title: Can Eigen do data slice operation like pandas.Dataframe?
body: <p>I want to do some matrix numeric operation in <code>c++</code>.</p>
  <p>I think <code>Eigen</code> is a very powerful tools.</p>
  <p>but can <code>eigen</code> support this kind of operation:</p>
  <p>get the subset matrix by some rule, for example, according to the columns value?</p>
  <p>let's explain it by <code>pandas</code> <code>DataFrame</code>:</p>
  <pre><code>df = pd.DataFrame([[1,2,3], [2,3,4], [3,4,5]])
  wanted_df = df[df[0] &gt; 1]
  </code></pre>
  <p>assume i initialize a matrix like this:</p>
  <pre><code>#include &lt;iostream&gt;
  #include &lt;eigen3/Eigen/Dense&gt;
   
  int main()
  {
    Eigen::MatrixXd m(3,3);
    m &lt;&lt; 1,2,3, 2,3,4, 3,4,5;
    std::cout &lt;&lt; m &lt;&lt; std::endl;
  }
  </code></pre>