When is it better to use an NSSet over an NSArray?

Question

I have used NSSets many times in my apps, but I have never created one myself.

When is it better to use an NSSet as opposed to an NSArray and why?

Answer 1

When the order of the items in the collection is not important, sets offer better performance for finding items in the collection.

The reason is that a set uses hash values to find items (like a dictionary) while an array has to iterate over its entire contents to find a particular object.