title: Coalesce two string columns with alternating missing values to one
body: <p>I have a data frame with two columns "a" and "b" with alternating missing values (<code>NA</code>)  </p>
  
  <pre><code>a      b
  dog    &lt;NA&gt;
  mouse  &lt;NA&gt;
  &lt;NA&gt;   cat
  bird   &lt;NA&gt;
  </code></pre>
  
  <p>I want to "merge" / combine them to a new column c that looks like this, i.e. the non-<code>NA</code> element in each row is selected:</p>
  
  <pre><code>c
  dog
  mouse
  cat
  bird
  </code></pre>
  
  <p>I tried <code>merge</code> and <code>join</code>, but neither worked as I wanted. Maybe because I do not have an id with which to merge? For integers I would just circumvent this and add both columns, but how in my case?    </p>