How to resize a multidimensional array without using an arraylist (java)

Question

I need to reset the size of an array from w[i][j][q][k] to w[i+2][j+2][q][k]. I don't want to use an array of lists as I would have to change large parts of my program. I read in some threads that it is possible to create a new array of the desired size, and copy the contents from the original array to the new array using java.lang.System.arraycopy(...).

I tried this as follows, but it does not work with my approach:

 int [][][][] w = new int [18][18][[Main.V+1][Main.k]; 

 (...)

 int[][][][] wNew = new int[20][20][Main.V+1][Main.k]; 

 for(int i=0; i<wNew.length; i++){
        for(int j=0; j<wNew[0].length; j++){
            for(int q=0; q<wNew[0][0].length; q++){
                for(int k=0; k<wNew[0][0][0].length; k++){
                    System.arraycopy(w, 0, wNew, 0, 18);
                }
            }
        }
    }

w = wNew;

(...)

when manipulating the array at the added positions, a java.lang.ArrayIndexOutOfBoundsException: 18 occurs (example below:

w[0][19][1][0] = 1; (this line now causes an error)

How does it not work with your approach?

Elliott Frisch
– Elliott Frisch

2017-01-10 02:12:19 +00:00
Commented Jan 10, 2017 at 2:12 — Elliott Frisch
– Elliott Frisch, Commented Jan 10, 2017 at 2:12
I added the error message.

Hendrik
– Hendrik

2017-01-10 02:24:28 +00:00
Commented Jan 10, 2017 at 2:24 — Hendrik
– Hendrik, Commented Jan 10, 2017 at 2:24

Bruno Delor · Accepted Answer · 2017-01-10 03:53:39Z

1

System.arraycopy(w, 0, wNew, 0, 20);

this 20 is the number of array element to copy. You've put the new size, use the old one. In your example it's 18.

Secondly your for loops are based on your new size. This is backward, you have to read your original array and insert into the new array, so you have to iterate on the original array size(for resizing up, of course to make it smaller it would be the other way around).

But more importantly you don't have to iterate on all the dimentions. I'll past you some code you'll can execute to see for yourself.

import java.util.Arrays; import java.util.Random;

public class Test {

    public static final int SIZE_DIM1 = 2;
    public static final int SIZE_DIM2 = 2;
    public static final int SIZE_DIM3 = 5;
    public static final int SIZE_DIM4 = 5;
    private static final int INCREMENT = 2;

    public static void main(String[] args) {
        int[][][][] w = new int[SIZE_DIM1][SIZE_DIM2][SIZE_DIM3][SIZE_DIM4];

        randomFill(w);

        display(w);

        int[][][][] wNew = new int[SIZE_DIM1 + INCREMENT][SIZE_DIM2 + INCREMENT][SIZE_DIM3][SIZE_DIM4];

        for (int i = 0; i < w.length; i++) {
            for (int j = 0; j < w[i].length; j++) {
                System.arraycopy(w[i][j], 0, wNew[i][j], 0, w[i][j].length);
            }
        }

        display(wNew);

        w = wNew;

        w[0][3][4][4] = 1;

    }

    public static void randomFill(int[][][][] w) {
        Random random = new Random();
        for (int[][][] w2 : w) {
            for (int[][] w3 : w2) {
                for (int[] w4 : w3) {
                    for (int i = 0; i < w4.length; i++) {
                        w4[i] = random.nextInt();
                    }
                }
            }
        }
    }

    public static void display(int[][][][] w) {
        System.out.println("Printing---------------------------------------------------------------------------------");


        System.out.print("[\n");
        for (int[][][] w2 : w) {
            System.out.print("\t[\n");
            for (int[][] w3 : w2) {
                System.out.print("\t\t[\n");
                for (int[] w4 : w3) {
                    System.out.print("\t\t\t[");
                    for (int element : w4) {
                        System.out.print(element + " ");
                    }
                    System.out.print("]\n");
                }
                System.out.print("\t\t]\n");
            }
            System.out.print("\t]\n");
        }
        System.out.print("]\n");

    }

}

As you can see you don't have to iterate on all the sub arrays. just on the ones that have their sizes changing.

Execute it and it will be obvious.

edited Jan 10, 2017 at 3:53

answered Jan 10, 2017 at 2:41

Bruno Delor

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4 Comments

Hendrik Over a year ago

That works. The problem is that I get an a java.lang.ArrayIndexOutOfBoundsException: 18 in the subsequent lines when first manipulating w again

Bruno Delor Over a year ago

In your for loops you should use w.length. You're copying stuff so you only have to 'read' from your source. If you need to initialize the destination's element do it before the main loop also

Hendrik Over a year ago

Thank you for the illustration. But how do I make w = wNew in terms of size?

Bruno Delor Over a year ago

I've updated my answer again, it's been a while I haven't used plain old arrays. it happens that Systrem.arraycopy(...) don't keep the "extra" space in the indexes it copies. This mean the w[0][19] won't work but w[19][19] will with my initial answer. To fix this refers to my updated answer. You need to iterate on the changing dimensions only

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