title: R data.table behavior while filtering rows
body: <p>I am creating a data.table in R and setting a column to be used as key. When I try to retrieve  values from the data table; for the rows where there is no match I get NA values back. I typically dont want that behavior in my search. Example below</p>
  
  <pre><code>library(data.table) 
  dt &lt;- data.table('foo'=seq(10),bar=sample(letters,10))
  setkey(dt,bar)
  dt[sample(letters,5)]
  
  
  &gt; dt[sample(letters,5)]
     b foo
  1: x   4
  2: q   2
  3: u   8
  4: s  NA
  5: b  NA
  </code></pre>