Reusing math.random

Question

I am making an app in Android studio that creates a series of random numbers and then uses them to make an equation, that the user answers.

The problem I have is that after they have awnsered correctly and press a button to generate a new equation, the program uses the same numbers it randomized the first time.

Things I have tried: putting the array outside the method, under class.

Using both Math.random and Random rng = new Random() .

Make sure that the method is started several times, using System.out.println .

I have also tried the code in Eclipse, where it works like it should.

The randomiser in my code:

public void SkapaUppgift() {

    System.out.println(test);
    test=test+1;

    for (int i = 0; i < tal.length; i++) {
        Random rng= new Random();
        tal[i] = (int) (Math.random() * ((200 - 0) + 1) + 0);
        //tal[i] = rng.nextInt(200);
    }
}

edit: to clarify. I have used both Random rng AND math.random, and both have been specified to a range between 0 and 200. The problem is that every single time i call the method, the same random number gets pulled. For example: I start the app and get the number 20, 25, 200 and 3. The next time I call the method, I get 20, 25, 200 and 3. The third time I get 20, 25, 200 and 3. If I close the app and start it again, I get 4 different numbers. I want to get different numbers each time I call the method, not just when I restart the app.

Answer 1

You're creating a new Random instance, but not using it. Try using rng.nextInt() instead of Math.random() .

The reason why this is not working is because Math.random() returns a random double between 0 (inclusive) and 1 (exclusive), but it's being downcast to an int, and, thus, being truncated, leaving you with only 1 possible outcome.

EDIT: In that case, you will need to include more code. Where is this being called from? Where are the results being printed out? I have a feeling that this method is working properly and you're actually just printing the same array each time.

Remember that objects in Java are passed by value, not by reference.

Answer 2

You should get the results you want from rng.nextInt(). You just have to make sure you only make it one time and use the same instance forever after . In other words, make it a Singleton.