copy a 2d array in java

Question

Hi guys - i have a 2d array called matrix of type int that i want to copy to a local variable in a method so i can edit it

whats the best way to copy the array, i am having some troubles

for example

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

    //do some stuff here
    return true;
}

Answer 1

You are not initializing the local 2D array.

int[][] myInt = new int[matrix.length][];
for(int i = 0; i < matrix.length; i++)
{
  myInt[i] = new int[matrix[i].length];
  for (int j = 0; j < matrix[i].length; j++)
  {
    myInt[i][j] = matrix[i][j];
  }
}

Answer 2

Actually, there are two good ways to copy array is to use clone and System.arraycopy().

Here is how to use close for 2D case:

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

For System.arraycopy(), you use:

int [][] myInt = new int[matrix.length][];
for(int i = 0; i < matrix.length; i++)
{
  int[] aMatrix = matrix[i];
  int   aLength = aMatrix.length;
  myInt[i] = new int[aLength];
  System.arraycopy(aMatrix, 0, aLength, myInt, 0);
}

I don't have a benchmark but I can bet with my 2 cents that they are faster and less mistake-prone than doing it yourself. Especially, System.arraycopy() as it is implemented in native code.

Hope this helps.

Answer 3

you can code like this also myInt = matrix.clone();