C Exercises: String is a palindrome or not

Last update on August 07 2025 12:45:53 (UTC/GMT +8 hours)

4. Palindrome String Callback Variants

Write a C program to check if a string (case-sensitive) is a palindrome or not using a callback function.

Sample Solution:

C Code:

#include <stdio.h>

#include <string.h>

#include <ctype.h>

// Define a function pointer type that takes two characters and returns an integer.
typedef int( * compare_func_t)(char, char);

// This function checks if a given string is a palindrome.
// It takes a string, its length, and a function pointer as arguments.
int is_palindrome(char * str, size_t len, compare_func_t compare) {
  // Loop through the first half of the string.
  for (size_t i = 0; i < len / 2; i++) {
    // Compare the i-th character from the start and end of the string using the provided comparison function.
    // If they are not equal, the string is not a palindrome.
    if (compare(str[i], str[len - i - 1]) != 0) {
      return 0;
    }
  }
  // If we make it through the loop without finding any non-matching characters, the string is a palindrome.
  return 1;
}

// This function compares two characters in a case-sensitive manner.
// It returns -1 if the first character is less than the second, 0 if they are equal, and 1 if the first character is greater than the second.
int compare_chars(char a, char b) {
  if (a < b) {
    return -1;
  } else if (a > b) {
    return 1;
  } else {
    return 0;
  }
}
// This function compares two characters in a case-insensitive manner.
// It returns -1 if the first character is less than the second, 0 if they are equal, and 1 if the first character is greater than the second.
// The function uses the tolower() function to convert the characters to lowercase before comparing them.
int compare_case_insensitive(char a, char b) {
  a = tolower(a);
  b = tolower(b);
  if (a < b) {
    return -1;
  } else if (a > b) {
    return 1;
  } else {
    return 0;
  }
}

int main() {
  char str1[] = "Madam";
  size_t len = strlen(str1);
  printf("String: %s", str1);

  // Test the string for palindrome-ness using the case-sensitive comparison function.
  if (is_palindrome(str1, len, compare_chars)) {
    printf("%s is a palindrome (case-sensitive).\n", str1);
  } else {
    printf("\n%s is not a palindrome (case-sensitive).\n", str1);
  }
  // Test the string for palindrome-ness using the case-insensitive comparison function.
  if (is_palindrome(str1, len, compare_case_insensitive)) {
    printf("%s is a palindrome (case-insensitive).\n", str1);
  } else {
    printf("%s is not a palindrome (case-insensitive).\n", str1);
  }

  char str2[] = "aba";
  len = strlen(str2);
  printf("\nString: %s", str2);
  // Test the string for palindrome-ness using the case-sensitive comparison function.
  if (is_palindrome(str2, len, compare_chars)) {
    printf("\n%s is a palindrome (case-sensitive).\n", str2);
  } else {
    printf("\n%s is not a palindrome (case-nsensitive).\n", str2);
  }
  // Test the string for palindrome-ness using the case-insensitive comparison function.
  if (is_palindrome(str2, len, compare_case_insensitive)) {
    printf("%s is a palindrome (case-insensitive).\n", str2);
  } else {
    printf("%s is not a palindrome (case-insensitive).\n", str2);
  }
  return 0;
}

Sample Output:

String: Madam
Madam is not a palindrome (case-sensitive).
Madam is a palindrome (case-insensitive).

String: aba
aba is a palindrome (case-sensitive).
aba is a palindrome (case-insensitive).

Explanation:

The above program defines three functions: is_palindrome(), compare_chars(), and compare_case_insensitive(). It also defines a function pointer type called compare_func_t.

is_palindrome() takes a string, its length, and a function pointer as arguments. It checks if the string is a palindrome by comparing its characters using the provided comparison function. The function returns 1 if the string is a palindrome, and 0 otherwise.

compare_chars() and compare_case_insensitive() are comparison functions that take two characters as arguments and return an integer (-1, 0, or 1) based on their relative values.

Flowchart:

For more Practice: Solve these Related Problems:

Write a C program to check if a string is a palindrome by reversing it using a callback function.
Write a C program to determine if a string is a palindrome after removing all non-alphanumeric characters via a callback.
Write a C program to compare a string with its reversed copy using a callback function for character comparison.
Write a C program to check if a string is a palindrome using a callback that applies a custom case-insensitive comparator.

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