Creating a copy of a multi-dimensional array

Question

I'm trying to create a copy of a multi-dimensional array such that any modification to the new array will not affect the original array.

Here is my code:

public static int[][] getNextGenUniverse(int[][] validUniverse) {

    // Create a copy of the original array

    int[][] nextUniverse = new validUniverse[][];

    // Iterate through all the elements of the array

    for (int i = 0; i < nextUniverse.length; i++) {

      // Iterate through all the elements of the subarrays

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

      nextUniverse[i][j] = validUniverse[i][j];
      }
    }
}

I'm having trouble declaring the length of the new array. Could someone help me please?

What do you mean with "any modification to the new array will not affect the original array"? Do you mean a shallow copy, where changing the new array's content does not change the other array's content, or do you mean a deep copy, where modifying objects _inside_ the new array do not modify ojects _inside_ the old array? — Mike 'Pomax' Kamermans, Mar 28 '19 at 00:52
@Mike'Pomax'Kamermans I mean that changing an element of the new array will not change the corresponding element of the original array. — Elizabeth, Mar 28 '19 at 00:57
there is still some ambiguity there, because "changing an element" usually means doing something like `arr[i].setValue()`, which will update _that object_ irrespective of what it's in. So did you mean a shallow copy, where the arrays are different, but they point to the same objects, or did you mean a deep copy, where _all the data involved_ gets copied, and immediately after copying there is already no relation between any of the data that either array points to? (e.g. "do you only want a copy of the library's indexing book, or do you want to copy the entire library including all the books") — Mike 'Pomax' Kamermans, Mar 28 '19 at 01:08

score 1 · Answer 1 · answered Mar 28 '19 at 00:50

1

It should be the same length as validUniverse. And you can use Arrays.copyOf(int[], int) to perform the copy. Like,

public static int[][] getNextGenUniverse(int[][] validUniverse) {
    int[][] r = new int[validUniverse.length][];
    for (int i = 0; i < validUniverse.length; i++) {
        r[i] = Arrays.copyOf(validUniverse[i], validUniverse[i].length);
    }
    return r;
}

answered Mar 28 '19 at 00:50

Elliott Frisch

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My code should be fine if I only change the declaration of the size of the new array? The Arrays.copyOf(int[], int) statement is optional, right? – Elizabeth Mar 28 '19 at 00:55
More or less, yes. However, Elliott's solution works for copying all matrices that aren't hellbent on destroying your computer. – Leftist Tachyon Mar 28 '19 at 01:01
@ElliotFrisch Don't you have to also initialize the length of the sub-arrays because I still get a NullPointerException – Elizabeth Mar 28 '19 at 13:00

Creating a copy of a multi-dimensional array

1 Answers1