How can I never repeat a already generated random number?

Question

How do you make it so that a random generated number in an arc4random() set is never repeated. For example, in arc4random(100) in 1 is chosen, it will never be brought up again.

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var card: UIImageView!

    @IBOutlet weak var nextButton: UIButton!


    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }




    @IBAction func nextCardTapped(sender: UIButton) {



        //Edit number of cards here for the random card to show up
        var randomCard = arc4random_uniform(100)
        // Edit number of cards here for the random card to show up


        var randomCardString:String = String(format: "card%i", randomCard)


        self.card.image = UIImage(named: randomCardString)

Answer 1

Are you going to be generating a few of these numbers, or a lot of them?

If you are only generating a few, then it might be worthwhile to keep a list of the ones you have already received and try again whenever you get one of them again. Something like...

NSMutableArray *uniqueNumbers = [[[NSMutableArray alloc] init] autorelease];

...

randomCard = arc4random_uniform(100);
while ([uniqueNumbers containsObject:[NSNumber numberWithInt:randomCard]]) {
    // try again
    randomCard = arc4random_uniform(100);
}
[uniqueNumbers addObject:[NSNumber numberWithInt:r]];

If you intend to generate many random numbers this way, it will get pretty inefficient. For instance, on the 99th number you will be looping repeatedly until by random chance you finally get one of the 2 numbers that aren't already in your list. If you want to generate a lot of numbers it might be more efficient to create a collection (array, stack, etc) with all of the numbers in it (1-100) and shuffle it. Then you can just pop them off the top one at a time.

NSMutableArray *uniqueNumbers = [[[NSMutableArray alloc] init] autorelease];
// build list
for(int i = 1; i <= 100; i = i + 1)
{
    [uniqueNumbers addObject:[NSNumber numberWithInt:i]];
}
// shuffle list
for(int i = 0; i < 100; i = i + 1)
{
    int n = arc4random() % 100;
    [uniqueNumbers exchangeObjectAtIndex:i withObjectAtIndex:n];
}

Answer 2

What you are doing is not generating random numbers but displaying a set of numbers in a random order.

There is a very easy algorithm (and quick too) called the Fisher-Yates shuffle. It randomly shuffles an array. The only downside is that it means that any number in the array cannot be in the place it started out. (This may or may not be a downside in your case).

Anyway, you should create your array of numbers. 1-300 and then shuffle them.

Then when you iterate the array you will get the full set of 300 numbers without any repeats and in a random order each time you do it.

Here's the code I used. (Bare in mind it's 3am and I'm tired but I had to do it).

import UIKit

var numbers: [Int] = []

for number in 1...52 {
    numbers.append(number)
}

for i in 0..<(numbers.count - 1) {
    let j = Int(arc4random_uniform(UInt32(numbers.count - i))) + i
    swap(&numbers[i], &numbers[j])
}

println(numbers)

There may be a neater way to write it.

Edited the function as mine wasn't working. Adapted from this answer.

Answer 3

Add the following category on NSMutableArray (this is the modified Fisher-Yates shuffle):

- (void) shuffle
{
    for (NSInteger i = [self count] - 1; i > 0; --i)
    {
        [self exchangeObjectAtIndex:arc4random() % (i+1) withObjectAtIndex: i];
    }
}

Generate the range of numbers sequentially (e.g., 1-300) and place them in an NSMutableArray (as NSNumber). Then, use the shuffle method above.

Then, pull the results sequentially from the NSMutableArray (indexes 0-299). You'll be drawing numbers randomly without replacement, which I think is what you're after.

Answer 4

Xcode 8 • Swift 3

extension Array {
    var shuffled: Array {
        var elements = self
        return elements.shuffle()
    }
    @discardableResult
    mutating func shuffle() -> Array {
        indices.dropLast().forEach { a in
            guard case let b = Int(arc4random_uniform(UInt32(count - a))) + a, b != a else { return }
            swap(&self[a], &self[b])
        }
        return self
    }
}

---

usage

let numbers = Array(1...100)

let shuffled = numbers.shuffled   // [5, 60, 15, 13, 94, 70, 17, 80, 95, 11, 76, 55, 96, 45, 1, 4, 23, 97, 48, 21, 47, 38, 51, 64, 59, 44, 19, 79, 98, 61, 35, 67, 49, 71, 83, 82, 54, 69, 36, 100, 73, 99, 93, 6, 62, 91, 24, 32, 10, 31, 41, 84, 46, 72, 12, 87, 20, 65, 43, 22, 85, 68, 86, 89, 88, 58, 33, 18, 30, 63, 52, 90, 57, 14, 16, 81, 2, 29, 37, 8, 50, 78, 9, 66, 3, 7, 77, 53, 39, 56, 28, 42, 25, 27, 34, 92, 26, 74, 75, 40]

let get10randomNumbers = numbers.shuffled.prefix(10)   // [71, 23, 33, 48, 73, 81, 75, 18, 19, 67]