Memory management issues when passing items between arrays - iphone

Question

I'm attempting to move a group of strings between three different places by calling this function getNextRandomItem. This works the first time I call it, but then I get an access error the second time. Can you explain what I'm doing wrong?
New Items:binNew (NSMutable Array) -> Current Item (NS String) -> Old Items:binOld (NSMutable Array)
-(NSString *) getNextRandomItem {
if (binNew.count > 0){
if (currentItem) {
[binUsed addObject:currentItem];
}
int r = floor(arc4random() % binNew.count);
currentItem = [binNew objectAtIndex:r];
[binNew removeObjectAtIndex:r];
return currentItem;
}
return #"No more items!";
}

Answer 1

You have to retain r when it is removed from the array. Now you remove it and set its pointer to currentItem. The retain count should become 0 so that probably will cause the error. You have two options.
1) Add a property in your interface:
#property (retain) NSString *currentItem;
and add this to your implementation
#synthesize currentItem;
then use
self.currentItem = ...
instead of
currentItem = ...
2) Use correct memory management. See altered code:
-(NSString *) getNextRandomItem {
if (binNew.count > 0){
if (currentItem) {
[binUsed addObject:currentItem];
[currentItem release]; //See here
currentItem = nil; //See here
}
int r = floor(arc4random() % binNew.count);
currentItem = [[binNew objectAtIndex:r] retain]; //See here
[binNew removeObjectAtIndex:r];
return currentItem;
} else {
[currentItem release];
currentItem = nil;
}
return #"No more items!";
}
Note currently you never set your currentItem to nil. You should do that.

Answer 2

This line
currentItem = [binNew objectAtIndex:r];
should be
currentItem = [[[binNew objectAtIndex:r] retain] autorelease];
When you remove object from container with [binNew removeObjectAtIndex:r], it receives release message and without retain-ing it, you get an invalid pointer (because it points to released object).

Answer 3

-(NSString *) getNextRandomItem {
if (binNew.count > 0){
if (currentItem) {
[binUsed addObject: currentItem];
[currentItem release]; //here
currentItem = nil;//not really necessary
}
int r = floor(arc4random() % binNew.count);
currentItem = [[binNew objectAtIndex:r] retain]; //and here
[binNew removeObjectAtIndex:r];
return currentItem;
}
return #"No more items!";
}