Circular Linked List in Cocoa

Question

Is there something similar to a circular linked list available in Cocoa?

I know that an NSArray is ordered - but I don't think I can use 'nextItem' or 'previousItem' - correct? Furthermore, I need the nextItem of the last item to be the first item.

I could add my own nextItem and previousItem methods, but I'm surprised if Apple haven't implemented something suitable already. I can't find it though if they have.

Answer 1

I'm not aware of any such circular list data structure. Your idea about implementing it manually seems like a good idea. I would use a category:

@implementation NSArray (myCircularList)

-(id)nextItem;
-(id)previousItem;

Answer 2

While you can certainly use a category to add the behavior to NSArray (as @darren suggests), it's possible that you might actually need a true circular buffer. If that's the case, check out the CHDataStructures framework. Besides a CHCircularBufferStack, there's also a CHCircularBufferQueue and a CHCircularBufferDeque.

Answer 3

You could also use something like

id foo = [array objectAtIndex:i % [array count]];

So to get from item i to the next item (or in case of last item to the very first item) just increase the index i by one. (for the previous item, simply decrease i by one) The modulo operator paired with the array length makes the array wrap around.

While not a very clean approach, it should do the trick without subclassing/categories. You'd always need to know the current index, though. :(