2D random walk, Java

Question

I'm writing a two-dimensional random walk that takes command line arguments. It's supposed to estimate how long it takes the random walker to hit the boundary of a 2N-by-2N square centered at the starting point.

What I got so far is:

public class RandomWalk 
{
    public static void main(String[] args) 
    {
        int N = Integer.parseInt(args[0]);
        int reps = Integer.parseInt(args[1]);
        int x = 0;       
        int y = 0;       
        double r;
        int steps = 0;

        while (x*x + y*y <= N*N) {
            steps++;
            r = Math.random();

            if      (r <= 0.25) x++;
            else if (r <= 0.50) x--;
            else if (r <= 0.75) y++;
            else if (r <= 1.00) y--;
        }

        System.out.println(steps);
   }

}

Just want to check if you guys think im doing it wrong.

Are you having problems? This might be more appropriate for [Code Review](http://codereview.stackexchange.com/). — Beau Grantham, Sep 06 '12 at 18:02
Yea you might be right. Dont know if im having any problems since im not nearly experienced enough on this to know. Just wanted to get a quick answer from someone that might know. Im asked to print it with N = 1000 and reps = 1000 and im getting 1528331 wich im guessing the steps the walker has to take. — bangalo, Sep 06 '12 at 18:05
what is the second command line argument supposed to represent? — Eric, Sep 06 '12 at 18:37

Matzi · Accepted Answer · 2012-09-06T18:52:40.420

3

Your program terminates only when the random walk reaches a treshold. Not the boundary of the area.

while (x*x + y*y <= N*N) {

An example: N = 100, x = 90, y = 90 ==> 90*90 + 90*90 = 16 200 > 10 000.

Switch it to:

while (x > -N && x < N && y > -N && y < N) {

This will be better. And your reps variable is newer used. Not even once, beside the setting it's value.What should be the purpose ot that?

edited Sep 06 '12 at 18:52

answered Sep 06 '12 at 18:14

Matzi

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+1 Nice answer - to answer your question, I think he is supposed to take the average over the number of reps - so I think he's missing another loop in there. – jeff Sep 06 '12 at 22:28

Eric · Answer 2 · 2012-09-06T18:35:54.273

1

Your boundary condition, x*x + y*y <= N*N describes a circle, not a square. Also, the phrasing "hit" implies the condition does not include the boundary (<).

You want Math.abs(x) < N && Math.abs(y) < N

edited Sep 06 '12 at 18:35

answered Sep 06 '12 at 18:29

Eric

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2D random walk, Java

2 Answers2