NSArray* set1 = [NSArray arrayWithObjects:@"one",@"two",@"three",nil];
NSArray* set2 = [NSArray arrayWithObjects:@"two",@"three",@"four",nil];

NSMutableArray* set3 = [NSMutableArray arrayWithCapacity:MIN(set1.count,set2.count)];

for (NSString* element in set1) {
  if( [set2 containsObject:element] ) {
    [set3 addObject:element];
  }
}

// set3 => [ @"two", @"three" ]