title: setdiff two single column data frames
body: <p>I'm having a problem with a very simple issue and I don't know how to sort it out. Here's the deal. I have two one column data frames</p>
  
  <pre><code>a &lt;- data.frame(C=c("c1","c2","c3","c4","c5","c6","c7","c8"))
  b &lt;- data.frame(C=c("c1","c4","c5","c8"))
  </code></pre>
  
  <p>I would like to get one column dataframe with the entries that do NOT appear in b but they are in a. ie. a dataframe with "c2","c3","c6","c7".
  I tried</p>
  
  <pre><code>c &lt;- setdiff(a,b)
  </code></pre>
  
  <p>but I got the a dataframe and also with</p>
  
  <pre><code>c &lt;- merge(a,b,all.x=TRUE)
  </code></pre>
  
  <p>I don't get what I want it. so do you know where I am wrong?</p>