title: Python Pandas Dataframe duplicate and reorder a column
body: <p>I have a (probably) pretty easy problem. I have the following DataFrame:</p>
  
  <p>c1 | c2 | c3</p>
  
  <p>a    |    b    |   4<br>
     b    |    a    |   1</p>
  
  <p>Now column3 depends only on column1 and column2 has the same items as column1, but in different order. All I am trying to do is add a column4 with the same values of column3 but in order of column2. So the result would look like this:</p>
  
  <pre><code>c1| c2| c3|c4
  
  a | b | 4 | 1             
  b | a | 1 | 4
  </code></pre>
  
  <p>Do you have any ideas?</p>
  
  <p>Thanks in advance!</p>