Replacing multiple array contents with a single object?

Question

I have an NSMutableArray with contents I want to replace with NSNull objects.

This is what I do:

NSMutableArray* nulls = [NSMutableArray array];

for (NSInteger i = 0; i < myIndexes.count; i++)
    [nulls addObject:[NSNull null]];

[stageMap replaceObjectsAtIndexes:myIndexes withObjects:nulls];

How can I do this more efficiently? Is there a way to enumerate an NSIndexSet, so I can replace the array content one by one?

Solved

Suggested method turns out to be 2x faster (avg 0.000565s vs 0.001210s):

if (myIndex.count > 0)
{
    NSInteger index = [myIndex firstIndex];

    for (NSInteger i = 0; i < myIndex.count; i++)
    {
        [stageMap replaceObjectAtIndex:index withObject:[NSNull null]];
        index = [myIndex indexGreaterThanIndex:index];
    }
}

Is this behaving inefficiently? i.e., when you profile your application, do you see large slowdowns in this code fragment? — Jason Coco
– Jason Coco, Commented Apr 12, 2009 at 6:48

Peter Hosey · Accepted Answer · 2009-04-12 07:27:33Z

1

You can use a for loop. Start with the first index, use indexGreaterThanIndex: to get the next index, and stop after you hit the last index.

Don't forget to account for an empty index set. Both the first and last index will be NSNotFound in that case. The easiest way is to test the index set's count; if it's zero, don't loop.

Also, what Jason Coco said about profiling. Don't worry too much about efficiency until your program works, and don't go optimizing things until you have run Shark (or Instruments, if that's your thing) and found exactly what is slow.

answered Apr 12, 2009 at 7:27

Peter Hosey

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1 Comment

hyn Over a year ago

Thanks for the tip. I will try to benchmark this and see if the new approach is any faster.

Tony Million · Accepted Answer · 2012-07-03 21:07:53Z

0

I realise this is a very old question but I'm posting here in case anyone else finds this question you could use:

    [indexes enumerateIndexesUsingBlock:^(NSUInteger idx, BOOL *stop) {
        NSLog(@"index: %d", idx);
        [objectArray replaceObjectAtIndex:idx 
                               withObject:newObject];
    }];

Which is a lot more succinct.

answered Jul 3, 2012 at 21:07

Tony Million

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