Array stored in array gets overwritten to the contents of the last one added

Question

Maybe it's something simple I'm missing, but I can't seem to figure out why my array of arrays is being overwritten with the last value in that is put into the array.

This is my code:

import java.util.Arrays;
import class.ArgsProcessor;

public class Dice {

    public static void main(String[] args) {
        ArgsProcessor ap = new ArgsProcessor(args);
        int D = ap.nextInt("How many dice will be thrown each time?");
        int T = ap.nextInt("How many throws?");

        int[] dieThrown = new int[D];
        int [][] thrown = new int[T][D]; 
        double randomNum;

        for (int t = 0; t < T; t++){
            for (int d = 0; d < D; d++){
                randomNum = Math.random();
                if (randomNum < 1.0/6.0)
                    dieThrown[d] = 1;
                else if (randomNum < 2.0/6.0)
                    dieThrown[d] = 2;
                else if (randomNum < 3.0/6.0)
                    dieThrown[d] = 3;
                else if (randomNum < 4.0/6.0)
                    dieThrown[d] = 4;
                else if (randomNum < 5.0/6.0)
                    dieThrown[d] = 5;
                else
                    dieThrown[d] = 6;           
            }
            System.out.println("On throw " + (t+1) + " we rolled:");
            System.out.println("\t" + Arrays.toString(dieThrown));
            thrown[t] = dieThrown;
        }
        System.out.println(Arrays.deepToString(thrown));
    }
}

Here's an example output for 3 dice thrown 3 times:

On throw 1 we rolled:
    [3, 4, 2]
On throw 2 we rolled:
    [1, 5, 4]
On throw 3 we rolled:
    [6, 5, 3]
[[6, 5, 3], [6, 5, 3], [6, 5, 3]]

However, I'm expecting something like this:

On throw 1 we rolled:
    [3, 4, 2]
On throw 2 we rolled:
    [1, 5, 4]
On throw 3 we rolled:
    [6, 5, 3]
[[3, 4, 2], [1, 5, 4], [6, 5, 3]]

How can I get it to add the right values?

Answer 1

You are adding the same array object dieThrown to the array thrown multiple times. Each iteration through the d for loop, you are overwriting the values in the same array, so the last iteration's values are the ones that remain. The thrown array is full of references to the same array object referred to by dieThrown .

Create a new array in each iteration, separate from any previous iteration's array. You can do this by moving the declaration of the dieThrown array:

int[] dieThrown = new int[D];

to the first line of the body of the t for loop, before the d for loop.

Answer 2

dieThrown is the same object throughout the program. You fill up your thrown array with references to the same object, and just replace its content in each iteration. So all your thrown entries basically point to the same place, and show the last content that has been assigned to dieThrown . To avoid that, use

dieThrown = new int[D]:

inside your outer loop, so that each element will be a separate array that will keep the content.

Answer 3

You could also get rid of dieThrown . You can directly add it to thrown . Following is the code

public class dice {

public static void main(String[] args) {
    int D = 3;
    int T = 3;

    int [][] thrown = new int[T][D];
    double randomNum;

    for (int t = 0; t < T; t++){
        for (int d = 0; d < D; d++){
            randomNum = Math.random();
            if (randomNum < 1.0/6.0)
                thrown[t][d] = 1;
            else if (randomNum < 2.0/6.0)
                thrown[t][d] = 2;
            else if (randomNum < 3.0/6.0)
                thrown[t][d] = 3;
            else if (randomNum < 4.0/6.0)
                thrown[t][d] = 4;
            else if (randomNum < 5.0/6.0)
                thrown[t][d] = 5;
            else
                thrown[t][d] = 6;
        }
        System.out.println("On throw " + (t+1) + " we rolled:");
        System.out.println("\t" + Arrays.toString(thrown[t]));
        //thrown[t] = dieThrown;
    }
    System.out.println(Arrays.deepToString(thrown));
}


} 

Notice I am adding to thrown directly

Output

On throw 1 we rolled:
    [5, 4, 4]
On throw 2 we rolled:
    [4, 4, 2]
On throw 3 we rolled:
    [6, 6, 6]
[[5, 4, 4], [4, 4, 2], [6, 6, 6]]

Answer 4

You are re-using dieThrown each loop without creating a new array each iteration. You can solve this by moving the initialisation of dieThrown to the start of the outer loop (see it running here: IDEONE ); or, a better solution, is to get rid of dieThrown and update the thrown array directly.

You can also simplify everything using java.util.Random#nextInt(int) instead of Math.random .

IDEONE

public static void main( String[] args ){
    int D = 3;
    int T = 4;

    int [][] thrown = new int[T][D]; 
    Random rand = new Random();

    for (int t = 0; t < T; t++){
        for (int d = 0; d < D; d++){
            thrown[t][d] = rand.nextInt(6)+1;
        }
        System.out.println("On throw " + (t+1) + " we rolled:");
        System.out.println("\t" + Arrays.toString(thrown[t]));
    }
    System.out.println(Arrays.deepToString(thrown));
}