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)

Answer 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.