Rotating image 90 degree counter clockwise in Java

Question

public static int[][] rotate(int[][] array){
    int height = array.length;
    int width = array[0].length;
    int[][] rotatedArray = array;

    for(int col = 0; col < width; col++){
        for(int row = 0; row < height; row++){
            rotatedArray[row][col] = array[col][row];
        }
    }
    return rotatedArray;
}

This is my code as method to rotate image 90 degree counter-wise, but it doesn't work. I have no idea how to arrange new rows and columns and rotate it properly, how can I fix it? Thanks!

Just a random image and try to rotate it 90 degree counter-clockwise. — yolam, Jan 20 '16 at 21:20
Possible duplicate of [How do you rotate a two dimensional array?](http://stackoverflow.com/questions/42519/how-do-you-rotate-a-two-dimensional-array) — Stephen Rosenthal, Jan 20 '16 at 21:22
You did not answer in what way the image is represented. Is the 2D array holding the pixels of the image? — user1803551, Jan 20 '16 at 21:25
Yes it is, and sorry I am just trying to write only method to it. — yolam, Jan 21 '16 at 00:39
Thanks a lot user1803551! I figure it out with answer below! :) — yolam, Jan 21 '16 at 01:20

John Sensebe · Answer 1 · 2016-01-20T21:32:54.070

1

Try rotatedArray[row][col] = array[col][height - row - 1];.

Also, you need to define rotatedarray as a new array. Right now, you're assigning it array, which means they are both referencing the same data.

Here's how you can do it:

public static int[][] rotate(int[][] array) {
    int height = array[0].length;
    int width = array.length;
    int[][] rotatedArray = new int[height][];

    for(int row = 0; row < height; row++) {
        rotatedArray[row] = new int[width];
        for(int col = 0; col < width; col++) {
            rotatedArray[row][col] = array[col][height - row - 1];
        }
    }
    return rotatedArray;
}

Note that the height of the original array becomes the width of the new array and vice versa.

edited Jan 20 '16 at 21:32

answered Jan 20 '16 at 21:12

John Sensebe

1,386
8
11

1

John, i kindly suggest that before guessing an answer try it yourself. You are responsible for what you answer, it is not up to the asker to decipher and debug your answer. – user1803551 Jan 20 '16 at 21:24
I have discovered a greater flaw in the original code an updated my answer accordingly. – John Sensebe Jan 20 '16 at 21:35
Thanks a lot!! I draw a picture and finally understand it. :) – yolam Jan 21 '16 at 01:19
This really works like a charm. Thank you! – Lucas Sousa Dec 22 '20 at 20:04

score 0 · Answer 2 · answered Jan 20 '16 at 21:18

0

By transposing the row & column indices with rotatedArray[row][col] = array[col][row], you are mirroring the image along the diagonal, instead of rotating it. Think about it - any entry where both indices are matching such as array[0][0] or array[1][1] is unchanged. That's not what you want!

I would recommend drawing a picture to see what pattern you see. You can start with very small examples, 2-by-2 or 3-by-3.

answered Jan 20 '16 at 21:18

Stephen Rosenthal

773
4
15

Thanks and I draw a picture, understanding it now. – yolam Jan 21 '16 at 01:21

Rotating image 90 degree counter clockwise in Java

2 Answers2