Shuffling an array using random numbers

Question

My program runs sometimes, but other times it runs infinitely because the random number generator keeps generating the same number again and again.

public static String[] shuffleElements(final String[] elements) {
    String[] shuffledArray = new String[elements.length];
    Set<Integer> generatedIndices = new HashSet<Integer>();
    for(int i=0;i < elements.length; i++) {
        int index = -1;
        do{
            index = r.nextInt(elements.length);
            System.out.println(i + ", " + index);
        }while(index == i || generatedIndices.contains(index));
        generatedIndices.add(index);
        assignments[i] = elements[index];
    }
    return shuffledArray;
}

How do I fix this problem? Is there a better way of doing this?

Coding is fun, but research is often more productive. Assuming this is an academic exercise, I'd recommend reading about the Fisher–Yates shuffle which has been around for ~50 years. — Chris Martin
– Chris Martin, Commented Aug 25, 2014 at 6:39
Thank you, Chris. Yes, I've been reading about the Fisher-Yates shuffle, but I just wanted to see if I could come up with something myself. Can you please point out why my program doesn't work? Is there a way to fix it? — Prasanna
– Prasanna, Commented Aug 25, 2014 at 6:43
If you want to improve this algorithm with a minimum of changes, you could try to keep a list of unassigned indices and generate each index like this: "index = r.nextInt(elements.length - generatedIndices.size())" — sstendal
– sstendal, Commented Aug 25, 2014 at 6:58
Thank you sstendal. I will try that. Do you know why the program goes on an infinite loop? — Prasanna
– Prasanna, Commented Aug 25, 2014 at 20:19

Rahul · Accepted Answer · 2014-08-25 06:38:05Z

1

You can do a very simple shuffling of elements using the Collections#shuffle(list) method.

public static String[] shuffleElements(final String[] elements) {
    List<String> list = Arrays.asList(elements);
    Collections.shuffle(list);
    return (String[]) list.toArray();
}

This method uses the default source of randomness. But if you want to specify your own source for the randomness, you can do so by using this overloaded Collections#shuffle(list, random) method.

answered Aug 25, 2014 at 6:38

Rahul

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Evgeniy Dorofeev · Accepted Answer · 2014-08-25 06:45:43Z

0

Try this

Collections.shuffle(Arrays.asList(elements));

answered Aug 25, 2014 at 6:45

Evgeniy Dorofeev

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