Question

NSString *temp = [self randomBallPick:temp];

the following error:
Pass-by-value argument in message expression is undefined

why?

-(NSString*) randomBallPick:(NSString*) oneFilename
{

    //create an array of filenames
    NSMutableArray *imageArray = [[NSMutableArray alloc] init];
    for (int c=0;c<37;c++)
    {
        NSString *imageName = [NSString stringWithFormat:@"ball_%d.png", c];
        [imageArray addObject: imageName];
    }

    //pick one filename
    int numFileNames = [imageArray count];
    int chosen = arc4random() % numFileNames;
    oneFilename = [imageArray objectAtIndex: chosen];
    [imageArray release];
    return oneFilename;
}

Answer 1

As far as I can tell you’re creating an array of 37 file names and then return one of them randomly.

I think the following provides the same functionality:

- (NSString*)randomBallPick
{
    [NSString stringWithFormat:@"ball_%d.png", arc4random() % 37];
}

If you want to keep the array around and reuse it, I’d do something like this:

- (NSString*)randomBallPick
{
    static NSMutableArray *imageArray;

    if (!imageArray) {
        imageArray = [[NSMutableArray alloc] init];
        for (int c = 0; c < 37; c++)
        {
            NSString *imageName = [NSString stringWithFormat:@"ball_%d.png", c];
            [imageArray addObject:imageName];
        }
    }

    //pick one filename
    NSUInteger numFileNames = [imageArray count];
    NSUInteger chosen = arc4random() % numFileNames;
    return [imageArray objectAtIndex:chosen];
}