Rotate matrix in place

Question

I'm solving the rotate an NxN matrix in place question.

It seems that my code makes a rotation, but leaves an X over the image.
So I'm guessing it's rotating the edges incorrectly. I'm attaching two images as sample input and output.

What's wrong with my code:

public static void rotateRight(float[][] img){
    for (int i=0; i<N/2; i++){
        for (int j=i; j<N-i; j++){
            int J_COMP = N-j-1; //complement of J
            int LEFT = i;
            int RIGHT = N-i-1;
            int TOP = i;
            int BOTTOM = N-i-1;

            float temp = img[J_COMP][LEFT];
            img[J_COMP][LEFT] = img[BOTTOM][J_COMP];
            img[BOTTOM][J_COMP] = img[j][RIGHT];
            img[j][RIGHT] = img[TOP][j];
            img[TOP][j] = temp;
        }
    }       
}

Answer 1

You are rotating main diagonals twice.

Fix inner loop (see "fix" comment)

package tests.StackOverflow;

public class Question_20773692 {

    private static int N;

    public static void main(String[] args) {

        float[][] img;
        int count;

        N=3;
        count = 0;
        img = new float[N][N];
        for(int i=0; i<N; ++i) {
            for(int j=0; j<N; ++j) {
                img[i][j] = count++;
            }
        }

        printImg(img);

        rotateRight(img);

        printImg(img);

    }

    public static void printImg(float[][] img) {
        for(int j=0; j<N; ++j) {
            System.out.print("-");
        }
        System.out.println();
        for(int i=0; i<N; ++i) {
            for(int j=0; j<N; ++j) {
                System.out.print((int)(img[i][j]));
            }
            System.out.println();
        }
        for(int j=0; j<N; ++j) {
            System.out.print("-");
        }
        System.out.println();   }

    public static void rotateRight(float[][] img){
        for (int i=0; i<N/2; i++){
            for (int j=i; j<N-i; j++){
            //for (int j=i+1; j<N-i; j++){ //fix
                int J_COMP = N-j-1; //complement of J
                int LEFT = i;
                int RIGHT = N-i-1;
                int TOP = i;
                int BOTTOM = N-i-1;

                float temp = img[J_COMP][LEFT];
                img[J_COMP][LEFT] = img[BOTTOM][J_COMP];
                img[BOTTOM][J_COMP] = img[j][RIGHT];
                img[j][RIGHT] = img[TOP][j];
                img[TOP][j] = temp;
            }
        }       
    }
}

Answer 2

In-place rotation of matrix.... I guess this should work.

public void rotate(ArrayList<ArrayList<Integer>> a) {
    int n=a.size()/2;
    for(int i=1;i<=n;i++){
        ArrayList<Integer>temp=a.get(i-1);
        a.set(i-1,a.get(a.size()-i));
        a.set(a.size()-i,temp);
    }
    for(int i=0;i<a.size();i++){
        for(int j=i+1;j<a.get(0).size();j++){
            a.get(i).set(j,a.get(i).get(j)^a.get(j).get(i));
            a.get(j).set(i,a.get(i).get(j)^a.get(j).get(i));
            a.get(i).set(j,a.get(i).get(j)^a.get(j).get(i));
    }
}
}

Answer 3

If you are about rotating picture, I would recommend to use java.awt.geom.AffineTransform

If this is logical-only question about matrix rotation, I believe you can correct your result by replacing (one or both?) < to <=. Also, note that your picture has size 309x309 (odd numbers!), and you left the middle not corrected by N/2. Possible solution is add 1 - replace N/2 to

N % 2 == 0 ? N / 2 : N / 2 + 1