Why is iterating through NSArray is faster than iterating through NSSet?

Question

I was wondering Why is iterating through NSArray is faster than iterating through NSSet? I imaging that it has something to do with the fact that NSArray is ordered while the NSSet is not but I what a certified answer instead of just guessing.

Edit:

my question was: why is it faster which is not explained in that topic. and not if its faster.

http://www.cocoawithlove.com/2008/08/nsarray-or-nsset-nsdictionary-or.html — GoodSp33d, Jul 02 '15 at 10:24
possible duplicate of [Is NSSet faster than NSArray?](http://stackoverflow.com/questions/22679418/is-nsset-faster-than-nsarray) — GoodSp33d, Jul 02 '15 at 10:24
@GoodSp33d the question is about different thing, the poster already knows that NSArray iteration is faster, he wonders why. — pronebird, Jul 02 '15 at 10:26
@Andy That post has sufficient information with stats. (which was part of that linked answer) — GoodSp33d, Jul 02 '15 at 10:33
In these posts, he dose not explain why is iterating through nsarray is faster he only shows benchmarks, which is not what i have asked. — AmirZ, Jul 04 '15 at 11:20

score 9 · Answer 1 · answered Nov 12 '16 at 07:25

@Amin's answer is not accurate in some aspects.

We are talking about Iterating, not searching.
Searching in a Set is of course faster than searching in an Array, as the first takes only O(1) time, while the second takes O(n) time.
In my opinion, NSArray is faster than NSSet(Iterating), because NSArray's accessing time is exact O(1), though NSSet is O(1) too, but it's amortized.
You can not even insert an object into NSSet

Can you explain a bit more about "Obviously the extra condition for sets is more expensive than the extra condition for arrays."

score 8 · Accepted Answer · answered Jul 02 '15 at 11:57

First: You cannot say that NSArray is faster than NSSet. As you get from the link in the comments, it depends on what you are doing. Searching for an object in an instance of NSSet is faster by far. And this is, what you want to do, when you choose NSSet.

There are two differences between sets and arrays.

arrays has to keep the order, sets don't.
sets has to take care about uniqueness, arrays don't.

So it seems to be clever, to have completely different implementations for both. This can lead to different runtime behavior. So the correct Q would be: Where is the surprise?

Obviously the extra condition for sets is more expensive than the extra condition for arrays.

BTW: It is impossible for an instance of NSSet, to fulfill the promise about uniqueness it makes. This is, because uniqueness is only checked, when you insert an object into the set. When you change an object after inserting it, it can become equal to another object in the set.

You're last paragraph is interesting. Do you have any documentation to back that up I can read please? — James Webster, Jul 10 '15 at 12:49
No, this is not documented. Simply try it with two mutable strings in an array. http://stackoverflow.com/questions/30535292/adding-strings-to-nsmutablesets — Amin Negm-Awad, Jul 12 '15 at 01:26
Question talks about iterating and not searching. So scott is right — adev, Jul 15 '17 at 19:44

Why is iterating through NSArray is faster than iterating through NSSet?

2 Answers2