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

I believe NSIndexSet stores its indexes using ranges, so there isn't necessarily a quick way to return the nth index. You could enumerate keeping a counter until your counter reaches your target index:

NSUInteger index = [indexSet firstIndex];

for (NSUInteger i = 0, target = 4; i < target; i++)
  index = [indexSet indexGreaterThanIndex:index];

That should give you the 4th index. You could even add the method as a category method if you want:

- (NSUInteger)indexAtIndex:(NSUInteger)anIndex
{
    if (anIndex >= [self count])
      return NSNotFound;

    NSUInteger index = [indexSet firstIndex];
    for (NSUInteger i = 0; i < anIndex; i++)
      index = [self indexGreaterThanIndex:index];
    return index;
}

But, as you said, this may not be the best data structure to use so do consider that more before going with something like this.