random number

[tyhooke cologne]

anyone know a slick way to create a random number between a negitive number and it's corosponding positive one?
i.e. gimme a random # between -5 and 5
thanks
-tyhooke

[Henry Wong]

How about this?

Random r = new Random();
int i = r.nextInt(11) - 5;

Henry

[Jeff Bosch]

A more generic solution may be:

import java.util.Random;
 
class RandomTest 
{
  public static void main(String[] args) 
  {
    RandomTest rt = new RandomTest();
    for (int n = 1; n < 11; n++)
    {
      rt.showNext( n );
    }
  }
 
  public void showNext( int number ){
    Random r = new Random();
    int random = r.nextInt( 2 * number ) - number;
    System.out.println("Random for n = " + number + " is " + random);
  }
}
  

The output:

---------- Java Runtime ----------
Random for n = 1 is 0
Random for n = 2 is 1
Random for n = 3 is -3
Random for n = 4 is 3
Random for n = 5 is -5
Random for n = 6 is -6
Random for n = 7 is 1
Random for n = 8 is 6
Random for n = 9 is -9
Random for n = 10 is -10
 
Output completed (0 sec consumed) - Normal Termination

[tyhooke cologne]

these help. thank you both!
-Tyhooke

[Jeff Bosch]

You're welcome!

[Patrick Joseph]

How about:

class RandomNum {
public static void main(String[] args) {
int[] numList = {-5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5};

int randIndex = (int)(Math.random() * numList.length);
int randNum = numList[randIndex];
System.out.println("Random number: " + randNum);
}
}

This gives you a random index (range of 0 to [array length - 1]) that will then give you the random contents (-5 to 5).
[ January 12, 2005: Message edited by: PJ Chin ]

[Jeff Bosch]

Sure, but what happens when the user comes back and says, "Oh, now we need -10 to 10"? The hard-coded values method would need to be rewritten, where the general method would not. I always try to think "Reusability" when I write code, and part of that is not having any hard-coded values (except perhaps in the testing/driver program)...

[David Harkness]

Originally posted by Jeff Bosch:
int random = r.nextInt( 2 * number ) - number;

Keep in mind that Random.nextInt(number) returns a value in the range [0, number), meaning 0, 1 ... number - 1. It will not return number itself. Thus the general solution should be changed to

int random = r.nextInt( 2 * number + 1 ) - number;

[Jeff Bosch]

Ah, good point! I'd whipped that solution up in haste and something about it just didn't feel right, but I didn't really have time to go back to it.

[Layne Lund]

To add more flexibility, you may want to write a method with the following signature:

int randomInt(int min, int max)

This would return a random integer between min and max. I'll leave the implementation as the proverbial exercise for the reader. However, it shouldn't be difficult to modify Jeff's program above for this purpose.

Layne