title: How to concat two columns into one in R
body: <p>Say I have this data frame in R.</p>
  
  <pre><code>df &lt;- data.frame( col1 = c(3,4,'NA','NA'), col2 = c('NA','NA',1,5))
    col1 col2
  1    3   NA
  2    4   NA
  3   NA    1
  4   NA    5
  </code></pre>
  
  <p>I would like to have new column like this</p>
  
  <pre><code>  col1 col2 col3
  1    3   NA    3
  2    4   NA    4
  3   NA    1    1
  4   NA    5    5
  </code></pre>
  
  <p>How shall I do that?</p>