Swift random number

Question

I'm having problems with this drawRandomCard function.

It works just like it should for some time, but eventually it crashes the application.

Here is the code:

import Foundation


var cardDeck = Array<PlayingCard>()

class Deck {

    func addCard(card : PlayingCard , atTop : Bool = false){

        if atTop {
            cardDeck.insert(card, atIndex: 0);
        }else{
            cardDeck += card
        }
    }

    func drawRandomCard() -> PlayingCard{
        var card = PlayingCard()
        var randomNumber : Int = Int(arc4random()) % (cardDeck.count - 1)
        card = cardDeck[randomNumber]
        cardDeck.removeAtIndex(randomNumber)
        return card
    }

}

How does your code crash? Is there a stack trace or error message? Your code as posted on Github seems to have a number of compilation errors. — Craig Otis, Jun 09 '14 at 11:56

score 37 · Accepted Answer · edited Dec 01 '15 at 16:12

37

Use arc4random_uniform to avoid modulo bias. Like following:

let randomNumber = arc4random_uniform(150)

For your example, it will be:

let randomNumber = Int(arc4random_uniform(UInt32(cardDeck.count)))

edited Dec 01 '15 at 16:12

orkoden

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answered Jun 09 '14 at 11:58

Adam

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Factually correct, but entirely unrelated to the question. – Kevin Jun 09 '14 at 15:02
3

@Kevin Not well explained, but it _is_ related to the question. Adam's answer won't overflow a Swift `Int` – pjs Jun 09 '14 at 15:12

score -4 · Answer 2 · answered Jun 09 '14 at 11:54

-4

Try this: Int(rand()) instead of arc4random()

answered Jun 09 '14 at 11:54

Alexey Sidorov

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How would this alleviate the crashing? I'm not saying it doesn't, but without an explanation I don't see the difference. – Craig Otis Jun 09 '14 at 11:57
thanks for the suggestion, worked really well. – vyudi Jun 09 '14 at 12:19
the arc4random() don't work well with Int type – vyudi Jun 09 '14 at 12:20
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missing any kind of explanation why to use rand instead of arc4random – Vojtech Vrbka Dec 31 '14 at 10:58
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The amazing thing about this really bad answer is that it has two up votes! To be clear:` rand()` is a terrible random number generator and requires seeding, it should not be used. `arc4random()` and `arc4random_uniform()` generate cryptographically good random numbers and require no seeding. – zaph Jun 01 '15 at 23:28

Swift random number

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