title: calculate the sum of diagonals in a matrix
body: <p>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 like to know if there is another version which I can understand. here is the code which does the job:</p>
  
  <pre><code>cout&lt;&lt;"Jepi rangun e  matrices"&lt;&lt;endl;  // pra bejme manipulim me matrice katrore ku rreshtat=kolonat
  cin&gt;&gt;n;
  cout&lt;&lt;"Tani jepi elementet e matrices"&lt;&lt;endl; // lexohet matrica
  
  for(i=1;i&lt;=n;i++)
  {
       for(j=1;j&lt;=n;j++)
          cin&gt;&gt;a[i][j];
  }
  
  d=0;
  s=0; // ketu e keni kushtin si dhe mbledhjen per te dy diagonalet me dy variabla te ndryshme
  
  for(i=1;i&lt;=n;i++)
      for(j=1;j&lt;=n;j++)
      {
          if(i==j)
              d=d+a[i][j];
          if(j==n-i+1 || i==n-j+1) 
              s=s+a[i][j];
      }
  </code></pre>
  
  <p>The part that is difficult to understand is</p>
  
  <pre><code>if(j==n-i+1 || i==n-j+1) 
      s=s+a[i][j];
  </code></pre>
  
  <p>Here is the entire code that I changed but it doesnt work for the secondary diagonal:</p>
  
  <pre><code>#include &lt;iostream&gt;
  using namespace std;
  
  int main()
  {
      int d=0,s=0; // ketu e keni kushtin si dhe mbledhjen per te dy diagonalet me dy variabla te ndryshme
      int i,j,n;
      int a[5][5];
  
      cout&lt;&lt;"Jepi rangun e  matrices"&lt;&lt;endl;  // pra bejme manipulim me matrice katrore ku rreshtat=kolonat
      cin&gt;&gt;n;
      cout&lt;&lt;"Tani jepi elementet e matrices"&lt;&lt;endl; // lexohet matrica
  
      for(i=1;i&lt;=n;i++)
      {
          for(j=1;j&lt;=n;j++)
              cin&gt;&gt;a[i][j];
      }
  
      for(i=1;i&lt;=n;i++)
      {
          for(j=1;j&lt;=n;j++)
          {
              if(i==j) 
                  d+=a[i][j]; //principal diagonal 
              if(i+j==n-1)
                  s+=a[i][j];//secondary diagonal
  
          }
      }
  
      cout &lt;&lt; d &lt;&lt; endl;
      cout &lt;&lt; s &lt;&lt; endl;
      cin.get();
      cin.get();
      return 0;
  }
  </code></pre>