Question

Aim: To obtain an NSArray containing unique keys for given NSDictionary(s) using elegant code

Example Code with Current Working Solution:

NSArray *data = [[NSArray alloc] initWithObjects:
                 [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithInt:1], @"a", [NSNumber numberWithInt:2], @"b", nil],
                 [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithInt:3], @"b", [NSNumber numberWithInt:4], @"c", nil],
                 [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithInt:5], @"a", [NSNumber numberWithInt:6], @"c", nil],
                 [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithInt:7], @"b", [NSNumber numberWithInt:8], @"a", nil],
                 [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithInt:8], @"c", [NSNumber numberWithInt:9], @"b", nil],
                 nil];

// create an NSArray of all the dictionary keys within the NSArray *data
NSMutableSet *setKeys = [[NSMutableSet alloc] init]; 
for (int i=0; i<[data count]; i++) {
    [setKeys addObjectsFromArray:[[data objectAtIndex:i] allKeys]];
}
NSArray *arrayKeys = [setKeys allObjects];
NSLog(@"arrayKeys: %@", arrayKeys);

Which returns the desired array of keys:

2012-06-11 16:52:57.351 test.kvc[6497:403] arrayKeys: (
    a,
    b,
    c
)

Question: Is there a more elegant way of approaching this? Surely there must be some KVC approach that can get all the keys without having to iterate through the array? I’ve been looking at Apple Developer Documentation and can’t see a solution. Any ideas? (looking at purely elegance of code rather than performance).

Answer 1

Normally you could use KVC by doing something like this:

NSArray *uniqueKeys = [data valueForKeyPath:@"@distinctUnionOfArrays.allKeys";

However NSDictionary overrides the valueForKey: selector which is used by the KVC internals, so this will not work correctly.

The documentation for NSDictionary’s valueForKey: method tells us that:

If key does not start with “@”, invokes objectForKey:. If key does start with “@”, strips the “@” and invokes [super valueForKey:] with the rest of the key.

So we just insert an @ before allKeys:

NSArray *uniqueKeys = [data valueForKeyPath:@"@distinctUnionOfArrays.@allKeys"];

And we get what we want:

(lldb) po [data valueForKeyPath:@"@distinctUnionOfArrays.@allKeys"]
(id) $14 = 0x07bb2fc0 <__NSArrayI 0x7bb2fc0>(
c,
a,
b
)