Question

Hey folks, beneath is a piece of code i used for a school assignment.
Whenever I enter a word, with an O in it (which is a capital o), it fails!
Whenever there is one or more capital O’s in this program, it returns false and logs : sentence not a palindrome.

A palindrome, for the people that dont know what a palindrome is, is a word that is the same read left from right, and backwards. (e.g. lol, kayak, reviver etc)
I found this bug when trying to check the ‘oldest’ palindrome ever found: SATOR AREPO TENET OPERA ROTAS.

When I change all the capital o’s to lowercase o’s, it works, and returns true.
Let me state clearly, with this piece of code ALL sentences/words with capital O’s return false. A single capital o is enough to fail this program.

-(BOOL)testForPalindrome:(NSString *)s position:(NSInteger)pos {
    NSString *string = s;
    NSInteger position = pos;
    NSInteger stringLength = [string length];
    NSString *charOne = [string substringFromIndex:position];
    charOne = [charOne substringToIndex:1];

    NSString *charTwo = [string substringFromIndex:(stringLength - 1 - position)];
    charTwo = [charTwo substringToIndex:1];
    if(position > (stringLength / 2)) {
        NSString *printableString = [NSString stringWithFormat:@"De following word or sentence is a palindrome: \n\n%@", string];
        NSLog(@"%@ is a palindrome.", string);
        [textField setStringValue:printableString];
        return YES;
    }
    if(charOne != charTwo) {
        NSLog(@"%@, %@", charOne, charTwo);
        NSLog(@"%i", position);
        NSLog(@"%@ is not a palindrome.", string);
        return NO;
    }
    return [self testForPalindrome:string position:position+1]; 
}

So, is this some weird bug in Cocoa?
Or am I missing something?

• B

Answer 1

This of course is not a bug in Cocoa, as you probably knew deep down inside.

Your compare method is causing this ‘bug in Cocoa’, you’re comparing the addresses of charOne and charTwo. Instead you should compare the contents of the string with the isEqualToString message.

Use:

if(![charOne isEqualToString:charTwo]) {

Instead of:

if(charOne != charTwo) {

Edit: tested it in a test project and can confirm this is the problem.