NSCountedSet

I just got a helpful comment on an old post (here) suggesting the use of an NSCountedSet. I'd never heard of this class, but it's described in the Apple docs about Collections (here). Besides the usual characters

NSArray NSMutableArray NSDictionary NSMutableDictionary NSIndexSet

there are several others including:

NSPointerArray NSPointerFunctions NSMapTable NSHashTable NSCountedSet

Let's show off the last one, from PyObjC. Here's the listing:

from Foundation import * L = list('xyyzzz') S = NSCountedSet.alloc().initWithArray_(L) def show(S): e = S.objectEnumerator() while True: obj = e.nextObject() if not obj: break print obj, S.countForObject_(obj) print show(S) N = S.countForObject_('z') for i in range(N): S.removeObject_('z') print 'z', S.countForObject_('z') print 'w', S.countForObject_('w')

and the output:

x 1 y 2 z 3 z 2 z 1 z 0 w 0

It feels a bit strange, not only can you get the count of an object that's not in the set (as in Python), but as the docs say:

removeObject: does nothing if anObject is not present in the set