Rotate M*N Matrix (90 degrees)

Question

How can I rotate matrix

|3 4 5 6 8|
|5 4 3 2 6|
|3 3 7 8 9|

to

|8 6 9|            
|6 2 8|
|5 3 7|
|4 4 3|
|3 5 3|

Because all algorithms I've seen was for N*N matrix.

Answer 1

If your matrix is represented by an array matrix[i, j], where the i are the rows and the j are the columns, then implement the following method:

static int[,] RotateMatrixCounterClockwise(int[,] oldMatrix)
{
    int[,] newMatrix = new int[oldMatrix.GetLength(1), oldMatrix.GetLength(0)];
    int newColumn, newRow = 0;
    for (int oldColumn = oldMatrix.GetLength(1) - 1; oldColumn >= 0; oldColumn--)
    {
        newColumn = 0;
        for (int oldRow = 0; oldRow < oldMatrix.GetLength(0); oldRow++)
        {
            newMatrix[newRow, newColumn] = oldMatrix[oldRow, oldColumn];
            newColumn++;
        }
        newRow++;
    }
    return newMatrix;
}

This works for matrices of all sizes.

Edit: If this operation is too expensive, then one could try changing the way one reads the matrix instead of changing the matrix itself. For example, if I am displaying the matrix as follows:

for (int row = 0; row < matrix.GetLength(0); row++)
{
    for (int col = 0; col < matrix.GetLength(1); col++)
    {
        Console.Write(matrix[row, col] + " ");
    }

    Console.WriteLine();
}

then I could represent a 90-degree counterclockwise rotation by changing the way I read the matrix:

for (int col = matrix.GetLength(1) - 1; col >= 0; col--)
{
    for (int row = 0; row < matrix.GetLength(0); row++)
    {
        Console.Write(matrix[row, col] + " ");
    }

    Console.WriteLine();
}

This access pattern could be abstracted in a class, too.

Answer 2

The simplest way is to create another matrix, which has the dimensions N * M (if the original has M * N) and then use nested loops to copy values from one matrix to another... Just mind the correct index usage.