Fill 1D array from matrix

Question

I have a matrix, my mission is to fill 1D array from my matrix.

Example:

1 2 3

1 2 3

1 2 3

I need to sum the columns and fill the sum of every column in a 1D array Here is my code (that doesn't work), (int[,] mat) is the matrix that the function gets.

public static int sumcolumn(int[,] mat)
{
    int sum = 0;
    int[] sumcol = new int[mat.GetLength(0)];
    for (int y = 0; y < mat.GetLength(0); y++)
    {
        for (int i = 0; i < mat.GetLength(0); i++)
        {
           for (int j = 0; j < mat.GetLength(1); j++)
           {
               sum = sum + mat[j, i];
           }
           sumcol[i] = sum;
           return sum;
           sum = 0;
        }
        return sum;
    }
    return sumcol;
}

How should I do this mission?

Thanks in advance.

Answer 1

Linq approach

int[,] array = new int[3, 3] { { 1, 2, 3 }, 
                               { 1, 2, 3 }, 
                               { 1, 2, 3 } };  

int[] result = Enumerable.Range(0, array.GetUpperBound(1) + 1)
                         .Select(y => Enumerable.Range(0, array.GetUpperBound(0) + 1)
                         .Select(x => array[x, y]).Sum()).ToArray(); // [3,6,9]

Answer 2

You need only 2 for loops. For each column run through all rows and sum up the content. Write the sum at the proper col index. Then after each column reset the sum. You also need to return the array with the sums. So I changed the return value:

Also it helps if you call the index variables with meaningful names.

public static int[] sumcolumn(int[,] mat)
{
    int[] sumcol = new int[mat.GetLength(1)];

    for (int col = 0; col < mat.GetLength(1); col++)
    {
        for (int row = 0; row < mat.GetLength(0); row++)
        {
             // since sumcol is initially filled with zeros you can just  
             // sum up the values from mat onto the zeros in each cell
            sumcol[col] += mat[row, col];
        }
    }
    return sumcol;
}

In the main you can test it like this:

void Main()
{
    int[,] array = { 
    { 1, 2, 3 },
    { 1, 2, 3 },
    { 1, 2, 3 },};

    // this is just for test display
    Console.WriteLine(String.Join(" ", sumcolumn(array)));

    // normally you would call it like this and catch the return value in a new array
    int[] result = sumcolumn(array);

}

Answer 3

public static int[] sumColumn(int[,] mat)
{
    //int sum = 0;
    int colCount = mat.GetLength(0);
    int[] sumCol = new int[colCount];

    for (int y = 0; y < colCount; y++)
    {
        int rowCount = mat.GetLength(1);
        sumCol[y] = 0;

        for (int x = 0; x < rowCount; x++)
        {
            sumCol[y] += mat[y, x];
        }

        //sum += sumCol[y];
    }

    //return sum;
    return sumCol;
}

Answer 4

So you need to evaluate a 2D matrix to get the column wise sum to a 1D array. So first thing you have to do is change the return type of the method to int[] instead for int.

Let me quote Few things that you have to notice before moving to a fix:

• If you execute a return during iteration of the loop rest of iterations will not be executed.
• A function can return only one value in a single call.
• Let i and j be two positive unequal integers then a[i,j] and a[j,i] will points to two different elements in the matrix a.

As a whole you have to modify the method signature like the following:

public static int[] sumcolumn(int[,] mat)
{
    int sum = 0;
    int[] sumcol = new int[mat.GetLength(1)];

    for (int i= 0; i< mat.GetLength(1); i++)
    {
        sum = 0; // reset sum for next colomn
        for (int j= 0; j< mat.GetLength(0); j++)
        {
            sum += mat[i, j];
        }
        // iteration of column completed
        sumcol[i] = sum;
    }
    return sumcol;
}