Java - Generate a random -1 or 1?

Question

Does anyone have a quick way to randomly return either 1 or -1?

Something like this probably works, but seems less than ideal:

return Random.nextDouble() > .5 ? 1 : -1;

you're asking for ideal solutions when working with random numbers? ;) — asgs, Jun 14 '11 at 18:10
@mre : http://stackoverflow.com/questions/453479/how-good-is-java-util-random/454433#454433 — leonbloy, Jun 14 '11 at 19:04
@Mike: seeing Bozho's answers I can certainly see what's wrong with *nextDouble() > .5 > 1 : -1*. — SyntaxT3rr0r, Jun 14 '11 at 20:17

Bozho · Answer 1 · 2011-06-14T18:17:21.117

33

how about:

random.nextBoolean() ? 1 : -1;

where random is an instance of java.util.Random

edited Jun 14 '11 at 18:17

answered Jun 14 '11 at 18:10

Bozho

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+1 Because this is the simplest of all the answers (some are really clever). However, just want to add that it is likely that *the same Random instance* should generally be used instead of always creating a new one ... otherwise leads to interesting "duplicate values" in degenerate situations. – Jun 14 '11 at 18:14
@pst agreed on that. I just wanted to fit it on one line. I'll remove the instantiation. – Bozho Jun 14 '11 at 18:17
1

Warning: you don't want to code this line inside a loop, it would not work! A single random object should be created outside. – leonbloy Jun 14 '11 at 18:17
@leonbloy just addressed that. The example was simplified, now it's more complete. – Bozho Jun 14 '11 at 18:17
Just remember that `java.util.Random` class generates poor quality random numbers. It is suitable only for "casual" use. – Jarek Przygódzki Jun 14 '11 at 18:43
"poor quality" is relative. For most typical uses it's ok. For critical applications, it's not. http://stackoverflow.com/questions/453479/how-good-is-java-util-random/ – leonbloy Jun 14 '11 at 19:06

score 2 · Answer 2 · answered Jun 14 '11 at 18:09

2

return Random.nextInt(2) * 2 - 1;

answered Jun 14 '11 at 18:09

Peter Alexander

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@JustinKSU: No it wouldn't. The `nextInt` produces 0 or 1, which doubles to 0 or 2, which then becomes -1 or 1. The `nextInt` value is exclusive. – Peter Alexander Jun 14 '11 at 18:12

score 0 · Answer 3 · answered Jun 14 '11 at 18:09

0

new Random.nextInt(2) == 0 ? -1 : 1;

answered Jun 14 '11 at 18:09

JustinKSU

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nextInt(integer) is between 0 (inclusively) and int (exclusively). thus you need Random.nextInt(2) – iliaden Jun 14 '11 at 18:13

score 0 · Answer 4 · answered Jun 14 '11 at 18:10

0

How about

Random r = new Random();
n = r.nextInt(2) * 2 - 1;

answered Jun 14 '11 at 18:10

Ted Hopp

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score 0 · Answer 5 · answered Jun 14 '11 at 18:10

0

return Math.floor(Math.random()*2)*2-1;

answered Jun 14 '11 at 18:10

jwl

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score 0 · Answer 6 · answered Jun 14 '11 at 18:25

import java.util.Random;

public class RandomTest {

public static void main(String[] args) {
    for (int i = 0; i < 100; i++) {
        System.out.println(randomOneOrMinusOne());

    }
}
static int randomOneOrMinusOne() {
    Random rand = new Random();
    if (rand.nextBoolean()) return 1;
    else return -1;
}

}

Java - Generate a random -1 or 1?

6 Answers6