Issue with Random numbers in Java

Question

I wrote a piece of code to move an object in a Random Direction in Java. There are two functions.

FindRandomDirection - Gets a random direction from 8 possible directions (Directions are represented by the numbers 1,2,3,4,6,7,8,9 in a numpad). Check is done to see if the object is near any of the borders. If so the object will move in a direction away from the border.
MoveObject - Changes the (X, Y) co-ordinates of the object by moving by a constant STEP.

But what ever values I give for X, Y; after repeating the process several times (700 or more), the values of X, Y becomes {X: 20-50} and {Y: 450-465}.

I.e.

Case 1: (x:35,y:65) becomes (x:35, y:465)
Case 2: (x:30, y:455) becomes (x:30, y:460)
Case 3: (x:435, y:65) becomes (x:25, y:460)
Case 4: (x:430, y:465) becomes (x:40, y:460)

I would like to why the after several iterations the x, y points converge towards these values even though I generate the numbers at random.

Below is the code for the same.

import java.io.*;
public class bug
{
    //public static int x = 35;
    //public static int y = 60;

    //public static int x = 35;
    //public static int y = 460;

    //public static int x = 435;
    //public static int y = 60;

    public static int x = 435;
    public static int y = 460;

    public static final int NORTH = 8;
    public static final int EAST = 6;
    public static final int WEST = 4;
    public static final int SOUTH = 2;
    public static final int NORTHEAST = 9;
    public static final int NORTHWEST = 7;
    public static final int SOUTHWEST = 1;
    public static final int SOUTHEAST = 3;
    public static final int STEP = 5;

    //Function to move the object in a specified direction.
    public static void moveObject(int direction)
    {
            double nX = 0, nY=0;
            switch(direction)
            {
            case  NORTH:
                nY = y- STEP; 
                nX = x;
                break;
            case  SOUTH:
                nY = y+ STEP; 
                nX = x;
                break;
            case  EAST:             
                nY = y; 
                nX = x +  STEP;
                break;
            case  WEST:
                nY = y; 
                nX = x- STEP;               
                break;
            case  NORTHEAST:
                nX = x +  STEP;
                nY = y- STEP; 
                break;
            case  NORTHWEST:
                nX = x- STEP;
                nY = y- STEP; 
                break;
            case  SOUTHEAST:
                nX = x +  STEP;
                nY = y+ STEP; 
                break;
            case  SOUTHWEST:
                nX = x- STEP;
                nY = y+ STEP; 
                break;
            }
            x = (int) nX;
            y = (int) nY;
            System.out.println("Direction: "+direction+"; X: "+x+"; Y: "+y);
        }
//Function to move the object in a random direction
//Also if wall(Border) is present the object should move in proper direction
    public static int findRandomDirection(int objObjectX, int objObjectY)
    {
        int[] move = {1,2,3,4,0,6,7,8,9};
        int randDir=0;
        //Generate a random direction to move. Generate new direction if the objected can not be moved in a direction 
        do
        {
            java.util.Random randomGenerator = new java.util.Random();
            randDir = randomGenerator.nextInt(8);

            //If the object lies near East Border, it can not move in that direction
            if(objObjectX <= 25)
            {
                move[0] = 0;
                move[3] = 0;
                move[6] = 0;
            }

            //If the object lies near West Border, it can not move in that direction
            if(objObjectX >= 465)
            {
                move[2] = 0;
                move[5] = 0;
                move[8] = 0;                
            }

            //If the object lies near North Border, it can not move in that direction
            if(objObjectY <= 25)
            {
                move[6] = 0;
                move[7] = 0;
                move[8] = 0;
            }

            //If the object lies near South Border, it can not move in that direction
            if(objObjectY >= 465)
            {
                move[0] = 0;
                move[1] = 0;
                move[2] = 0;                
            }
        } while(move[randDir]==0);
        return move[randDir];       
    }
    public static void main(String[] args)
    {
        for(int i = 0; i<1000;i++)
        {
        int dir=findRandomDirection(x,y);   
        moveObject(dir);
        }
    }
}

So over time, my object moves towards the lower left corner of the board. Kindly help me in finding the bug.

You should never re-initialize a pseudo-random number generator in a loop. Create the object once for the class and re-use it. While java.util.Random includes code to deal with this mistake it's still better not to pick up bad habits. — Joey
– Joey, Commented Mar 13, 2012 at 21:25

VeeArr · Accepted Answer · 2012-03-13 21:23:14Z

6

Since you are using nextInt(8), the value returned will always be between 0 and 7 (inclusive). Since 8 is never returned, movement is biased towards the opposite direction. You probably want to use nextInt(9), to return a value between 0 and 8 (inclusive).

edit: To clarify, since "8" is never chosen as your random direction, and moves[8]==9, the object never moves in the NORTHEAST direction, which means over time it will tend to travel SOUTHWEST.

Also, as @Joey said above, you shouldn't reinitialize the Random object every time, but that isn't what was causing the drifting behavior.

answered Mar 13, 2012 at 21:23

VeeArr

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1 Comment

ggrigery Over a year ago

Funny how this question sits here for 45 minutes without an answer (I know, I was trying to figure it out), then your answer manages to attract 2 more that are almost the same :)

akgood · Accepted Answer · 2012-03-13 21:26:46Z

When you select a direction, you select a value between 0 and 7 inclusive. These values (in your mapping) correspond to:

public static final int SOUTHWEST = 1;
public static final int SOUTH = 2;
public static final int SOUTHEAST = 3;
public static final int WEST = 4;
public static final int EAST = 6;
public static final int NORTHWEST = 7;
public static final int NORTH = 8;

But note that this will never be chosen:

public static final int NORTHEAST = 9;

So, it's not surprising to me that your code seems biased toward the southeast...

erickson · Accepted Answer · 2012-03-13 21:28:28Z

1

The nextInt(n) method return numbers from zero, inclusive, to n, exclusive. Since you are passing the value 8, the results are from the set of 8 values, 0–7. Thus, the ninth element of the array, which represents the northeast direction, is never selected.

This bias against the northeast results in an eventual migration to the southwest.

answered Mar 13, 2012 at 21:28

erickson

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