title: Save Eigen array row in a variable
body: <p>I'm trying to extract the <code>j</code>-th row from an <code>Eigen::ArrayXXi</code> and store it in a variable. This is what I tried.</p>
  
  <pre><code>Eigen::Array&lt;int, arr.rows(), arr.cols()&gt; row = arr.row(j);
  </code></pre>
  
  <p>But I get the following error highlighting <code>arr.rows()</code></p>
  
  <blockquote>
    <p>Non-type template argument of type 'Eigen::Index' (aka 'long') is not an integral constant expression</p>
  </blockquote>
  
  <p>I am unable to find what the return type of <code>arr.row()</code> is supposed to be. According to the documentation, it's <code>RowExpr</code>, which means nothing to me.</p>