How can I sum the integers of a column in this 2D array?

Question

The following code calculate the sum of each row of 'int[,] a'. What I want is to modify it so it calculates the sum per column; 1+1+1, 2+2+2, 3+3+3, 4+4+4 instead of per row. These values will be put inside 'int[] output'

static void Main(string[] args)
        {
            int[,] a = {{ 1, 2, 3, 4},
                        { 1, 2, 3, 4},
                        { 1, 2, 3, 4} };

            int[] b = SumColumn(a);
        }

 public static int[] SumColumn(int[,] a)
        {
            int[] output = new int[a.GetLength(1)];
            int sum = 0;
      
            for (int i = 0; i < a.GetLength(0); i++)
            {
                for (int j = 0; j < a.GetLength(1); j++)
                {
                    sum += a[i, j];
                }

                output[i] = sum;
                sum = 0;
            }
            return output;
        }

So i tried adding a counter and using a boolean in the nested loop (and otther methods), however it did not work for me.

Answer 1

The code is almost identical in both cases. Only the loops get flipped

    static void Main(string[] args)
    {
        int[,] a = {{ 1, 2, 3, 4},
                    { 1, 2, 3, 4},
                    { 1, 2, 3, 4} };

        int[] rowSums = RowSums(a);
        int[] colSums = ColumnSums(a);

        Console.WriteLine($"Row Sums: {string.Join(",", rowSums)}");
        Console.WriteLine($"Col Sums: {string.Join(",", colSums)}");
    }

    public static int[] ColumnSums(int[,] matrix)
    {
        int rowCount = matrix.GetLength(0);
        int colCount = matrix.GetLength(1);

        int[] result = new int[colCount];
        for (int j = 0; j < colCount; j++)
        {
            int sum = 0;
            for (int i = 0; i < rowCount; i++)
            {
                sum += matrix[i, j];
            }
            result[j] = sum;
        }
        return result;
    }
    public static int[] RowSums(int[,] matrix)
    {
        int rowCount = matrix.GetLength(0);
        int colCount = matrix.GetLength(1);

        int[] result = new int[rowCount];
        for (int i = 0; i < rowCount; i++)
        {
            int sum = 0;
            for (int j = 0; j < colCount; j++)
            {
                sum += matrix[i, j];
            }
            result[i] = sum;
        }
        return result;
    }

PS. In your code there is no need to set sum=0; in the end of the outer loop, as it will get set to zero in the beginning of the next loop.

The output should be

Row Sums: 10,10,10
Col Sums: 3,6,9,12