How to deep copy an irregular 2D array

How can I deep copy an irregularly shaped 2D array in Java?

Ie.

int[][] nums =  {{5},
                 {9,4},
                 {1,7,8},
                 {8,3,2,10}}

I'm unable to use Arrays.arrayCopy() for some reason (versioning?)

Solution 1:

int[][] copy = new int[nums.length][];

for (int i = 0; i < nums.length; i++) {
    copy[i] = new int[nums[i].length];

    for (int j = 0; j < nums[i].length; j++) {
        copy[i][j] = nums[i][j];
    }
}

You can replace the second loop with System.arraycopy() or Arrays.copyOf().

Solution 2:

I wrote this in Eclipse, tested it, came back and found that João had beaten me to almost exactly the same solution. I upvoted him, but here's mine for comparison. I guess it's instructive to see the very slight details people choose to do differently.

private static int[][] copy2d(int[][] nums) {
    int[][] copy = new int[nums.length][];

    for (int i = 0; i < copy.length; i++) {
        int[] member = new int[nums[i].length];
        System.arraycopy(nums[i], 0, member, 0, nums[i].length);
        copy[i] = member;
    }

    return copy;
}

For extra credit, try writing one that copies an n-dimensional array where n is arbitrary.

Solution 3:

N-dimensional deep copy

public class ArrayTest extends TestCase {

    public void testArrays() {
        Object arr = new int[][]{
                {5},
                {9, 4},
                {1, 7, 8},
                {8, 3, 2, 10}
        };

        Object arrCopy = copyNd(arr);
        int height = Array.getLength(arr);
        for (int r = 0; r < height; r++) {
            Object rowOrigonal = Array.get(arr, r);
            Object rowCopy = Array.get(arrCopy, r);
            int width = Array.getLength(rowOrigonal);
            for (int c = 0; c < width; c++) {
                assertTrue(rowOrigonal.getClass().isArray());
                assertTrue(rowCopy.getClass().isArray());
                assertEquals(Array.get(rowOrigonal, c), Array.get(rowCopy, c));
                System.out.println(Array.get(rowOrigonal, c) + ":" + Array.get(rowCopy, c));
            }
        }
    }

    public static Object copyNd(Object arr) {
        if (arr.getClass().isArray()) {
            int innerArrayLength = Array.getLength(arr);
            Class component = arr.getClass().getComponentType();
            Object newInnerArray = Array.newInstance(component, innerArrayLength);
            //copy each elem of the array
            for (int i = 0; i < innerArrayLength; i++) {
                Object elem = copyNd(Array.get(arr, i));
                Array.set(newInnerArray, i, elem);
            }
            return newInnerArray;
        } else {
            return arr;//cant deep copy an opac object??
        }
    }
}

Solution 4:

Some folks suggest clone() -- just to be extra clear, clone() on a multi-dimensional array is only a shallow clone. original.clone()[0] == original[0]. But (for primitives) you can use clone() instead of System.arraycopy() once you're down to one-dimensional arrays.

Solution 5:

Here's one that specializes to deeply cloning int[][]. It also allows any of the int[] to be null.

import java.util.*;

public class ArrayDeepCopy {

    static int[][] clone(int[][] arr) {
        final int L = arr.length;
        int[][] clone = new int[L][];
        for (int i = 0; i < clone.length; i++) {
            clone[i] = (arr[i] == null) ? null : arr[i].clone();
        }
        return clone;
    }

    public static void main(String[] args) {
        int[][] a = {
            { 1, },
            { 2, 3, },
            null,
        };
        int[][] b = a.clone();
        System.out.println(a[0] == b[0]); // "true", meaning shallow as expected!

        b = clone(a); // this is deep clone!
        System.out.println(Arrays.deepEquals(a, b)); // "true"
        System.out.println(a[0] == b[0]); // "false", no longer shallow!
    }
}