title: Removing rows by reference using data.table?
body: <p>I am trying to figure out how to remove a group of rows from a dataset by reference. For example, with this data set:</p>
  
  <pre><code>testset &lt;- data.table(date=as.Date(c("2013-07-02","2013-08-03","2013-09-04","2013-10-05","2013-11-06")), 
             yr = c(2013,2013,2013,2013,2013), 
             mo = c(07,08,09,10,11), 
             da = c(02,03,04,05,06), 
             plant = LETTERS[1:5], 
             product = as.factor(letters[26:22]), 
             rating = runif(25))
  </code></pre>
  
  <p>I want to remove all rows where the product is "y". I have no idea how to go about this.</p>