Efficient array elements un-ordering in Java

Question

I just wanted to know what is the best way to un-order the elements in an ordered array in Java, Thanks

http://stackoverflow.com/questions/180979/using-collections-api-to-shuffle — Matt Ball, Oct 14 '10 at 00:59

score 6 · Accepted Answer · answered Oct 14 '10 at 00:58

6

I think you want Collections.shuffle(List)? If not that, you will need to give us more details about what you're trying to do.

answered Oct 14 '10 at 00:58

wkl

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And you can be fairly sure that the shuffle algorithm of the Collections.shuffle() is pretty good. The standard JavaSE api is incredibly well tested. (Whoa... didn't want to sound like a sell speech) – Eric-Karl Oct 14 '10 at 01:07

score 2 · Answer 2 · answered Oct 14 '10 at 01:00

2

You want to shuffle that array using a good algorithm.

I would trust Collections#shuffle to implement this properly. If you need it to work on the array directly, implement the algorithm in your own helper method.

answered Oct 14 '10 at 01:00

Thilo

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score 1 · Answer 3 · answered Oct 14 '10 at 00:59

I don't know much Java, so there may be better way, but this'll do the trick nicely.

Fisher-Yates shuffle from Wikipedia:

static Random rng = new Random();
public static void shuffle(int[] array) {
    // i is the number of items remaining to be shuffled.
    for (int i = array.length; i > 1; i--) {
        // Pick a random element to swap with the i-th element.
        int j = rng.nextInt(i);  // 0 <= j <= i-1 (0-based array)
        // Swap array elements.
        int tmp = array[j];
        array[j] = array[i-1];
        array[i-1] = tmp;
    }
}

Efficient array elements un-ordering in Java

3 Answers3