C Project: Palindrome Checker

What you will build

A palindrome reads the same forwards and backwards. radar is one. A single letter such asC is one. StudyGrid is not. The checker must ignore case so Radar andradar both count as yes.

C strings are char arrays that end with a null byte. You will use strlen from<string.h> and tolower from <ctype.h>. Compile withTry it in C at /c/try,.

Two indices, one string

Walk from both ends. Index i starts at 0. Index j starts at length minus one. Compare the characters. If they differ, the string is not a palindrome. If they match, move i up andj down until the indices meet.

#include <stdio.h>
#include <string.h>

int main(void) {
  char word[] = "radar";
  int i = 0;
  int j = (int)strlen(word) - 1;
  int ok = 1;

  while (i < j) {
    if (word[i] != word[j]) {
      ok = 0;
      break;
    }
    i++;
    j--;
  }

  printf("%s: %s\n", word, ok ? "yes" : "no");
  return 0;
}

This version is case-sensitive. Radar would fail because 'R' is not'r'. The next listing folds both sides to lowercase before comparing.

Ignore case with tolower

tolower expects a value that fits in unsigned char (or EOF). Cast each character before you call it. Compare the folded values, not the originals. The letters in the string stay unchanged; only the test is case-insensitive.

#include <stdio.h>
#include <string.h>
#include <ctype.h>

int is_palindrome(const char *s) {
  int i = 0;
  int j = (int)strlen(s) - 1;
  while (i < j) {
    unsigned char a = (unsigned char)s[i];
    unsigned char b = (unsigned char)s[j];
    if (tolower(a) != tolower(b)) {
      return 0;
    }
    i++;
    j--;
  }
  return 1;
}

int main(void) {
  printf("%s\n", is_palindrome("Radar") ? "yes" : "no");
  printf("%s\n", is_palindrome("grid") ? "yes" : "no");
  return 0;
}

Test radar, C, and StudyGrid

Put the required samples in an array of string pointers. Print yes or no for each. A one-character string is a palindrome because the two indices never pass each other: length 1 givesj == 0, so the loop body does not run and the function returns 1.

#include <stdio.h>
#include <string.h>
#include <ctype.h>

int is_palindrome(const char *s) {
  int i = 0;
  int j = (int)strlen(s) - 1;
  while (i < j) {
    unsigned char a = (unsigned char)s[i];
    unsigned char b = (unsigned char)s[j];
    if (tolower(a) != tolower(b)) {
      return 0;
    }
    i++;
    j--;
  }
  return 1;
}

int main(void) {
  const char *samples[] = {"radar", "C", "StudyGrid"};
  int n = (int)(sizeof samples / sizeof samples[0]);

  for (int k = 0; k < n; k++) {
    printf("%s: %s\n", samples[k], is_palindrome(samples[k]) ? "yes" : "no");
  }
  return 0;
}

Expected output: radar yes, C yes, StudyGrid no. If StudyGrid prints yes, the indices are not moving, or you compared only the first character.

A few extra checks

Empty text is a palindrome by the same loop rule: length 0 makes j equal to -1, thewhile condition fails, and the function returns 1. Mixed-case palindromes such asRacecar should print yes only after tolower is in place.

#include <stdio.h>
#include <string.h>
#include <ctype.h>

int is_palindrome(const char *s) {
  int i = 0;
  int j = (int)strlen(s) - 1;
  while (i < j) {
    unsigned char a = (unsigned char)s[i];
    unsigned char b = (unsigned char)s[j];
    if (tolower(a) != tolower(b)) {
      return 0;
    }
    i++;
    j--;
  }
  return 1;
}

int main(void) {
  const char *samples[] = {
      "radar", "C", "StudyGrid", "Racecar", "Ada", "",
  };
  int n = (int)(sizeof samples / sizeof samples[0]);

  for (int k = 0; k < n; k++) {
    const char *label = samples[k][0] == '\0' ? "(empty)" : samples[k];
    printf("%s: %s\n", label, is_palindrome(samples[k]) ? "yes" : "no");
  }
  return 0;
}

Common mistakes

• Comparing with == on two arrays. That compares addresses. Walk characters, or usestrcmp only when you mean exact full-string equality.
• Forgetting tolower, so Radar fails. The lead for this project is case-insensitive.
• Calling tolower on a plain char that might be negative. Cast tounsigned char first.
• Using j = strlen(s) without subtracting one. Then the last index is the null terminator and every non-empty string fails.

Practice

1. Skip spaces so a phrase such as never odd or even can count as a palindrome if you want that rule.
2. Print the length of each sample beside yes or no.
3. Add level, python, and AbBa to the sample list and write the expected yes/no lines before you compile.