title: Paste one column to another with NA conditional
body: <p>I would like to paste <code>col3</code> to <code>col1</code> however avoiding <code>NAs</code> in <code>col1</code>.</p>
  
  <p>My input:</p>
  
  <pre><code>df &lt;- "col1 col2 col3
         100   x   102
         105   x   106
         101   x   NA"
  df &lt;- read.table(text=df, header=T)
  </code></pre>
  
  <p>My expected output:</p>
  
  <pre><code>dfout &lt;- "col1 col2 col3
         102   x   102
         106   x   106
         101   x   NA"
  dfout &lt;- read.table(text=dfout, header=T)
  </code></pre>
  
  <p>Any ideas?</p>