How to turn an NSArray of strings into an array of unique strings, in the same order?

Question

If you have an NSArray of strings

{ @\"ONE\", @\"ONE\", @\"ONE\", \"TWO\", @\"THREE\", @\"THREE\" }

How would I turn that into

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Answer 1

My initial thought was that you could do:

NSArray * a = [NSArray arrayWithObjects:@"ONE", @"ONE", @"ONE", @"TWO", @"THREE", @"THREE", nil];
NSLog(@"%@", [a valueForKeyPath:@"@distinctUnionOfObjects.self"]);

But that does not maintain order. Therefore, you have to do it manually:

NSArray * a = [NSArray arrayWithObjects:@"ONE", @"ONE", @"ONE", @"TWO", @"THREE", @"THREE", nil];
NSMutableArray * unique = [NSMutableArray array];
NSMutableSet * processed = [NSMutableSet set];
for (NSString * string in a) {
  if ([processed containsObject:string] == NO) {
    [unique addObject:string];
    [processed addObject:string];
  }
}

I use an NSMutableSet for determining if I've already come across this entry before (as opposed to [unique containsObject:string], since a set will have O(1) lookup time, and an array has O(n) lookup time. If you're only dealing with a small number of objects, then this won't matter. However, if the source array is very large, then using the set to determine uniqueness may add a bit of a speed boost. (however, you should use Instruments to profile your code and see if it's necessary)