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

The image from Apple's Documentation describes it very well:

Array is an ordered (order is maintained when you add) sequence of elements

[array addObject:@1];
[array addObject:@2];
[array addObject:@3];
[array addObject:@4];
[array addObject:@6];
[array addObject:@4];
[array addObject:@1];
[array addObject:@2];

[1, 2, 3, 4, 6, 4, 1, 2]

Set is a distinct (no duplicates), unordered list of elements

[set addObject:@1];
[set addObject:@2];
[set addObject:@3];
[set addObject:@4];
[set addObject:@6];
[set addObject:@4];
[set addObject:@1];
[set addObject:@2];

[1, 2, 6, 4, 3]