title: Matrix with unknown number of rows and columns Eigen library
body: <p>I want to read data from a file into a matrix in Eigen. I have coded everything but there is one problem that I encounter. I don't know yet beforehand how many data points are in the file, so I want to be able to initialize a matrix without specifying its size. I know that the following way of intializing a matrix works in Eigen:</p>
  
  <pre><code>MatrixXd A;
  </code></pre>
  
  <p>But now if I then do for instance</p>
  
  <pre><code>A &lt;&lt; 1, 2,
       4, 7;
  </code></pre>
  
  <p>It doesn't work. I had hoped that it would recognise it as a 2x2 matrix in this example, so that I could then work with it. So basically my question is, how can I add data to A, without having to specify its size?</p>