Touch Code Magazine

Comparing strings Objective-C

This is just a quick debugging tip: sometimes for no reason string comparison in Objective-C won’t work:) You are sure myString cannot be @”this and that”, but then comparing the 2 strings results succeeds. And here is your code which looks perfect, but still says myString equals to @”this and that” :

if ([myString compare:@"TEST"]==NSOrderedSame)

Well check if myString is nil, then actually the condition in the if is true. In Objective-C you can send messages to nil objects, calling “compare” on a nil object will return 0 (zero) when compared to NSOrderedSame (which, haha, is as well the number 0), so this is how you end up with the result that actually a nil string is equal to any string you compare it. Keep that in mind.

ALWAYS when you compare a constant and an NSString put the constant first like this- saves a lot of debugging time:

if ([@"TEST" compare:myString]==NSOrderedSame)

This is absolutely the same condition, but this time you send the “compare” message to an instance of NSString containing the text “TEST” and then you compare to a nil object – wooop, they are not the same. In short- for correct results make sure you are not sending messages to nil objects.

So, a short discussion, but I hope it’s been helpful to understand why the problem happens and how to avoid it.

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