nothing
   }
   return rtn;
}
int check_bounds(char lower, char upper) 
{
   if (isdigit(lower) && isdigit(upper)) {
      int l = lower - ZERO;
      int u = upper - ZERO;
      
      return (l <= u);
   }
   
   return 0;
}

int count_digits(char *s, char lower, char upper) 
{
   int count = 0;
   for (; *s != '\0'; s++) {
      if (*s >= lower && *s <= upper)
         count++;
   }

   return count;
}