How to use NSIndexSet

it would be the proper way:

NSIndexSet *indexSet = [NSIndexSet indexSetWithIndexesInRange:NSMakeRange(0, 3)];

or you can use the NSMutableIndexSet for the random indexes:

NSMutableIndexSet *mutableIndexSet = [[NSMutableIndexSet alloc] init];
[mutableIndexSet addIndex:0];
[mutableIndexSet addIndex:2];
[mutableIndexSet addIndex:9];

etc.