Inverting a two-dimensional array

2
votes

I'm trying to invert and flip a two-dimensional array, but something goes wrong! Flipping works ok, but inverting is not. Can't find a mistake right here:

public int[][] flipAndInvert(int[][] A) {
    int row = -1;
    int col = -1;
    int[][] arr = A;

    for (int i = 0; i < arr.length; i++) {
        row++;
        col = -1;
        for (int j = arr[i].length - 1; j >= 0; j--) {
            col++;
            arr[row][col] = A[i][j];
        }
    }
    for (int i = 0; i < arr.length; i++) {
        for (int j = 0; j < arr[i].length; j++) {
            if (arr[i][j] == 1) {
                arr[i][j] = 0;
            } else {
                arr[i][j] = 1;
            }
        }
    }
    return arr;
}
int[][] A = { { 0, 1, 1 },{ 0, 0, 1 },{ 0, 0, 0 } };

After proceeding the output should be: After inverting: {1,1,0},{1,0,0},{0,0,0} After flipping: {0,0,1,},{0,1,1},{1,1,1}

Thanks to all a lot, the problem was here: int[][] arr = A; The reference of the array is being passed to arr.

3
javaarraysreverseflipinversion
Can you share a sample 2D array and the result you're trying to get for it? It would help make the question clearer. - Mureinik
What goes wrong? How does the actual output differ from your expected output? - mypetlion
@Mureinik added - Dmytro Z.
Your flipping does not work. I got { {1,1,1}, {1,1,1}, {1,1,1} from input array { {0,1,1}, {0,1,1}, {0,1,1} }. That doesn't seem like what you want. - mypetlion
@mypetlion first two "for" loops are doing invert, and the next two flip the array, so I can't find a mistake right here - Dmytro Z.

3 Answers

2
votes

What I think is that since you are using this line:

int[][] arr = A;

The reference of the array is being passed to arr, and hence the line:

arr[row][col] = A[i][j];

is equivalent to:

A[row][col] = A[i][j];

as arr has an reference to A and they both now refer to the same memory location (or they are both different names to a single variable)

You can fix this by either using the new keyword with arr and then initializing it:

int[][] arr = new int[someRows][someCols];
//use for loop to assign the value to each element of arr

Or you can run the for loop till arr[i].length/2 - 1:

    for (int i = 0; i < arr.length; i++) {
    row++;
    col = -1;
    for (int j = arr[i].length / 2 - 1; j >= 0; j--) { //here changed arr[i].length to arr[i].length / 2
        col++; 
        arr[row][col] = A[i][j]; //for this you do not need arr and you can directly work on A and return it
    }
}
1
votes

The problem of your code should be this line:

int[][] arr = A;

You are assigning the reference of array A to arr and from then when you modify one you modify both of the arrays or better they are modified together because they refer to the same address.

0
votes

Using apache commons lang:

    int[][] matrix = new int[3][3];
    for (int i = 0; i < matrix.length; i++) {
        matrix[i][0] = 3 * i;
        matrix[i][1] = 3 * i + 1;
        matrix[i][2] = 3 * i + 2;
    }

    System.out.println(Arrays.toString(matrix[0]));
    System.out.println(Arrays.toString(matrix[1]));
    System.out.println(Arrays.toString(matrix[2]));

    ArrayUtils.reverse(matrix);

    System.out.println(Arrays.toString(matrix[0]));
    System.out.println(Arrays.toString(matrix[1]));
    System.out.println(Arrays.toString(matrix[2]));

Hope this helps.