Simulate the roll of a bias or weighted dice.

Recently I can across this problem and find it really interesting, so i thought why not write a code and share it.

There is a probability distribution given for a n sided dice in form an array. Write a code in java to simulate the roll of dice?

I came up with a solution initially which I am not proud of involving caching the outcomes and then based on that rolling the dice. But some googling pointed me finally to this Stackoverflow post:

http://stackoverflow.com/questions/5850445/simulating-a-roll-with-a-biased-dice

Which explains the algorithm for the same:

4down voteaccepted

In general, if your probabilities are {p1,p2, ...,p6}, construct the following helper list: {a1, a2, ... a5} = { p1, p1+p2, p1+p2+p3, p1+p2+p3+p4, p1+p2+p3+p4+p5} Now get a random number X in [0,1] If X <= a1 choose 1 as outcome a1 < X <= a2 choose 2 as outcome a2 < X <= a3 choose 3 as outcome a3 < X <= a4 choose 4 as outcome a4 < X <= a5 choose 5 as outcome a5 < X choose 6 as outcome Or, more efficient pseudocode if X > a5 then N=6 elseif X > a4 then N=5 elseif X > a3 then N=4 elseif X > a2 then N=3 elseif X > a1 then N=2 else N=1 Edit This is equivalent to the roulette wheel selection you mention in your question update as shown in this picture:

I decide why don't i write a Java code for the hep of others:

package com.biplav.algorithms;

import java.util.Random;

public class WeightedDiceSimulation {

private static Random r= new Random();

public static int roll(int[] n) {

int[] wieghtedSum = new int[n.length];

wieghtedSum[0] = n[0];

for(int i=1;ilength

;i++) {

wieghtedSum[i] = wieghtedSum[i-1]+n[i];

}

int p = r.nextInt(100);

for(int i=0;ilength

-1;i++) {

if(p <= wieghtedSum[i]) 

return i+1;

else if(p > wieghtedSum[i] && p<=wieghtedSum[i+1]) {

return i+2;

}

}

return n.length;

}

public static void main(String[] args) {

int[] n = new int[] {20,30,10,0,20,20};

for(int i=0;i<10 i="" p="">

System.out.println(roll(n));

}

}

}