Question

I want to insert a object at a given index. 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

Answer 1

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.