title: Counting how many rows satisfy a condition for each id
body: <p>I have a dataframe containing multiple features. In the same dataset the features are grouped by 'id' column from 1 to 10. I need to count <strong>the number of rows of 'vout' which are > 0.2</strong> for <strong>each group id</strong> and save it for every single id.</p>
  
  <pre><code>    id    freq    zr         zi           z        vout    
  1   1   4641.60 0.010534    -0.002541   0.010687    0.63490
  2   1   2154.40 0.010787    -0.000516   0.010786    0.63471
  3   1   1000.00 0.011431    0.000674    0.011366    0.63451
  4   1   464.16  0.012167    0.001527    0.012232    0.63432
  </code></pre>
  
  <p>the result must be something like this:</p>
  
  <pre><code>id 1 -&gt; numbRows = 1304
  id 2 -&gt; numbRows = 3234
  ...
  id 10 -&gt;numbRows = 223
  </code></pre>
  
  <p>Thanks in advice.</p>