Java Math.random() concern

Question

so I have a 700x700 screen and I want to create an object (an asteroid) to travel from the left side of the screen to the right side. As soon as it gets out of screen, I want it to spawn back at a random Y ana do the same thing. So far, I have created the object to spawn at a random Y and travel to the right, however, between 0:100 pixels and 600:700 pixels, I have objects and I do not want my object to spawn and travel through those objects. I want the random Y to be between let's say 101 and 599.

if (centerX > width + 200) {
                isTravelling = false;
                centerX = -200;
                centerY = (int)(Math.random());
            }

`Math.random()` returns a positive `double` in the range `[0.0, 1.0)`. May wanna double-check your math. See http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html. — Bucket, Nov 22 '13 at 21:18

ljgw · Accepted Answer · 2013-11-22T21:36:44.113

5

try

import java.util.Random;

public class YourClass {
    Random rnd = new Random();
    // insert variable declarations here...

    public void yourMethod() {
        if (centerX > width + 200) {
            isTravelling = false;
            centerX = -200;
            centerY = rnd.nextInt(499) + 101;
        }
    }
}

edited Nov 22 '13 at 21:36

answered Nov 22 '13 at 21:19

ljgw

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Creating a new Random object every time you need a random integer is a poor practice, instead create the random object once and reuse it. – dimo414 Nov 22 '13 at 21:22
I agree and will fix my example – ljgw Nov 22 '13 at 21:25
I decided and I want my max Y to be 450 and my min Y to be 100, but it's still not working. – MrSilent Nov 22 '13 at 21:44
1

try `centerY = rnd.nextInt((max - min) + 1) + min;` with the values of your choice in `max` and `min`. – ljgw Nov 22 '13 at 21:47
This is the answer, thank you. – MrSilent Nov 22 '13 at 21:49

score 1 · Answer 2 · answered Nov 22 '13 at 21:18

1

Math.floor(Math.random() * 499 + 101) is what you're looking for

answered Nov 22 '13 at 21:18

Andrei Nicusan

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This is inadvisable, as it does not correctly generate a uniformly random value from min to max. – dimo414 Nov 22 '13 at 21:21

score 1 · Answer 3 · answered Nov 22 '13 at 21:19

1

You could do the following:

min + ((int) (Math.random() * (max - min + 1))

Math.random() generates a double value in the range [0,1) (i.e. including 0 and excluding 1).

answered Nov 22 '13 at 21:19

Chthonic Project

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This is inadvisable, as it does not correctly generate a uniformly random value from min to max. – dimo414 Nov 22 '13 at 21:21
I just found [this explanation](http://stackoverflow.com/questions/738629/math-random-versus-random-nextintint). I didn't know the difference. Thanks for sharing the knowledge :-) – Chthonic Project Nov 22 '13 at 21:26

dimo414 · Answer 4 · 2013-11-22T21:51:11.187

Math.random() returns a double in the range [0-1), which is not what you want.

You can create a new Random() object and call its nextInt(max) method in order to get a uniformly random number between 0 and max (not including max).

private final int MAX = 600;
private final int MIN = 101;
private Random rnd = new Random();

public void doStuff() {
  // before
  if (centerX > width + 200) {
    isTravelling = false;
    centerX = -200;
    centerY = rnd.nextInt(MAX-MIN+1)+MIN;
  }
  // after
}

Java Math.random() concern

4 Answers4