title: Why does an empty dataframe fail an is.null() test?
body: <p>please excuse me if my question is quite basic. 
   I created an empty data frame by <code>df &lt;- data.frame()</code> and obviously the data frame is NULL (empty). when I try to check if the data frame is empty by <code>is.null(df)</code>, the result comes FALSE. Is there any difference between <code>NULL</code> and <code>empty</code> in R. In this case if the data frame is not <code>NULL</code> , then what is in the empty data frame and  when it will be <code>NULL</code>. Thanks</p>