How can I create and display an arraylist of random numbers in java?

Question

I'm trying to generate an arraylist of random numbers and display it. I'm not sure where I'm going wrong. I think my showArray method isn't working properly because it's displaying two random numbers and then repeating the second one n-2 times.

private static ArrayList<Integer> RandomArray(int n)
    {

        ArrayList<Integer> arrayRandom = new ArrayList<Integer>(n);

        for (int i=0; i<n; i++)
        {
            Random rand = new Random();
            rand.setSeed(System.currentTimeMillis());
            Integer r = rand.nextInt() % 256;
            arrayRandom.add(r);

        }

        return arrayRandom;

    }

private static void ShowArray(ArrayList<Integer> randomArray)
{
    int n = randomArray.size();

    ArrayList<Integer> showArray = new ArrayList<Integer>(n);

    for (int i = 0; i<n; i++)
    {
        int r = randomArray.get(i);
        showArray.add(r);
    }
    System.out.println(showArray);

}

public static void main(String args[])
    {

        ShowArray(RandomArray(5));

    }

So for example this would produce an output of

[132, 152, 152, 152, 152]

Any help is much appreciated. Thanks in advance

Jan Thomä · Accepted Answer · 2010-11-29 19:35:47Z

6

Take the random object out of your loop and don't set the seed everytime.

ArrayList<Integer> arrayRandom = new ArrayList<Integer>(n);

Random rand = new Random();
rand.setSeed(System.currentTimeMillis());
for (int i=0; i<n; i++)
{
    Integer r = rand.nextInt() % 256;
    arrayRandom.add(r);
}

This should work better.

answered Nov 29, 2010 at 19:35

Jan Thomä

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2 Comments

cadolphs Over a year ago

Exactly! Otherwise, you'll end up re-setting the random number generator with the same seed for a couple of times (because it'll run faster than one millisecond)

Sparky123 Over a year ago

Thanks so much! This works a treat. The most obvious thing to someone with more experience is so distreessing for a novice like me!

Tim Bender · Accepted Answer · 2010-11-29 19:37:10Z

1

You problem is that you keep resetting the seed and thus restarting the pseudo random number generator (PRNG) sequence.

Do this:

Random rand = new Random();
rand.setSeed(System.currentTimeMillis());
for (int i=0; i<n; i++)
{
    Integer r = rand.nextInt(256);
    arrayRandom.add(r);

}

answered Nov 29, 2010 at 19:37

Tim Bender

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Comments

Falmarri · Accepted Answer · 2010-11-29 19:38:20Z

0

Don't set the seed every iteration

        Random rand = new Random();
        rand.setSeed(System.currentTimeMillis());

Do it once.

answered Nov 29, 2010 at 19:38

Falmarri

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