title: Replace blank cells with character
body: <p>I'm working on a data set that looks as follow:</p>
  
  <pre><code>191  
  282 A
  
  202  
  210 B
  </code></pre>
  
  <p>I would like to replace those empty cells at the second column with a character, say 'N'. How can I efficiently do this in R? </p>
  
  <p>Appreciate it. </p>