NSMutableArray - replacing an item causes exception...help!

Question

I am a newbie in Obj-C. I need to learn this better so please tell me what I am doing wrong..

I have an array of Images.... at various points of exec I need to replace the last element with one of the preceding images... so the last image always replicates one of the images before. When I do the replacement, it throws an exception! If I remove the call to setCorrectImage, it works.

Am not able to figure this out for the last few hours now :-(

---

The declaration in the controller.h is as follows-

NSMutableArray  *imageSet;
UIImage *img, *img1, *img2, *img3, *img4, *img5;

The array is initialized in the controller -

-(void)loadStarImageSet
{

    NSString *imagePath = [[NSBundle mainBundle] pathForResource:AWARD_STAR_0 ofType:@"png"], 
    *imagePath1 = [[NSBundle mainBundle] pathForResource:AWARD_STAR_1 ofType:@"png"],
    *imagePath2 = [[NSBundle mainBundle] pathForResource:AWARD_STAR_2 ofType:@"png"],
    *imagePath3 = [[NSBundle mainBundle] pathForResource:AWARD_STAR_3 ofType:@"png"],
    *imagePath4 = [[NSBundle mainBundle] pathForResource:AWARD_STAR_4 ofType:@"png"],
    *imagePath5 = [[NSBundle mainBundle] pathForResource:AWARD_STAR_5 ofType:@"png"] 
    ;

    img  = [[UIImage alloc] initWithContentsOfFile:imagePath];
    img1 = [[UIImage alloc] initWithContentsOfFile:imagePath1];
    img2 = [[UIImage alloc] initWithContentsOfFile:imagePath2];
    img3 = [[UIImage alloc] initWithContentsOfFile:imagePath3];
    img4 = [[UIImage alloc] initWithContentsOfFile:imagePath4];
    img5 = [[UIImage alloc] initWithContentsOfFile:imagePath5];


    if(imageSet != nil)
    {
     [imageSet release];
    }
    imageSet = [NSArray arrayWithObjects:img, img1, img2, img3, img4, img5, img, nil];

    [imageSet retain];
}

When the view appears this is what happens -

(void)viewDidAppear:(BOOL)animated
{
    [self processResults];

    [self setCorrectImage];

    [self animateStar];
}


-(void)setCorrectImage
{
    // It crashes on this assignment below!!!!!

    [imageSet replaceObjectAtIndex:6 withObject:img4]; // hard-coded img4 for prototype... it will be dynamic later
}

-(void) animateStar
{
    //Load the Images into the UIImageView var - imageViewResult
    [imageViewResult setAnimationImages:imageSet];

    imageViewResult.animationDuration = 1.5;
    imageViewResult.animationRepeatCount = 1;
    [imageViewResult startAnimating];
}

Answer 1

imageSet = [NSArray arrayWithObjects:img, img1, img2, img3, img4, img5, img, nil];

You are creating an NSArray (non-mutable array) object here and assigning it to your imageSet variable. This is very bad because imageSet was declared to be of type NSMutableArray *, and the object you created has type NSArray, and NSArray is not a subtype of NSMutableArray.

So the error occurs because the object is actually an NSArray object, not NSMutableArray (or subclass thereof), and therefore does not support a replaceObjectAtIndex:withObject: message.

You should create an NSMutableArray object instead:

imageSet = [NSMutableArray arrayWithObjects:img, img1, img2, img3, img4, img5, img, nil];