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

How about I try to explain this version? :D

There are 3 important parts of the code:

• inputing the matrix
• calculating major diagonal ( \ direction)
• calculating minor diagonal ( / direction)

And here they are, explained:

// input elements
for(i=1;i<=n;i++) // from left to right
{
    for(j=1;j<=n;j++) // from up to down
        cin>>a[i][j]; // input element at (i,j) position
}

Here, d and s contain the inter-values of major and minor diagonal respectively. At the end of 2 loops, they will contain the results

for (i=1;i<=n;i++)
     for (j=1;j<=n;j++)
     {
        if(i==j)          // major diagonal - if coordinates are the same
           d=d+a[i][j];   // e.g. (1,1), (2,2)
        if(j==n-i+1 || i==n-j+1)  // coordinates of the minor diagonal - check
           s=s+a[i][j];           // e.g. n=3 (3,1) (2,2) ...
      }

Hope this helps.

Note that this code starts matrix coordinates at 1 instead of 0, so you will actually need to allocate (n+1)x(n+1) space for the matrix:

double a[n+1][n+1];

before using it.

Also, the code you gave is not most effective. It has O(n^2) complexity, while the task can be done in O(n) like so:

// matrix coordinates now start from 0
for (int i=0; i < n; ++i){
    d += a[i][i]; // major
    s += a[i][n-1-i]; // minor
}