Mutable array contents are lost between methods - objective-c

Question

I have this code in my .m file, which is a Cocos 2D CCLayer class. I initialize an array in the init method and then I try to use contents of this array in the nextFrame method. But when the nextFrame method gets called, the contents of the array seem empty. When I try to get the first item, I receive an error message saying:
Program received signal "EXC_BAD_ACCESS"
How can I successfully access the contents of this array in my nextFrame method?
NSMutableArray *cars;
-(id) init {
cars = [NSMutableArray array];
Car *car;
car = [[Car alloc] init];
[cars addObject:car];
self.isTouchEnabled = YES;
}
- (void) nextFrame:(ccTime)dt {
Car *car = [cars objectAtIndex:i]; // Program received signal "EXC_BAD_ACCESS"
}
Car.h
#import <Foundation/Foundation.h>
#import "cocos2d.h";
#interface Car : NSObject {
NSInteger type;
CCSprite *sprite;
}
#property (readwrite, assign) NSInteger type;
#property (retain) CCSprite *sprite;
#end
Car.m
#import "Car.h"
#implementation Car
#synthesize type;
#synthesize sprite;
- (id)init
{
self = [super init];
if (self) {
// Initialization code here.
}
return self;
}
- (void) dealloc {
[sprite release];
[super dealloc];
}
#end

Answer 1

You're assigning the result of [NSMutableArray array] to an instance variable. That is an autoreleased object, which essentially means it doesn't have any owners and thus will feel free to go away after the current runloop iteration*. You need to retain it if you want it to stick around (or just use [[NSMutableArray alloc] init], which returns an object you own).
* Basically. You should see the Cocoa memory mangement guide for more details. It's pretty short but full of essential information.