Shuffle a deck of cards in Java

Question

I've read many things on the topic, but I am still having misgivings about it. Please, help!

I create a deck of 52 cards:

int[] deck = new int[52]; 
for (int i = 0; i < deck.length; i++) {deck[i] = i;}

Now, I'd like to shuffle the deck properly, not by following any pattern or function.

I am going to use the Math.random() method in my algorithm:

for (int i = 0; i < deck.length; i++) {
    int j = (int)(Math.random() * deck.length); // Get a random index out of 52
    int temp = deck[i]; // Swap the cards
    deck[i] = deck[j];
    deck[j] = temp;
}

The problem I am having here, however, is that I sometimes might get duplicates; as a result, some cards/values are missing altogether.

I am almost certain that I am not discarding the indices/values I have already used, which is probably the core of the abovementioned problem. But how do I go about it?

Any suggestions? Possibly, following my same train of thought. Thanks a bunch!

screenshot 1

screenshot 2

duplicates of what? duplicate values of j? of i and j ? of what? — Sharon Ben Asher, Sep 18 '16 at 12:39
duplicates of the ones I've randomly generated and/or already assigned — Illy P., Sep 18 '16 at 12:47
Can you show us an example of your output with duplicates? There doesn't seem to be any mistake in the code that would generate duplicates. So maybe the way you are testing for duplicates is wrong. — Amin J, Sep 18 '16 at 12:48
When you generate random numbers you get duplicates. Nothing wrong with it when shuffling. — Guy, Sep 18 '16 at 12:48
`deck` should be a `List`, then you could just `Collections.shuffle()` it. — Bohemian, Sep 18 '16 at 12:53
I've uploaded a picture here: https://drive.google.com/file/d/0BzCVEMaRSqK5N21aUWRtVG85OG8/view?usp=sharing — Illy P., Sep 18 '16 at 12:59

felipeptcho · Answer 1 · 2016-09-18T13:11:12.013

Simply use a new array for the shuffled cards and fill it using random cards removed from the prior array.

import java.util.ArrayList;

public class Test {
    private static final int DECK_SIZE = 52;

    public static void main(String args[]) {
        ArrayList<Integer> deck = new ArrayList<Integer>();

        for (int i = 0; i < DECK_SIZE; ++i) {
            deck.add(i);
        }

        ArrayList<Integer> shuffledDeck = new ArrayList<Integer>();

        while (deck.size() > 0) {
            int index = (int) (Math.random() * deck.size());
            shuffledDeck.add(deck.remove(index));
        }

        System.out.println(shuffledDeck.toString());
    }
}

Or just use Collections.shuffle():

import java.util.ArrayList;
import java.util.Collections;

public class Test {
    private static final int DECK_SIZE = 52;

    public static void main(String args[]) {
        ArrayList<Integer> deck = new ArrayList<Integer>();

        for (int i = 0; i < DECK_SIZE; ++i) {
            deck.add(i);
        }

        Collections.shuffle(deck);

        System.out.println(deck);
    }
}

score 0 · Answer 2 · answered Sep 18 '16 at 12:51

0

Please look if this helps:

boolean init[] = new boolean[52];
Random r = new Random();

int getNextCard(){
    int i = r.nextInt(52);
    while(init[i])
        i = r.nextInt(52);
    init[i] = true;
    return i;
}

void shuffleCards() {   

    int[] deck = new int[52];
    for (int i = 0; i < deck.length; i++) {
        deck[i] = getNextCard();
    }
    System.out.println(Arrays.toString(deck));
}

Idea is to use a Random to generate a random number between 0 and 52. Once a number is taken, we will mark it as taken in the boolean array init.So, next time the same number comes up, we again generate the random number until we get a number we haven't already taken.

answered Sep 18 '16 at 12:51

Fayaz

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2
7

This is not shuffling, this is repopulating. – Guy Sep 18 '16 at 12:54
But this generates a shuffled set right? – Fayaz Sep 18 '16 at 12:59
True, the order is different. But this isn't the question. – Guy Sep 18 '16 at 13:00
As far as I understood, his shuffling approach is resulting in duplicates and hence missing some other values, I showed him in code an approach that is't resulting in duplicates – Fayaz Sep 18 '16 at 13:02
Thank you for your help! – Illy P. Sep 18 '16 at 13:10
Well I liked it anyway, it does the job so can't complain. – Hasen Jul 28 '20 at 17:47

Shuffle a deck of cards in Java

2 Answers2