Runtime error when NSString Object is released - cocoa

Question

In my Function below if I remove the tempString release statement it works just fine but with it, there is ALWAYS a runtime error. It is a simple function that displays an array in an NSTextField either _stackDisp1 or _stackDisp2 but for some reason releasing the string creates a runtime error Any help?
- (void) displayArr:(NSMutableArray*)stack{
NSTextField *myObj;
if([stack count] <= 10) myObj = _stackDisp1;
else myObj = _stackDisp2;
NSString *tempString = [[NSString alloc]initWithString:#""];
for(NSString *i in stack){
tempString = [NSString stringWithFormat:#"%#\n%#",tempString,i];
}
[myObj setStringValue:tempString];
[tempString release];
}

Answer 1

That's because
tempString = [NSString stringWithFormat:#"%#\n%#",tempString,i];
creates a new autoreleased object assigning it to your variable tempString. The pointer to the first object gets lost and you end up over-releasing an autoreleased object. Just change the initial assignment to
NSString *tempString = #"";
and remove the [tempString release] line.

Answer 2

In the for loop you're assigning tempString to an autoreleased string:
tempString = [NSString stringWithFormat:#"%#\n%#",tempString,i];
releasing it manually results in a BAD_ACCESS.
Also you are probably looking for this:
- (void) displayArr:(NSMutableArray*)stack{
NSTextField *myObj = ([stack count] <= 10) ? _stackDisp1 : _stackDisp2;
[myObj setStringValue:[stack componentsJoinedByString:#"\n"]];
}
The declaration/assignment of myObj was a bit too verbose for my taste,
so I used a ternary operator instead (it's use is not essiential though. Just a matter of style.).