Random number that doesn't repeat twice in a row

Question

I'm pretty new to coding, I've been looking for similar questions but none fits my needs. I'm working in a dice rolling app, and I need a a random number generator to "roll" the dice. arc4random seems perfect, but the problems that I can't have the same face occurring twice in a row. I have a method firing when I press the button with a timer

- (IBAction)dieRoll:(id)sender {

self.currentFace = 1;

_timer = [NSTimer scheduledTimerWithTimeInterval:0.25 target:self selector:@selector(roll) userInfo:nil repeats:YES];;

}

but I have to implement the 'roll' method where I get a random number different from the one already selected (the property self.currentFace).

any clue?

I just don't want it to repeat twice in a row, the generated number needs to be different from the one previously generated — Riccardo, Nov 14 '15 at 15:25
it's supposed to simulate the faces rolling rather then the roll result — Riccardo, Nov 14 '15 at 15:45

André Slotta · Answer 1 · 2015-11-14T15:32:44.083

4

this is how your implementation could look like:

@interface ViewController ()

@property (assign, nonatomic) NSInteger currentFace;

@end

@implementation ViewController

- (void)viewDidLoad {
  [super viewDidLoad];

  self.currentFace = -1;
}

- (IBAction)diceRoll:(id)sender {
  NSInteger newFace = -1;
  do {
    newFace = arc4random_uniform(6) + 1;
  } while (newFace == self.currentFace);

  self.currentFace = newFace;
}

@end

edited Nov 14 '15 at 15:32

answered Nov 14 '15 at 15:27

André Slotta

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just an initial value. the "nil" equivalent for a value type... ;) – André Slotta Nov 14 '15 at 15:29

o15a3d4l11s2 · Answer 2 · 2015-11-14T15:39:49.510

1

A simple solution would be to just remember the last rolled number and roll the dice until you get a different one. Pretty simple and you can keep arc4random.

An example:

- (NSUInteger)rollDiceWithLastFaceNumber:(NSUInteger)lastFaceNumber
{
    NSUInteger currentFaceNumber;

    do {
        currentFaceNumber = (arc4random_uniform(6) + 1);
    } while (currentFaceNumber == lastFaceNumber);

    return currentFaceNumber;
}

and how to use it:

[self rollDiceWithLastFaceNumber:3];

edited Nov 14 '15 at 15:39

answered Nov 14 '15 at 15:12

o15a3d4l11s2

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yeah, I thought so, but how how do I get it to roll until it's a different one (I set _currentFace exactly to remember the last rolled number) – Riccardo Nov 14 '15 at 15:18
Check the example that I added. In case this is not enough for your case, please explain further what should be your exact logic. I noticed the timer that you use, but I do not see where you are using the returned result. – o15a3d4l11s2 Nov 14 '15 at 15:23
Use `arc4random_uniform()`, it eliminates any mod bias. – zaph Nov 14 '15 at 15:36
Thanks for the suggestion, fixed the code. – o15a3d4l11s2 Nov 14 '15 at 15:40
thank you, it worked sweet!!! – Riccardo Nov 14 '15 at 17:38

Guilherme Amantea · Answer 3 · 2015-11-14T16:47:14.100

This solution avoids an unknown number of iterations until you get your result.

- (void)roll {
    NSUInteger next = [self nextFaceWithPreviousFace:self.currentFace];
    NSLog(@"%lu", next);
    self.currentFace = next;
}

- (NSUInteger)nextFaceWithPreviousFace:(NSUInteger)previous {
    NSMutableArray *candidates = [NSMutableArray arrayWithObjects:@1, @2, @3, @4, @5, @6, nil];
    [candidates removeObject:@(previous)];
    NSUInteger index = arc4random_uniform((unsigned)candidates.count);
    return [candidates[index] unsignedIntegerValue];
}

Random number that doesn't repeat twice in a row

3 Answers3