CODEWITHSUNDEEP
javaprobability
Soham Banerjee
Soham Banerjee

Reputation: 61

Different random output when run differently

I am trying to solve a problem:- A point is at an initial position X. It can be shifted either left or right. If it is moved either left or right with equal probability 10 times, what is the probability of it ending up in its initial position X? I used the following java program:-

import java.util.Random;

public class ProbMain {

int left;
int right;
int error;
double middle;

public ProbMain() {
    left = right = error = 0;
    middle = 0.0;
}

void pushLeft() {
    ++left;
}

void pushRight() {
    ++right;
}

void push() {
    int whichWay;
    Random rand = new Random();
    whichWay = rand.nextInt(2);
    if (whichWay == 0)
        pushLeft();
    else if (whichWay == 1)
        pushRight();
    else
        ++error;
}

public static void main(String[] args) {
    ProbMain obj = new ProbMain();
    for (int b = 0; b < 10000; b++) {
        for (int a = 0; a < 10000; a++) {
            for (int i = 0; i < 10; i++)
                obj.push();
            if (obj.left == obj.right)
                ++obj.middle;
            obj.left = obj.right = 0;
        }
    }
    System.out.println("Error: " + obj.error);
    System.out.println("Probability of middle: " + obj.middle / (10000*10000));
}

}

Weird thing is that when I run this on Eclipse I get result around 0.05 but when I run from command line I get result around 0.24. Why so? And which one is correct?

Upvotes: 1

Views: 67

Answers (1)

Cybran
Cybran

Reputation: 2313

You are creating a new Random object each time you want to retrieve a random number (in the push() method) - this can lead to very poor entropy and create strange results when the program is run with different timings - usually running from eclipse will be much slower due to the attached debugger, which will yield better random results when the RNG is initialized with a time value as seed.

You should change your program to use only ONE Random instance, for example by declaring a new Random member variable and initializing it once in your ProbMain constructor.

Upvotes: 2

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