Trying to make dice roll java app with soft-coding (array size based on input), getting an ArrayIndexOutOfBoundsException

Question

EDIT: Solved with simple change, solution at the bottom

I am supposed to make the program count how many times each value is rolled in a set of dice rolls and print that number out for each value. The array size is based on the ((number of dice * number of sides) + 1), but for some reason I still get this out of bounds exception with multiple different entries (for number of dice, sides, and rolls) I've tried. I don't see what could be throwing this exception.

import java.util.Scanner;
import java.lang.Math;

public class DiceRolls {
    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);
        int numRolls, numSides, numDice;
        int outcome = 0;

        /* prompt user for number of rolls */
        System.out.print("How many sides on each die? ");
        numSides = input.nextInt();
        System.out.print("How many dice should be rolled? ");
        numDice = input.nextInt();
        System.out.print("How many rolls? ");
        numRolls = input.nextInt();
        input.close();

        int arraySize = (numDice * numSides);

        int[] outcomes = new int[arraySize + 1];

        /* roll dice and add to outcomes */
        for (int roll = 0; roll < numRolls; roll++) {
            for (int dice = 0; dice < numDice; dice++) {
                outcome += (int)(numSides * Math.random() + 1);
            }
            outcomes[outcome] += 1;
        }

        /* show counts of outcomes */
        for (int i = numDice; i <= arraySize; i++) {
            System.out.println(i + ": " + outcomes[i]);
        }
    }
}

Solution

I added the outcome = 0; line to reset the outcome with each new roll. Otherwise the outcome would keep accumulating and reach values higher than what is possible in a single roll.

/* roll dice and add to outcomes */
for (int roll = 0; roll < numRolls; roll++) {
    outcome = 0; // <-----HERE
    for (int dice = 0; dice < numDice; dice++) {
        outcome += (int)(numSides * Math.random() + 1);
    }
    outcomes[outcome] += 1;
}

Indie Man Indie Man · Accepted Answer · 2016-03-16T15:25:56

Your problem is that your array isn't big enough. The problem lies within this line of code:

int[] outcomes = new int[arraySize + 1];

Your array is too small. You forgot to factor in the numRolls int into your array size calculation.

Trying to make dice roll java app with soft-coding (array size based on input), getting an ArrayIndexOutOfBoundsException

EDIT: Solved with simple change, solution at the bottom

Solution

1 Answers