Question

I’ve noticed that the enumerated output of a NSDictionary is not returning the same sequence as the order in which the items were added.

NSMutableArray *arrKeys;
NSMutableArray *arrValues;
NSDictionary *items;

arrKeys = [NSMutableArray arrayWithCapacity:5];
arrValues = [NSMutableArray arrayWithCapacity:5];

[arrKeys addObject:@"A"]; [arrValues addObject:@"First"];
[arrKeys addObject:@"B"]; [arrValues addObject:@"Second"];
[arrKeys addObject:@"C"]; [arrValues addObject:@"Third"];
[arrKeys addObject:@"D"]; [arrValues addObject:@"Fourth"];
[arrKeys addObject:@"E"]; [arrValues addObject:@"Last"];

items = [NSDictionary dictionaryWithObjects:arrValues forKeys:arrKeys];

for (NSString *aKey in items) {
    NSLog(@"%@=%@", aKey, [items valueForKey:aKey]);
}

Returns…

A=First
D=Fourth
B=Second
E=Last
C=Third

I appreciate that a Dictionary allows you to randomly access items by key, but is there some rationale for the arbitrary order they are returned while enumerating the collection?

Answer 1

That’s just that, an NSDictionary does not guarantee iteration order. This is consistent with the behavior you’d expect from a typical hash table — optimized for quick lookup, but not for ordered iteration.

Why not iterate through arrKeys that you already have?