How to safely remove object from a nsmutablearray while iterating through it?

Question

I am using a nsmutablearray in loop and want to remove its object (or assign nil) that has just traversed. But if I am doing so, I get an error as <__NSArrayM: 0x8c3d3a0> was mutated while being enumerated.' . The code is as below

- (TreeNode*)depthLimitedSearch:(TreeNode *)current costLimit:(int)currentCostBound {

NSMutableArray *children=[NSMutableArray arrayWithArray:[current expandNodeToChildren]];
for (TreeNode *s in children) {
    if (s.puzzleBox.isFinalPuzzleBox) {//checking for final puzzleBox
        return s;
    }
    /*exploredNodes++;
    if (exploredNodes %10000==0) {
        NSLog(@"explored nodes for this treshold-%d are %d",currentCostBound,exploredNodes);
    }*/

    int currentCost =[s.cost intValue]+[s.heuristicsCost intValue];

    if (currentCost <= currentCostBound) {
        //[s.puzzleBox displayPuzzleBox];

        TreeNode *solution = [self depthLimitedSearch:s costLimit:currentCostBound];
        if (solution!=nil){//&& (bestSolution ==nil|| [solution.cost intValue] < [bestSolution.cost intValue])) {
            bestSolution = solution;
            return bestSolution;
        }
    }else {
        if (currentCost < newLimit) {
            //NSLog(@"new limit %d", currentCost);
            newLimit = currentCost;
        }
    }
    // here I want to free memory used by current child in children
    [children removeObject:s]
}
children=nil;
return nil;
}

and I have commented the place where I want to release the space used by the child.

GO BACKWARDS -- an age-old programming trick. A fuller solution is how you do it in a database .. have another column (field, whatever) labelled please_delete_me, and proceed from there. Deletion is really complicated, in a broad sense, when you are dealing with threads, different consumers of the data, etc etc. — Fattie
– Fattie, Commented Jun 5, 2014 at 6:35

Fattie · Accepted Answer · 2014-06-05 06:38:05Z

3

You should not be using a for...in loop if you want to remove elements in the array. Instead, you should use a normal for loop and go backwards in order to make sure you don't skip any items.

for (NSInteger i = items.count - 1; i >= 0; i--) {
    if (someCondition) {
        [items removeObjectAtIndex:i];
    }
}

edited Jun 5, 2014 at 6:38

Fattie

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answered Jun 5, 2014 at 6:25

Senseful

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5 Comments

Revinder Over a year ago

why should not use forward as it will always hold index x for object that will be there for sure because i am not doing anything with other object.

Senseful Over a year ago

@Revinder: Assume the array has 10 elements and we are currently at index 3 and want to delete this index. We delete index 3, then the next iteration of the loop causes i to be 4. Since we deleted index 3 everything above it has been reindexed by -1. So now we are observing the item that was originally at position 5, and we have never looked at the item at position 4.

Fattie Over a year ago

Hi @Revinder. It's an absolute basic of programming that when you do deletions, you have to work backwards. It's the first thing you learn in CompSci 001 ! :) Please listen to Senseful's excellent explanation.

Revinder Over a year ago

hey @JoeBlow do you have knowledge about solving 15 puzzle using pattern database?

Fattie Over a year ago

@Revinder. I'm sorry, I have no clue what you are talking about. Be sure to ask a new question for further questions, as this one is closed now.

zoul · Accepted Answer · 2014-06-05 07:35:48Z

1

You can collect the items to be removed in another array and remove them in a single pass afterwards:

NSMutableArray *toRemove = [NSMutableArray array];
for (id candidate in items) {
    if (something) {
        [toRemove addObject:candidate];
    }
}
[items removeObjectsInArray:toRemove];

It’s easier than iterating over indexes by hand, which just asking for off-by-one errors. Not sure how this plays with your early returns, though.

edited Jun 5, 2014 at 7:35

answered Jun 5, 2014 at 6:33

zoul

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