title: count of entries in data frame in R
body: <p>I'm looking to get a count for the following data frame:</p>
  
  <pre><code>&gt; Santa
     Believe Age Gender Presents Behaviour
  1    FALSE   9   male       25   naughty
  2     TRUE   5   male       20      nice
  3     TRUE   4 female       30      nice
  4     TRUE   4   male       34   naughty
  </code></pre>
  
  <p>of the number of children who believe. What command would I use to get this?</p>
  
  <p>(The actual data frame is much bigger. I've just given you the first four rows...)</p>
  
  <p>Thanks!</p>