Objecive-C – Manipulating two arrays
NSMutableArray *web = [[NSMutableArray alloc] initWithObjects:@"2",@"3",@"4",nil];
NSMutableArray *disk = [[NSMutableArray alloc] initWithObjects:@"1",@"2",@"3",nil];

- (void)minusSet:(NSSet *)otherSet

NSMutableSet *web = [[NSMutableSet alloc] initWithObjects:@"2",@"3",@"4",nil];
NSMutableSet *disk = [[NSMutableSet alloc] initWithObjects:@"1",@"2",@"3",nil];

NSMutableSet * remArray = [[NSMutableSet setWithSet:disk] minusSet:web];
NSMutableSet * addArray = [[NSMutableSet setWithSet:web] minusSet:disk];

NSMutableArray * remArray = [NSMutableArray arrayWithArray:disk];
[remArray removeObjectsInArray:web];

- (NSArray *) differnceBetween: (NSArray *) array1 and: (NSArray *) array2 {
    NSMutableArray *difference = [[NSMutableArray alloc] init];
    for(id obj in array1) {
        if(![array2 contains:obj]) {
            [differnce addObject:obj];
        }
    }
    return difference;
}