Question

Hi I have two arrays one has 200 + objects ( never changes) and another which is an array of integers ( as NSNumber objects) what is the easiest way to create a third array which will be an array of all objects from array 1 that are included in the array of indexes from array 2?

Answer 1

One option would be to convert the array of indexes to an NSIndexSet and then use objectsAtIndexes:

NSArray *objects = @[@"A", @"B", @"C", @"D", @"E", @"F", @"G"];
NSArray *indexArray = @[@(0), @(2), @(6)];

NSMutableIndexSet *indexSet = [NSMutableIndexSet indexSet];
for (NSNumber *number in indexArray) {
    [indexSet addIndex:[number unsignedIntegerValue]];
}
NSArray *filtered = [objects objectsAtIndexes:indexSet];

Result: A, C, G

Of course, if you can, it would be better to directly use an index set, instead of an array.

Or you could just use a simple loop.

NSMutableArray *filtered = [[NSMutableArray alloc] 
                             initWithCapacity:indexArray.length];
for (NSNumber *number in indexArray) {
    [filtered addObject:objects[[number unsignedIntegerValue]]];
}