C program to check whether a given string is palindrome or not

A palindrome is a word, sentence, or sequence that reads the same forward and backward — ignoring differences in capitalization for word-level palindromes. Examples: “racecar”, “GADAG”, “level”, “9009”. The most efficient check uses a two-pointer approach: one pointer starts at the left end, one at the right, and they advance toward each other until they meet or find a mismatch.

The original post used conio.h, void main(), and a non-standard initializer revString[25]={''}. It also manually built a reversed copy then compared — which is correct but allocates an extra buffer unnecessarily. The two-pointer approach in this rewrite needs O(1) extra space.

C Program: Palindrome String Check (Two-Pointer)

/* Check if a string is a palindrome (without strrev)
 * Compile: gcc -ansi -Wall -Wextra palindrome_str.c -o palindrome_str */
#include <stdio.h>
#include <string.h>

int is_palindrome(const char *s)
{
    int left = 0;
    int right = (int)strlen(s) - 1;
    while (left < right) {
        if (s[left] != s[right])
            return 0;
        left++;
        right--;
    }
    return 1;
}

int main(void)
{
    char s[256];
    printf("Enter a string: ");
    if (fgets(s, sizeof(s), stdin) == NULL) return 1;
    s[strcspn(s, "\n")] = '\0';

    if (is_palindrome(s))
        printf("\"%s\" is a palindrome.\n", s);
    else
        printf("\"%s\" is NOT a palindrome.\n", s);
    return 0;
}

How to Compile and Run

gcc -ansi -Wall -Wextra palindrome_str.c -o palindrome_str
./palindrome_str

Sample Output

Enter a string: GADAG
"GADAG" is a palindrome.

Enter a string: racecar
"racecar" is a palindrome.

Enter a string: hello
"hello" is NOT a palindrome.

Enter a string: A
"A" is a palindrome.

Two-Pointer Trace — “racecar” (length 7)

Two-Pointer Trace — “hello” (length 5)

Code Explanation

• Two-pointer technique — start left at index 0 and right at strlen(s) - 1. Compare the characters at both ends. If they match, move both pointers inward. If they don’t match, the string is not a palindrome. When left and right cross or meet (left ≥ right), all pairs matched — it is a palindrome.
• O(n/2) comparisons, O(1) space — each pair is checked once. The loop runs at most n/2 iterations. No reversed copy of the string is needed — only two integer index variables. Compare to the original approach (build reversed string + strcmp) which uses O(n) extra memory.
• Single-character strings are palindromes — strlen("A") - 1 = 0, so right = 0 and left = 0. The condition left < right is false immediately — the loop never executes and the function returns 1 (palindrome). Correct by definition.
• Empty string edge case — strlen("") - 1 = -1. Since right is int, it becomes -1, and left < right (0 < -1) is immediately false. Returns 1 — the empty string is technically a palindrome.
• Case sensitivity — this program is case-sensitive: “Racecar” is NOT a palindrome because ‘R’ != ‘r’. For case-insensitive checking, convert each character with tolower() before comparing: if (tolower((unsigned char)s[left]) != tolower((unsigned char)s[right])).

What This Program Teaches

• The two-pointer pattern — two pointers moving toward each other from both ends of an array is one of the most useful patterns in competitive programming and interviews. It solves palindrome check, two-sum in a sorted array, container with most water, and many others in O(n) time with O(1) space.
• strlen returns size_t, not int — strlen() returns size_t, which is an unsigned type. Subtracting 1 from strlen("") would give a very large positive number if stored in size_t (unsigned underflow). Casting to int first gives the expected -1. Always be careful with unsigned arithmetic when computing lengths.
• Separate the logic into is_palindrome() — extracting the check into a function makes main() clean and makes the check reusable. A function that returns int (0 = false, 1 = true) is the idiomatic C boolean.

Related Programs

• Palindrome Number Check in C
• Substring Search with strstr() in C
• String Functions in C — Complete Guide
• Compare Strings in C
• Pointers in C — Complete Guide

Recommended book:
The C Programming Language — Kernighan & Ritchie (India) |
(US)
 | 
C Programming: A Modern Approach — K.N. King (India) |
(US)

Practice what you learned: C Aptitude Questions — or try our C Programming Quiz App on Android.