UITableView crashes when scrolling

First of all: I have been doing research and this is a problem many, many developers have, but unfortunately I haven't gotten a solution out of it yet.

Situation: My view is a UITabBar with a UINavigationController and a UITableView. When I scroll the table, it crashes. The error I'm getting is index 9 beyond bounds of empty array, and the ninth table entry happens to be the first one that's not (completely) visible. Anyway, a code snippet says more than a thousand words:

Code:

// Customize the appearance of table view cells.
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    
    static NSString *CellIdentifier = @"Cell";
    
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
    }
    
	NSArray *buildingNames = [self.buildings allKeys];
	NSArray *sortedBuildingNames = [buildingNames sortedArrayUsingSelector:@selector(caseInsensitiveCompare:)];
	
	cell.textLabel.text = [sortedBuildingNames objectAtIndex:indexPath.row];
    
    return cell;
}

I know it most likely has something to do with memory management, but I can't find it. I've tried retaining sortedBuildingNames because that seemed to be the solution for other people. Not in my case, though...

For some reason sortedBuildingNames becomes empty after the first eight rows are being displayed, but the array should be rebuilt for every cell (which is probably not very efficient?), so I don't understand how it could ever be empty.

Any experts who can shine a light on this?

 

Of course, completely forgot about that tool... I haven't worked with it yet, but there's got to be a first time for everything. Thanks, if I find the solution I'll let you know what it was and if I don't I'll be back with my findings

 

Ok, so I got to a point where I figured out what line is causing the error:

Code:

NSArray *buildingNames = [self.buildings allKeys];

I added a breakpoint before that line, and for every time the method was called, I tested the value of buildings which returned the dictionary (retrieved from a plist in viewDidLoad) as it should be. So far so good.

Then, when I scrolled, the breakpoint stopped code execution, and I tested buildings again, and now it's empty, or non-existent. I think it's the first, because it doesn't give me a "no object at address" error.

This means allKeys: is being called on an empty dictionary, which shouldn't be a problem as it just returns an empty array, but then I get the index 9 beyond bounds of empty array error (and, apparently randomly also "[NSCFString allKeys] unrecognized selector sent to instance". The latter error doesn't make any sense, as at no point buildings is an NSString. The second one makes more sense, as I know buildings is empty. But how is it possible that variable is suddenly empty?

I simply create buildings this way:

Code:

NSString *path = [[NSBundle mainBundle] pathForResource:@"Buildings" ofType:@"plist"];
buildings = [NSDictionary dictionaryWithContentsOfFile:path];

In my .h, all I do is declaring an instance variable and creating a property for it (which I then synthesize in my .m).

So, long story short: I got it narrowed down to "buildings is suddenly empty", but the only place I ever change the value of buildings is in viewDidLoad (where it loads the .plist), and viewDidUnload (where it sets self.buildings to nil).

 

I did read that, and I know the rules for memory management, but I figured "well, I'm creating a property that retains the object, so it being autoreleased can't really be a problem." However, I didn't think of the fact that I actually wasn't using the property to set the instance variable. Thanks to your and PhoneyDeveloper's reply I came to that realization, and then quickly solved my problem.

I know to you this was probably another noob question, but if it's of any consolation, I not only have my code working now, but I also have a (much) better understanding of properties. Thanks for that, I appreciate it guys!