Question

Very newbie obj-c question.

In ViewController I have four UIImages. They are imitating playing cards. User can see only their backside at start. By tapping one of them user will see the front side (animation for this is ready now). My task is to show random front side from four images with defined repetition rate. So I want to set one of the images (prize) will appear eight times less Eight times less than the chance of getting any one of the others, everytime the user will visit this ViewController.

If it’s possible, please show me the piece of code what can generate random but with possibility of setting repetition rate

Answer 1

- (NSUInteger)indexForImageGivenIndexSet:(NSIndexSet*)indexSet             // Set of indexes you want to select from
                         prizeImageIndex:(NSUInteger)prizeIndex      // The prize index
                        probabilityScale:(CGFloat)probabilityScale   // The scale for the prize (8.0 for 8x less than the others)
{
    // If invalid, return what was given
    if (![indexSet containsIndex:prizeIndex]) {
        return prizeIndex;
    }

    // Calculate our probabilities
    // For a set of 4, with a scale of 8 on the prize, our probabilities would be
    // 0.04 (prize), 0.32, 0.32, 0.32
    double prizeProbability = (1.0 / indexSet.count) * (1.0 / probabilityScale);

    double val = (double)arc4random() / RAND_MAX;

    if (val < prizeProbability) {
        return prizeIndex;
    }
    else {
        // Select from the others in range equally
        NSMutableIndexSet* newSet = [[NSMutableIndexSet alloc] initWithIndexSet:indexSet];
        [newSet removeIndex:prizeIndex];

        NSInteger selectedPosition = arc4random() % newSet.count;
        __block NSInteger count = 0;
        return [newSet indexPassingTest:^BOOL(NSUInteger idx, BOOL *stop) {
            if (count == selectedPosition) {
                *stop = YES;
                return YES;
            }
            else {
                count++;
                return NO;
            }
        }];
    }
}