NSIndexSet “-indexAtIndex:”?

Question

This feels like a dumb question because it seems to me like my use case must be quite common.

Say I want to represent a sparse set of indexes with an NSIndexSet (whi

Answer 1

Say I want to represent a sparse set of indexes with an NSIndexSet (which is of course what it's for).

[my emphasis]

Actually, no it's not. The documentation says this:

You should not use index sets to store an arbitrary collection of integer values because index sets store indexes as sorted ranges.

So if you are using it to store a sparse array of integers, it is quite inefficient. Also, the only way to get the nth index is to iterate from one end. You'd be better off using an array.