title: Minus operation of data frames
body: <p>I have 2 data frames  <code>df1</code> and <code>df2</code>. </p>
  
  <pre><code>df1 &lt;- data.frame(c1=c("a","b","c","d"),c2=c(1,2,3,4) )
  df2 &lt;- data.frame(c1=c("c","d","e","f"),c2=c(3,4,5,6) )
  
  &gt; df1
    c1 c2
  1  a  1
  2  b  2
  3  c  3
  4  d  4
  
  &gt; df2
    c1 c2
  1  c  3
  2  d  4
  3  e  5
  4  f  6
  </code></pre>
  
  <p>I need to perform set operation of these 2 data frames. I used <code>merge(df1,df2,all=TRUE)</code> and <code>merge(df1,df2,all=FALSE)</code> method to get the union and intersection of these data frames and got the required output. What is the function to get the minus of these data frames,that is all the positions existing on one data frame but not the other? I need the following output.</p>
  
  <pre><code> c1 c2
  1  a  1
  2  b  2
  </code></pre>