Subtract values in Array2 from Array1 while keeping remaining duplicates

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)

Paul.s · Accepted Answer · 2012-03-04 00:35:09Z

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.

edited Mar 4, 2012 at 0:35

answered Mar 4, 2012 at 0:04

Paul.s

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3 Comments

Vanny Over a year ago

This works great, thanks! Could you comment on what exactly NSNotFound is doing? After reading up on it I think I understand what's going on, but it's not 100% clear for me. Thanks again.

Paul.s Over a year ago

@Vanny I've added some clarification

Vanny Over a year ago

Thank for the very thorough answer, much appreciated. I wish I could upvote this answer, but I'm new.

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