</lurk> Hi Bob, I've been looking at using NSDictionary to hold Sparse Matrix information (generally used when you have a large world where only some locations have an entry). Where the use of NSDictionary seems to fall down for me is when you want to get a range of entries. It is easy to look up a location (x,y) - it is very expensive to get a list of locations within a certain range (x,y)-(x+n,y+n). I haven't found an easy solution to this at the moment, there are some scary algorithms about (some google searches on sparse matrixes and range finding will get them) but nothing concrete in Objective C. If anyone has any ideas (source code always welcome) I'd love to hear about it. Cheers, Matt