title: Merging two columns into one in R
body: <p>I have the following data frame, and am trying to merge the two columns into one, while replacing <code>NA</code>'s with the numeric values.</p>
  
  <pre><code>ID    A     B
  1     3     NA
  2     NA    2
  3     NA    4
  4     1     NA
  </code></pre>
  
  <p>The result I want is:</p>
  
  <pre><code>ID    New
  1     3
  2     2
  3     4
  4     1
  </code></pre>
  
  <p>Thanks in advance!</p>