Exercise 11-2 - CBD page 383 - Kurt Nørmark

/* Programmed by Kurt Normark, April 2003 */

#include <stdio.h>

int is_palindrome_len(char *str, int lgt);

int is_palindrome(char *str){
  int lgt = strlen(str);
  return is_palindrome_len(str, lgt);
}

int is_palindrome_len(char *str, int lgt){
  int result;

  if (lgt >= 2)
   result = 
     str[0] == str[lgt-1] &&
     is_palindrome_len(str+1, lgt-2);
  else
    result = 1;

  return result;
}


int main(void) {

  char str[100];

  do{
    printf("Enter string: ");
    scanf("%s", str);

    if (strcmp(str,"exit") != 0){
      if (is_palindrome(str))
        printf("%s is a palindrome\n", str);
      else
        printf("%s is NOT a palindrome\n", str);
    }
  }
  while (strcmp(str,"exit") != 0);

  return 0;
}

In this dissection we explain the predicate (boolean function) which finds out if a string is a palindrome. A palindrome reads the same both forward and backward.

 

	int is_palindrome(char *str){ int lgt = strlen(str); return is_palindrome_len(str, lgt); }
	The function is_palindrome uses a helping function is_palindrome_len, which also takes the string length as a parameter. The helping function is programmed in order to avoid repeated calculations of the length of the palindrome candiate string.

 

int is_palindrome_len(char *str, int lgt){ int result; if (lgt >= 2) result = str[0] == str[lgt-1] && is_palindrome_len(str+1, lgt-2); else result = 1; return result; }

The function is_palindrome_len does the real work. First, if the length of the string is less than two, the string is always considered as a palindrome. In that case, we just return true (1). In case the string length is 2 or more, the string is a palindrome if the first and last letter in the string are equal, and if the 'middle part of the string' recursively is a palindrome. As an example, the middle part of the string "computer" is "ompute". Notice the way we use pointer arithmetic to find the middle part of a string.

 

	int main(void) { char str[100]; do{ printf("Enter string: "); scanf("%s", str); if (strcmp(str,"exit") != 0){ if (is_palindrome(str)) printf("%s is a palindrome\n", str); else printf("%s is NOT a palindrome\n", str); } } while (strcmp(str,"exit") != 0); return 0; }
	In the main function we prompt the user for a string which is exercised for the palindrome property. Around this we have a simple loop, which terminates if we enter the string "exit".