title: Extract values from column under given conditions of other column
body: <p>I have the following given:</p>
  
  <pre><code>TR  Avg RE
  3   0
  3   0
  4   209.3524872
  3   185.6542898
  3   0
  3   0
  3   0
  4   136.7522375
  4   157.6887675
  4   0
  3   202.8994858
  3   0
  3   89.45242983
  4   0
  3   0
  3   218.4987273
  3   192.4212849
  </code></pre>
  
  <p>I want to extract only those values of RE where TR is equal to 4 - how can I do that?</p>