NSDictionary with ordered keys

Question

I'm curious if this is a situation any other folks have found themselves in. I have an NSDictionary (stored in a plist) that I'm basically using as an associative array (strings as keys and values). I want to use the array of keys as part of my application, but I'd like them to be in a specific order (not really an order that I can write an algorithm to sort them into). I could always store a separate array of the keys, but that seems kind of kludgey because I'd always have to update the keys of the dictionary as well as the values of the array, and make sure they always correspond. Currently I just use [myDictionary allKeys], but obviously this returns them in an arbitrary, non-guaranteed order. Is there a data structure in Objective-C that I'm missing? Does anyone have any suggestions on how to more elegantly do this?

Answer 1

The solution of having an associated NSMutableArray of keys isn't so bad. It avoids subclassing NSDictionary, and if you are careful with writing accessors, it shouldn't be too hard to keep synchronised.

Answer 2

If you're going to subclass NSDictionary you need to implement these methods as a minimum:

• NSDictionary
  • -count
  • -objectForKey:
  • -keyEnumerator
• NSMutableDictionary
  • -removeObjectForKey:
  • -setObject:forKey:
• NSCopying/NSMutableCopying
  • -copyWithZone:
  • -mutableCopyWithZone:
• NSCoding
  • -encodeWithCoder:
  • -initWithCoder:
• NSFastEnumeration (for Leopard)
  • -countByEnumeratingWithState:objects:count:

The easiest way to do what you want is to make a subclass of NSMutableDictionary that contains its' own NSMutableDictionary that it manipulates and an NSMutableArray to store an ordered set of keys.

If you're never going to encode your objects you could conceivable skip implementing -encodeWithCoder: and -initWithCoder:

All of your method implementations in the 10 methods above would then either go directly through your hosted dictionary or your ordered key array.

Answer 3

Matt Gallagher just wrote a blog post titled “OrderedDictionary: Subclassing a Cocoa class cluster” covering exactly this issue, complete with sample code.

Answer 4

Quick 'n dirty:

When you need to order your dictionary (herein called “myDict”), do this:

     NSArray *ordering = [NSArray arrayWithObjects: @"Thing",@"OtherThing",@"Last Thing",nil];

Then, when you need to order your dictionary, create an index:

    NSEnumerator *sectEnum = [ordering objectEnumerator];
    NSMutableArray *index = [[NSMutableArray alloc] init];
     id sKey;
     while((sKey = [sectEnum nextObject])) {
      if ([myDict objectForKey:sKey] != nil ) {
       [index addObject:sKey];
      }
     }

Now, the *index object will contain the appropriate keys in the correct order. Note that this solution does not require that all the keys necessarily exist, which is the usual situation we're dealing with...

Answer 5

I'm late to the game with an actual answer, but you might be interested to investigate CHOrderedDictionary. It's a subclass of NSMutableDictionary which encapsulates another structure for maintaining key ordering. (It's part of CHDataStructures.framework.) I find it to be more convenient than managing a dictionary and array separately.

Disclosure: This is open-source code which I wrote. Just hoping it may be useful to others facing this problem.