Copy a 2D Array in Java

How do I copy a 2 Dimensional array in Java?

current=old or old=current makes the two array refer to the same thing, so if you subsequently modify current, old will be modified too. To copy the content of an array to another array, use the for loop

for(int i=0; i<old.length; i++)
  for(int j=0; j<old[i].length; j++)
    old[i][j]=current[i][j];

PS: For a one-dimensional array, you can avoid creating your own for loop by using Arrays.copyOf

how to copy a multidimensional array to a single array?

Here is the correct way to use the arraycopy:

int copy[] = new int[arr[r].length];
System.arraycopy(arr[r], 0, copy, 0, copy.length);
return copy;

A shorter way of writing the above:

return Arrays.copyOf(arr[r], arr[r].length);

A third way:

return arr[r].clone();

All three ways will have the same result. As for speed, the first two ways may be a tiny bit faster than the third way.

Cloning two-dimensional Arrays in Java

you should iterate this array with two loops. This will helps you:

static double[][] clone(double[][] a) {
    double[][] b = new double[a.length][];
    for (int i = 0; i < a.length; i++) {
        b[i]= new double[a[i].length];
        for (int j = 0; j < a[i].length; j++)
            b[i][j] = a[i][j];
    }
    return b;
}

Java: copy a single array from a 2d array

If you want a reference to a row, you can use double[] selectedRow = P[rowToSelect]. The new keyword in your code is misplaced. If you want a real copy or better known as deep copy, you can use:

double[] selectedRow = new double[P[rowToSelect].length];
System.arraycopy(P[rowToSelect], 0, selectedRow, 0, P[rowToSelect].length);`

How do I do a deep copy of a 2d array in Java?

Yes, you should iterate over 2D boolean array in order to deep copy it. Also look at java.util.Arrays#copyOf methods if you are on Java 6.

I would suggest the next code for Java 6:

public static boolean[][] deepCopy(boolean[][] original) {
    if (original == null) {
        return null;
    }

    final boolean[][] result = new boolean[original.length][];
    for (int i = 0; i < original.length; i++) {
        result[i] = Arrays.copyOf(original[i], original[i].length);
        // For Java versions prior to Java 6 use the next:
        // System.arraycopy(original[i], 0, result[i], 0, original[i].length);
    }
    return result;
}