title: Fill matrix A ( Eigen::Matrix2d A) with numbers using loops
body: <p>Whenever I try this code I get an Assertion failed !
  How can I fill the matrix A using loops ? </p>
  
  <pre><code>#include &lt;iostream&gt;
  #include "../eigen3/Eigen/Dense"
  int main ()
  {
   Eigen::Matrix2d A;
   for (int row = 0; row &lt; 5; ++row)
    {
     for (int col = 0; col &lt; 5; ++col)
      {
  //     A(row,col)=10;   // ???
      }
    }
  
  }
  </code></pre>