How to insert an object at a given index in iphone-sdk

I want to insert a object at a given index开发者_开发问答. and If Is there no object at the index, I want to know that. Is NSDictionary good choice? Is there another good solution?

// Initailization
NSMutableDictionary *items = [NSMutaleDictionary dictionary];
...
// insert an object.
[items setObject:item forKey:[NSNumber numberWithInt:3]];
...
// I want to know that there is an object at a given index.
item = [items objectForKey:[NSNumber numberWithInt:4]];
if (item)
    // exist
else
    // non exist

Of course there is the NSArray class, for an indexed version of NSDictionary, kind of. However, the indexes in an NSArray should be subsequent, so the index begins at 0 and then increments with every object.

So when you want to use a random index, you should go with NSDictionary and you're good. The code you provided is absolutely valid and works correctly.