Complement of two arrays in Objective-C - ios

Question

Very simple problem, I am on the right track, I have to use predicates but couldn't find the solution.
I have two arrays:
A = [1, 2, 3, 4]
B = [1, 2]
I want to have an aray C = A \ B that is
C = [3, 4]

Answer 1

Use the right tool for the task. What you're describing here is the set operation "difference".
NSMutableSet *first = [NSMutableSet setWithObjects:#1, #2, #3, #4, nil];
NSSet *second = [NSSet setWithObjects:#1, #2, nil];
[first minusSet:second];
If you really really really need to work with arrays, you can convert between arrays and sets too:
NSArray *array = [someSet allObjects];
or
NSSet *set = [NSSet setWithArray:someArray];

Answer 2

You can achieve it with removeObjectsFromArray:
NSArray * arrayA = #[ #1, #2, #3, #4 ];
NSArray * arrayB = #[ #1, #2 ];
NSMutableArray * arrayC = [arrayA mutableCopy];
[arrayC removeObjectsInArray:arrayB];
EDIT
As suggested in the comments and also proposed by H2CO3, if you don't have a valid reason to use NSArray, the most convenient solution is to change data structure and switch to NSSet, which has the support for set difference, using the method -minusSet:.
EDIT 2
As Sven noted in the comments, your objects must respond properly to hash and isEqual: in order to use removeObjectsInArray:. In the above example everything will work just fined, since NSNumber has a default implementation for those methods, but in case you're using a custom class you need to explicitly override them.

Answer 3

Here's another approach:
NSArray * arrayA = #[ #1, #2, #3, #4 ];
NSArray * arrayB = #[ #1, #2 ];
NSIndexSet *indexSet = [arrayA indexesOfObjectsPassingTest:^BOOL(id obj, NSUInteger idx, BOOL *stop) {
if([arrayB containsObject:obj]){
return NO;
}
else{
return YES;
}
}];
NSArray *arrayC = [arrayA objectsAtIndexes:indexSet];