Objective-c – Collection <__NSArrayM: 0x76c11b0> was mutated while being enumerated

cocos2d-iphoneobjective c

I'm relatively new to obj-c, so I must be missing something, but my program crash when an enemy collides with a wall. I've located where I'm removing the enemy from the loop, while in the loop, but for the life of me, i can't figure out how to fix it.
My code is as follows:

(the error is "[allEnemies removeObject:enemyType1];")

//ALWAYS RUNNING
-(void) update:(ccTime)dt
{

for (CCSprite *enemyType1 in allEnemies) { //for every attacking unit in allEnemies

    //Adjust the collison box for each enemey depending on the height of the enemy
    float a;
    float b;
    float yOne = (wall.contentSize.height-enemyType1.position.y);
    float yTwo = (wall.contentSize.height);
    float xTwo = 30;
    a = (xTwo*(yOne/yTwo)); // always < 1
    b = xTwo-a;             // always > 1


    //Create the altered collison box 
    CGRect enemyType1Rect = CGRectMake (
                enemyType1.position.x - (enemyType1.contentSize.width/2), 
                enemyType1.position.y - (enemyType1.contentSize.height/2), 
                enemyType1.contentSize.width+b, 
                enemyType1.contentSize.height
                                       );


    //If the enemey hits the wall, stop it, then add it to the attacking enemies array
    if (CGRectIntersectsRect(enemyType1Rect, wall.boundingBox)) {
        [enemyType1 stopAllActions];
        [allEnemies removeObject:enemyType1];
        [attackingEnemies addObject:enemyType1];            
    }


}
//Wall Collison END

Best Answer

Well, just as the error states, you mutated the array while it was being enumerated. The easiest fix is to do for (CCSprite *enemyType1 in [[allEnemies copy] autorelease]) This way you're enumerating a copy of the array (this does NOT copy the elements, just gives you another container to enumerate them in), and can still modify the mutable array.

You can not modify containers while enumerating them.

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