objective c - How to do sparse array in Cocoa

Question

I have an undetermined size for a dataset based on unique integer keys.

I would like to use an NSMutableArray for fast lookup since all my keys are integer based.

I want to do this.

NSMutableArray* data = [NSMutableArray array]; // just create with 0 size

then later people will start throwing data at me with integer indexes (all unique) so I just want to do something like this...

if ([data count] < index)
    [data resize:index];  // ? how do you resize

and have the array resized so that i can then do...

[data insertObject:obj atIndex:index];

with all the slots between last size and new size being zero which will eventually be filled in later.

So my question is how do I resize an existing NSMutableArray?

Thanks, Roman

Answer 1

Use an NSPointerArray.

http://developer.apple.com/mac/library/documentation/Cocoa/Reference/Foundation/Classes/NSPointerArray_Class/Introduction/Introduction.html

NSPointerArray is a mutable collection modeled after NSArray but it can also hold NULL values, which can be inserted or extracted (and which contribute to the object’s count). Moreover, unlike traditional arrays, you can set the count of the array directly. In a garbage collected environment, if you specify a zeroing weak memory configuration, if an element is collected it is replaced by a NULL value.

If you were to use a dictionary like solution, use NSMapTable. It allows integer keys. The NSMutableDictionary based solution recommended has a tremendous amount of overhead related to all of the boxing & unboxing of integer keys.