title: Pandas return separate column value in current index if two separate columns match
body: <p>Say I have the following data frame:</p>
  <pre><code>  A    B    C
  0 n1   n2   n4
  1 n2   n3   n5
  2 n3   n1   n6
  </code></pre>
  <p>I have been trying to:</p>
  <ol>
  <li>Loop through <code>Column A</code> to find a matching value in <code>Column B</code></li>
  <li>If there is a match in <code>Column B</code> I want to grab the value in <code>Column C</code> <em>for the current index</em> and create a <code>Column D</code> with that value.</li>
  <li>Given the example data frame above, below would be the solution I'm trying to achieve.</li>
  </ol>
  <pre><code>  A    B    C    D
  0 n1   n2   n4   n6
  1 n2   n3   n5   n4
  2 n3   n1   n6   n5 
  </code></pre>
  <p>I've seen lots of answers for excel utilizing match and index, but I literally can't find anything to help me solve this problem. Any help would be appreciated.</p>