Reversing a matrix in C#

Question

Implementation:

static void ReverseMatrix<T>(T[,] matrix)
{
    int width = matrix.GetLength(1);
    int height = matrix.GetLength(0);
    int startRow = 0;
    int startCol = 0;
    int endRow = height-1;
    int endCol = width-1;

    while (true)
    {
        if (startRow == height-1 || endRow == 0) return;

        // swap
        var temp = matrix[startRow, startCol];
        matrix[startRow, startCol] = matrix[endRow, endCol];
        matrix[endRow, endCol] = temp;

        startCol++;
        endCol--;
        if (startCol == width)
        {
            startRow++;
            startCol = width - 1;
        }
        if (endCol == -1)
        {
            endRow--;
            endCol = 0;
        }
    }
}

Test program:

static void Main()
{
    var matrix = new[,]
    {
        {1, 2, 3},
        {4, 5, 6},
        {7, 8, 9}
    };

    ReverseMatrix(matrix);

    PrintMatrix(matrix);
}

static void PrintMatrix(int[,] matrix)
{
    for (int row = 0; row < matrix.GetLength(0); row++)
    {
        for (int col = 0; col < matrix.GetLength(1); col++)
        {
            Console.Write("{0}\t",matrix[row,col]);
        }
        Console.WriteLine();
    }
}

Input:

1 2 3
4 5 6
7 8 9

Output:

9 8 7
6 5 4
3 2 1

How can I improve this implementation?

Answer 1

I also changed the Reversal code and got rid of all the variables. from the looks of it you did what I did only you did it with out the use of for loops

I used one loop to go through the rows(or the first dimension of the multi-dimensional array) and then I nested a second loop inside the first to loop through the elements of each array. now in doing this I needed some independent variables to be incremented for the output matrix so I made sure that I declared them in the right places and incremented them in the right places. the output matrix was declared using the input matrix so that I didn't have to declare any magic numbers and so that I could minimize the number of variables that I had to declare as well.

enough chit chat here is the code.

    static int[,] ReverseMatrix(int[,] inputMatrix)
    {
        var outputMatrix = new int[inputMatrix.GetLength(0), inputMatrix.GetLength(1)];
        var x = 0;
        for (int i = inputMatrix.GetLength(0) - 1; i >= 0; i--)
        {
            var y = 0;
            for (int j = inputMatrix.GetLength(1) - 1; j >= 0; j--)
            {
                outputMatrix[x, y] = inputMatrix[i, j];
                y++;
            }
            x++;
        }
        return outputMatrix;
    }

_{Posted as a question also Reversing a Matrix}

---

one thing that I did differently was that I used an input and an output for my method, I personally don't like the way that your code changes a variable outside of the scope like that, if the purpose were to change the original then I wouldn't change my code at all, I would just change how I call the code, I would do it like this

matrix = ReverseMatrix(matrix);

I don't like the way that you did it, I am sure there is a reason for it and a good and bad way to do it, but I don't think this is a good place to use code like this.

Answer 2

My implementation was not correct. I've made some corrections and now it works.

static void ReverseMatrix<T>(T[,] matrix)
{
    int width = matrix.GetLength(1);
    int height = matrix.GetLength(0);
    int startRow = 0;
    int startCol = 0;
    int endRow = height-1;
    int endCol = width-1;

    while (true)
    {
        if (startRow > endRow) return;

        // swap
        var temp = matrix[startRow, startCol];
        matrix[startRow, startCol] = matrix[endRow, endCol];
        matrix[endRow, endCol] = temp;

        startCol++;
        endCol--;

        if (startCol == width)
        {
            startRow++;
            startCol = 0;
        }
        if(startCol == -1)
        {
            startRow++;
            startCol = width - 1;
        }
        if (endCol == -1)
        {
            endRow--;
            endCol = width - 1;
        }
        if (endCol == width)
        {
            endRow--;
            endCol = 0;
        }
    }
}

Answer 3

How can I improve this implementation?

Use a Stack, which is a LIFO (Last In First Out) collection.