Reversing a string and testing for a palindrome

This lesson covers reversing a string and testing for a palindrome in Objective-C, with the aim of understanding the basics of string handling.

A string that reads the same forwards and backwards is a palindrome. Objective-C has no dedicated method for reversing a string, so the usual approach is to take the characters one at a time and assemble them in reverse.

In the sample code, a reverseString() function loops from str.length - 1 down to 0, appending one character at a time with [reversed appendFormat:@"%C", ...]. Note the %C specifier for a single character.

A common early stumble is needing an NSMutableString. NSString is immutable, so building up characters one at a time calls for the mutable version.

Reversal and palindrome checks rarely come up directly in practice, but they are a classic subject for practising how strings are handled.

📖 Reference code

NSString *reverseString(NSString *str) {
    NSMutableString *reversed = [NSMutableString stringWithCapacity:str.length];
    for (NSInteger i = str.length - 1; i >= 0; i--) {
        [reversed appendFormat:@"%C", [str characterAtIndex:i]];
    }
    return reversed;
}

BOOL isPalindrome(NSString *str) {
    return [str isEqualToString:reverseString(str)];
}

NSLog(@"%d", isPalindrome(@"racecar"));
NSLog(@"%d", isPalindrome(@"hello"));

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run Objective-C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).