calculate the sum of diagonals in a matrix

Question

I need to calculate the sum of two diagonals in a matrix in C++, I already have a solution for that but I must be dumb because I cant understand what it is doing, so I would

Answer 1

you must use i + j == n + 1 instead of i + j == n - 1 for secondary diagonal i.e

for(i = 1; i <= n; i++)
{
    for(j = 1; j <= n; j++)
    {
        if(i == j) 
            d += a[i][j]; //principal diagonal 
        if(i + j == n+1)
            s += a[i][j];//secondary diagonal

    }
}