Why would the "if (j == TB_1.ColumnCount-k-1)" condition be checked for each "j"?

Question

C++ / C#

Completed the task "Transform the matrix by replacing the aij element with the arithmetic mean among the elements located in the i-th row and having a column number >= j". Here's how I did it:

int[,] m = new int[TB_3.RowCount,TB_3.ColumnCount];
            for (int i = 0; i < TB_3.RowCount; i++)
            {
                for (int j = 0; j < TB_3.ColumnCount; j++)
                {
                    m[i, j] = Convert.ToInt32(TB_3[j, i].Value);
                    
                }
            }
            for (int i = 0; i < TB_3.RowCount; i++)
            {
                for (int k = 0; k < TB_3.ColumnCount; k++)
                {
                    double s = 0;
                    for (int j = 0; j < TB_3.ColumnCount-k; j++)
                    {
                        s += m[i, k + j];
                        if (j == TB_3.ColumnCount-k-1)
                        {
                            double sum = s / (j + 1);
                            TB_2[k, i].Value = sum;
}

TB_3-original matrix, TB_2-transformed.
The teacher said: "calculation of the new value of the element was performed incorrectly (there are 2 errors)
- besides, why should the condition "if (j == TB_1.ColumnCount-k-1)" be checked for each "j". After all, it is known that it will be executed only once. Think about how to get rid of it completely."
Help me please.

Answer 1

int[,] m = new int[TB_3.RowCount,TB_3.ColumnCount];
            for (int i = 0; i < TB_3.RowCount; i++)
            {
                for (int j = 0; j < TB_3.ColumnCount; j++)
                {
                    m[i, j] = Convert.ToInt32(TB_3[j, i].Value);
                    
                }
            } 
            for (int i = 0; i < TB_3.RowCount; i++)
            {
                for (int k = 0; k < TB_3.ColumnCount; k++)
                {

                    double s = 0;
                    
                    for (int j = 0; j < TB_3.ColumnCount-k; j++)
                    {
                        
                        s += m[i, k + j];
                       }
                        
                            double sum = s / (j + 1);
                            TB_2[k, i].Value = sum;
                        
                    }
                }