Java random is always delivering a negative trend on the long run?

Question

I'm constructing an app and for that I have a function to fill it with test data. Short outline:

        HashMap<String, Long> iIDs = new HashMap<String, Long>();
        HashMap<String, Integer> vals = new HashMap<String, Integer>();

        long iID1 = addIndicator("I1", "i1", Color.RED);
        long iID2 = addIndicator("I2", "i2", Color.BLUE);
        long iID3 = addIndicator("I3", "i3", Color.GREEN);
        long iID4 = addIndicator("I4", "i4", Color.MAGENTA);

        iIDs.put("iID1", iID1);
        iIDs.put("iID2", iID2);
        iIDs.put("iID3", iID3);
        iIDs.put("iID4", iID4);

        int v1 = 80;
        int v2 = 30;
        int v3 = 25;
        int v4 = 40;

        vals.put("v1", v1);
        vals.put("v2", v2);
        vals.put("v3", v3);
        vals.put("v4", v4);

        int numDays = 500;
        int dateDistance = 14;

        Calendar c = Calendar.getInstance();

        for(int i=0;i<numDays;i++)
        {
            c.add(Calendar.DATE, dateDistance);
            for(int j=1;j<5;j++)
            {
                int currVal = vals.get("v"+j);
                int rand = new Random().nextInt(6);
                int newVal; 

                if(rand <= 2) // 0, 1, 2
                    newVal = currVal + rand;
                else          // 3, 4, 5
                    newVal = currVal - rand;

                pseudo: addPointForIndicator();
                vals.put("v"+j, newVal);
            }
        }

No matter how often I create the test data, the picture looks always like this:

So the trend of the random numbers is always negative. Why is that?

Earache

Why do you create a new Random object in each loop iteration? – Roach

Not that it necessarily explains your results, but your usage of Random is wrong. You must share the same instance across all calls, otherwise you are not getting a pseudorandom sequence. – Guerrero

Use the same instance of java.util.Random for generation of all random numbers. This will "increase randomization" by making the randomization true pseudo-randomization. – Chemosphere

Thanks, now I created the Random above the first for-loop, but as you said, the picture doesn't change. I'm just wondering... – Earache

Answer 1

It is quite clear from your logic that it must create a negative trend, even ignoring the fact that your usage of Random doesn't follow the contract. You add a number in the range [0,2] half of the time and subtract a number in the range [3,5] the other half of the time. The code is easy to fix, though:

if(rand <= 2) // 0, 1, 2
  newVal = currVal + rand;
else          // 3, 4, 5
  newVal = currVal - rand + 3;

And a cleaner fix would be

newVal = currVal + random.nextInt(7)-3;

This has the additional benefit that it allows the value to stay unchanged sometimes, which I believe should be a more proper way to simulate your data.

Guerrero

lol... you're right... :) ahh, logic, my enemy... thanks! I'll mark your post as answer in 5 min – Earache

@downvoter Why on Earth would you downvote this perfectly good, accepted answer? This is certainly nothing but vandalism, and the sorriest kind -- hiding behind your anonymity. – Guerrero

Answer 2

I am not sure what you are trying to do, but the following block would seem to produce a negative trend

if(rand <= 2) // 0, 1, 2
    newVal = currVal + rand;
else          // 3, 4, 5
    newVal = currVal - rand;

You are adding small numbers and subtracting larger ones.

Koumis

Right, Marko Topolnik just brought me to this... sometimes the most obvious is the hardest to see :P – Earache

Answer 3

I dont know what is your purpose, but try to make a lower bound. Like

2+random.nextInt()

Where random is your Random class instance. And as the others guys said, use the same instance, you cant generate a "correct" sequence like that.

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