Reverse a String in C

Many old books use strrev() to reverse a string. But this function is not part of standard C. It works in some Windows compilers only, and gives an error in GCC and Clang. So we write our own function, which works everywhere.

Method 1: print in reverse order

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

int main(void) {
    char s[] = "Hello";

    for (int i = strlen(s) - 1; i >= 0; i--) {
        printf("%c", s[i]);
    }
    printf("\n");
    return 0;
}

Output:

Method 2: reverse the string itself

We swap the first and last characters, then the second and second last, and so on until we reach the middle.

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

void reverse(char s[]) {
    int left = 0, right = strlen(s) - 1;

    while (left < right) {
        char temp = s[left];
        s[left] = s[right];
        s[right] = temp;
        left++;
        right--;
    }
}

int main(void) {
    char word[] = "Programming";

    reverse(word);
    printf("%s\n", word);
    return 0;
}

Output:

Palindrome check

A palindrome reads the same from both sides, like “madam”.

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

int main(void) {
    char s[] = "madam";
    int n = strlen(s), isPal = 1;

    for (int i = 0; i < n / 2; i++) {
        if (s[i] != s[n - 1 - i]) {
            isPal = 0;
            break;
        }
    }
    printf(isPal ? "Palindrome\n" : "Not a palindrome\n");
    return 0;
}

Output:

Next: C Convert String to Lowercase and Uppercase.

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