Question

Is it possible to subtract the values in one NSMutableArray from those in another NSMutableArray, while preserving any remaining like values (keep duplicate copies)? I don’t want to remove every instance of the value, just a 1 for 1 subtraction.

I’m working with CGPoints.

Array1
ccp(1,1)
ccp(1,1)
ccp(1,2)
ccp(1,3)

Array2
ccp(1,1)

Desired output: Array3
ccp(1,1)
ccp(1,2)
ccp(1,3)

Answer 1

Sorry I didn’t understand your question properly before. Maybe you could try something like:

NSMutableArray *points1 = [NSMutableArray arrayWithObjects:
                            [NSValue valueWithCGPoint:CGPointMake(1, 1)],
                            [NSValue valueWithCGPoint:CGPointMake(1, 1)],
                            [NSValue valueWithCGPoint:CGPointMake(1, 2)],
                            [NSValue valueWithCGPoint:CGPointMake(1, 3)], nil];

NSArray        *points2 = [NSArray arrayWithObject:
                            [NSValue valueWithCGPoint:CGPointMake(1, 1)]];

NSInteger index = NSNotFound;

for (NSValue *point in points2) {
    index = [points1 indexOfObject:point];

    if (NSNotFound != index) {
        [points1 removeObjectAtIndex:index];
    }
}

NSLog(@"%@", points1);

=> 2012-03-04 00:02:26.376 foobar[19053:f803] (
       "NSPoint: {1, 1}",
       "NSPoint: {1, 2}",
       "NSPoint: {1, 3}"
   )

---

Update

NSNotFound is defined in NSObjCRuntime.h. You can find this out by command + clicking on the symbol NSNotFound in Xcode.

The definition is

 enum {NSNotFound = NSIntegerMax};

The reason I knew to use this is by looking at the NSArray documentation for the indexOfObject: method, which reads:

Return Value
The lowest index whose corresponding array value is equal to anObject. If none of the objects in the array is equal to anObject, returns NSNotFound.