The 2D array won't transponse c#

Question

I have a simple task to transponse a square 2D array: (I need to do it in a very plain manner, no containers etc)

static void Main(string[] args)
    {


        double[,] a = new double[5, 5];
        Random random = new Random();

        for(int i = 0; i < 5; i++)
        {
            for(int j = 0; j < 5; j++)
            {
                a[i, j] = random.NextDouble();
            }
        }

        for (int i = 0; i < 5; i++)
        {
            for (int j = 0; j < 5; j++)
            {
                Console.Write(a[i, j] + " ");

            }
            Console.WriteLine();
        }


        for (int i = 0; i < 5; i++)
        {
            for (int j = 0; j < 5; j++)
            {
                double temp = a[i, j];
                a[i, j] = a[j, i];
                a[j, i] = temp;
            }
        }

        Console.WriteLine("\n\n\n");
        for (int i = 0; i < 5; i++)
        {
            for (int j = 0; j < 5; j++)
            {
                Console.Write(a[i, j] + " ");

            }
            Console.WriteLine();
        }

        Console.ReadKey();


    }
}

I was expecting a reversed array as the output. However, I got the same array here. Please, help me find out what did I do wrong?

Answer 1

This happens because you are executing both for loops from 0 to 5. So you are doing the transpose twice.

For example, for i=0 and j=1 you transpose a[0,1] with a[1,0] and when i=1 and j=0 the values of a[1,0] and a[0,1] going back to its original position.

You can make the inner for from 0 to i, so the positions are swapped just one time.

for (int i = 0; i < 5; i++)
{
    for (int j = 0; j < i; j++)
    {
        double temp = a[i, j];
        a[i, j] = a[j, i];
        a[j, i] = temp;
    }
}

Answer 2

The third loop swaps both diagonal halves of the array, you have to swap only the elements from one half:

for (int i = 0; i < 5; i++)
{
    for (int j = 0; j < i; j++)
    {
        double temp = a[i, j];
        a[i, j] = a[j, i];
        a[j, i] = temp;
    }
}