title: How can I use a value from one column to populate a nan in another column and viceversa in Pandas?
body: <p>There are 2 columns in my Pandas dataframe.</p>
  <pre><code>cola    colb  
  X         X     
  C        NaN    
  NaN       R     
  X         X     
  </code></pre>
  <p>I want to get rid of cola and colb and replace it with colc.</p>
  <pre><code>cola    colb   colc
  X         X     X
  C        NaN    C
  NaN       R     R
  X         X     X
  </code></pre>
  <p>Basically I need to populate colc with the value in cola and colb. If there is a nan in cola then it will pull from colb and if there is a nan in colb it will pull from cola. cola and colb should always be equal if it doesn't have a nan in one of them.</p>
  <p>How would I achieve this in pandas?</p>