Figure 10.4  Recursive Function to Count a Character in a String
/*
 *  Count the number of occurrences of character ch in string str
 */
int
count(char ch, const char *str)
{
 
      int ans;

      if (str[0] == '\0')                           /*  simple case  */
            ans = 0;
      else                      /*  redefine problem using recursion */
            if (ch == str[0])   /*  first character must be counted  */
                  ans = 1 + count(ch, &str[1]);
            else                /*  first character is not counted   */
                  ans = count(ch, &str[1]);

      return (ans);
}