How to rotate 2D array 90º anticlockwise?

Question

I need to rotate a given matrix 90º anticlockwise and I don't know how to start.

For example:

Go from this

5   10   8   9
16  30   25  41
7   17   50  12
45  8    22  34

to this:

9   41   12  34
8   25   50  22
10  30   17  8
5   16   7   45

score 1 · Accepted Answer · answered Dec 14 '20 at 20:20

This is like a matrix transpose. You can use a loop in a loop, or a stream in a stream:

int d = 4;
int[][] arr1 = {
        {5, 10, 8, 9},
        {16, 30, 25, 41},
        {7, 17, 50, 12},
        {45, 8, 22, 34}
};

int[][] arr2 = new int[d][d];
int[][] arr3 = new int[d][d];
int[][] arr4 = new int[d][d];

IntStream.range(0, d).forEach(i ->
        IntStream.range(0, d).forEach(j -> {
            // matrix transpose
            arr2[j][i] = arr1[i][j];
            // turn matrix 90º clockwise
            arr3[j][d - 1 - i] = arr1[i][j];
            // turn matrix 90º counterclockwise
            arr4[d - 1 - j][i] = arr1[i][j];
        }));

Arrays.stream(arr4).map(Arrays::toString).forEach(System.out::println);
// [9, 41, 12, 34]
// [8, 25, 50, 22]
// [10, 30, 17, 8]
// [5, 16, 7, 45]

score 1 · Answer 2 · 2020-12-24T01:05:02.227

Every cell [i][j] in the original matrix becomes a cell [4-1-j][i] in the rotated matrix:

int d = 4;
int[][] arr1 = {
        {5, 10, 8, 9},
        {16, 30, 25, 41},
        {7, 17, 50, 12},
        {45, 8, 22, 34}};

int[][] arr2 = new int[d][d];

for (int i = 0; i < d; i++)
    for (int j = 0; j < d; j++)
        arr2[d - 1 - j][i] = arr1[i][j];

for (int row = 0; row < d; row++)
    System.out.println(Arrays.toString(arr2[row]));
// [9, 41, 12, 34]
// [8, 25, 50, 22]
// [10, 30, 17, 8]
// [5, 16, 7, 45]

^{See also:

• How do I rotate a matrix 90 degrees counterclockwise?

• Is there a way to reverse specific arrays in a multidimensional array?}

How to rotate 2D array 90º anticlockwise?

2 Answers2