android - generate random numbers with no repeat

Question

can anyone please tell me how to generate random numbers with no repeat example

random (10) should(may) return 3,4,2,1,7,6,5,8,9,10 with no repeat

Thanks

it will help you: http://stackoverflow.com/questions/4040001/java-creating-random-numbers-with-no-duplicates — Thiru, Oct 11 '11 at 11:12

score 27 · Accepted Answer · answered Oct 11 '11 at 11:08

27

I would suggest adding the numbers to an ArrayList<Integer> and then use Collections.shuffle() to randomize their order. Something like this:

ArrayList<Integer> number = new ArrayList<Integer>();
for (int i = 1; i <= 10; ++i) number.add(i);
Collections.shuffle(number);

answered Oct 11 '11 at 11:08

Till Helge

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liked it! a question, if i execute Collection.Shuffle twice or thrice will it provide different order? – Yogesh Oct 11 '11 at 11:23
I'm pretty sure that it will. I can't find a reason why it shouldn't. Easy enough to check though, eh? ;) – Till Helge Oct 11 '11 at 11:25

score 4 · Answer 2 · answered Oct 11 '11 at 11:09

Make a list of generated numbers, when your newly generated number is already in this list you make a new random number.

Random rng = new Random(); // Ideally just create one instance globally
List<Integer> generated = new ArrayList<Integer>();
for (int i = 0; i < numbersNeeded; i++)
{
    while(true)
    {
        Integer next = rng.nextInt(max) + 1;
        if (!generated.contains(next))
        {
            // Done for this iteration
            generated.add(next);
            break;
        }
    }
}

unfortunately this method doesn't scale very well – Richard Le Mesurier Aug 19 '14 at 15:38 — Richard Le Mesurier, Aug 19 '14 at 15:38

score 2 · Answer 3 · answered Oct 11 '11 at 11:20

2

My two cents

public Collection<Integer> getRandomSubset(int max,int count){
    if(count > max){
        throw new IllegalArgumentException();
    }
    ArrayList<Integer> list = new ArrayList<Integer>();
    for(int i =  0 ; i < count ;i++){
        list.add(i);
    }       
    Collections.shuffle(list);
    return list.subList(0, count);
}

answered Oct 11 '11 at 11:20

Nikolay Ivanov

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I like the `subList()` enhancement in this solution - great for dealing out a hand of cards from a deck – Richard Le Mesurier Aug 19 '14 at 15:40

score -2 · Answer 4 · answered Oct 11 '11 at 11:11

If there are only a few numbers, less than 100, I think I solution could be create a boolean array and once you get a number, set the position of the array to true. I don't think that it takes long time until all the numbers appear. Hope it helps!

Cheers!

android - generate random numbers with no repeat

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