title: R Frequency table containing 0
body: <p>I'm working on a data.frame with about 700 000 rows. It's containing the ids of statusupdates and corresponding usernames from twitter. I just want to know how many different users are in there and how many times they've tweeted. So I thought this was a very simple task using tables. But know I noticed that I'm getting different results.</p>
  
  <p>recently I did it converting the column to character like this</p>
  
  <pre><code>&gt;freqs &lt;- as.data.frame(table(as.character(w_dup$from_user))
  &gt;nrow(freqs)
  [1] 239678
  </code></pre>
  
  <p>2 months ago I did it like that</p>
  
  <pre><code>&gt;freqs &lt;- as.data.frame(table(w_dup$from_user)
  &gt;nrow(freqs)
  [1] 253594
  </code></pre>
  
  <p>I noticed that this way the data frame contains usernames with a Frequency 0. How can that be? If the username is in the dataset it must occur at least one time. </p>
  
  <p>?table didn't help me. Neither was I able to reproduce this issue on smaller datasets.</p>
  
  <p>What I'm doing wrong. Or am I missunderstanding the use of tables? </p>