How can I swap keys and values in NSDictionary?

Question

I want to know how to invert a NSDictionary.

I've seen some crazy code like

NSDictionary *dict = ...;
NSDictionary *swapped  = [NSDictionary dictionaryWithObjects:dict.allKeys forKeys:dict.allValues];

which is according to documentation not safe at all since the order of allValues and allKeys is not guaranteed.

Answer 1

NSDictionary *dict = ...;    
NSMutableDictionary *swapped = [NSMutableDictionary new];
[dictionary enumerateKeysAndObjectsUsingBlock:^(id key, id value, BOOL *stop) {
    swapped[value] = key;
}];

Note that the values should also conform to the NSCopying protocol.

Answer 2

You're right, that code is crazy, but there are two ways to get an array of the values in the order given by an array of keys:

NSArray * keys = [dict allKeys];
NSArray * vals = [dict objectsForKeys:keys notFoundMarker:nil];

NSDictionary * inverseDict = [NSDictionary dictionaryWithObjects:keys
                                                         forKeys:vals];

Or

NSUInteger count = [dict count];
id keys[count];
id vals[count];
[dict getObjects:vals andKeys:keys];

NSDictionary * inverseDict = [NSDictionary dictionaryWithObjects:keys
                                                         forKeys:vals
                                                           count:count];

The former is obviously a lot nicer. As noted in hfossli's answer, the objects that were values in the original dictionary must conform to NSCopying in order to be used as keys in the inversion.