Question

I have an array that I populate with 6 randomly generated numbers. First it generates a random number between 1 and 49 and then checks it against the numbers in the array. If it finds a duplicate it should generate a random number again and then perform the check once again. If there are no duplicates then the number is added to the array.

Here’s the code:

public void populateArray()
{
    for(int i = 0; i < numberLine.length; i++)
    {
        randomNumber = 1 + randomGen.nextInt(49);
        for(int j = 0; j < i; j++)
        {
            if (numberLine[j] == randomNumber)
            {
                i--;
            }
            else
            {
                continue;
            }
        }
        if(i >= 0)
        {
            numberLine[i] = randomNumber;
        }
        else
        {
            continue;
        }
    }
    Arrays.sort(numberLine);
}

However, for some reason this still lets in a duplicate, though rarely (about 1 in 50 arrays), such as 6 6 16 24 34 46. But when I try to duplicate this by taking out the random number element and using a number like 30, I am unable to reproduce the result. What’s going wrong?

Answer 1

Actually since your domain is limited to integers between 1 and 49 it’s better to use an array of booleans to indicate whether the number was already drawn:

public void populateArray()
{
    count = 0;
    boolean[] used = new boolean[50];
    while (count < 6) {
        randomNumber = 1 + randomGen.nextInt(49);
        if (!used[randomNumber]) ++count;
        used[randomNumber] = true;
    }


    int j = 0;
    for (int i = 1; i < used.length; ++i) {
        numberLine[j++] = i;
    }
}

edit

That still has potential infinite loop.

You’re drawing 6 numbers out of 49 without duplicates. The correct solution would be:

 public void populateArray() {
    List<Integer> pool = new ArrayList<Integer>();
    for (int i = 0; i < 49; ++i) {
        pool.add(i + 1);
    }

    for (int i = 0; i < 6; ++i) {
        randomNumber = randomGen.nextInt(pool.size());
        numberLine[i] = pool.get(randomNumber);
        pool.remove(randomNumber);
    }

    Arrays.sort(numberLine);
}

Finite loops, the same probability distribution as the original one. Instead of retrying the draw when a duplicate is encountered you just eliminate the possibility of drawing the duplicate beforehand. It’s basically emulating the real lotto-like draw.