Both variables changing when I modify one

Question

I'm doing an algorithm in like this:

public int[][] moveLeft(int m[][], int[] index){
    Puzzle x = new Puzzle(m);
    System.out.println(x);
    int[][] p = m;
    int temp = p[index[0]][index[1]];
    p[index[0]][index[1]] = p[index[0]][index[1]-1];
    p[index[0]][index[1]-1] = temp;
    return p;
}

To be more specific, what i'm trying to do is to change the position of certain values and return the new matrix, but when i'm debuging, i noted that the value "m" also changues, even when im doing changes to the value p. What is wrong here?

Ok first of all, puzzle its just an object holding a matrix, done that for testing purposes, the moment when m starts "imitating" p happens at line 6, when i switch the positions in "p", the debugger shows that m switches positions too — chntgomez, Sep 20 '12 at 01:16

score 1 · Accepted Answer · edited May 23 '17 at 12:27

1

Here is your culprit:

int[][] p = m;

Once you do this, p and m refer to the same exact object, because arrays in Java are reference objects. Any change to p's content will be reflected in m, because p and m are two different names for a single object.

If you need p to be a copy of m, you should do it explicitly. Here is a link to an answer showing you how it is done.

edited May 23 '17 at 12:27

Community

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answered Sep 20 '12 at 01:17

Sergey Kalinichenko

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Thats the problem! Thanks a lot man... How can I "copy" the object? – chntgomez Sep 20 '12 at 01:21
@chntgomez Take a look at the answer that I linked, it shows you two ways of doing it (through cloning and through arraycopy). – Sergey Kalinichenko Sep 20 '12 at 01:25

score 0 · Answer 2 · answered Sep 20 '12 at 01:19

0

I think you should use :

int [][] p = new int[m.length][];
for(int i = 0; i < m.length; i++)
   p[i] = m[i].clone();

instead of int[][] p = m;

so that p and m refer to two different objects.

answered Sep 20 '12 at 01:19

Ouadie

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Nailed it with your solution. Thanks a lot! – chntgomez Sep 20 '12 at 01:26

Both variables changing when I modify one

2 Answers2