title: Assign value to specific data.table columns and rows
body: <p>still understanding this great package... Could anyone please explain me the reason of this error? Thanks!</p>
  
  <pre><code>library(data.table)
  
  DT &lt;- data.table(id   = LETTERS,
                   var1 = rnorm(26),
                   var2 = rnorm(26))
  
  &gt; DT[2, list(var1, var2)]
              var1          var2
  1: -0.8628479332 -0.2367492928
  &gt; DT[2, c(var1, var2)]
  [1] -0.8628479332 -0.2367492928
  &gt; 
  &gt; DT[2, list(var1, var2)] &lt;- DT[8, list(var1, var2)]
  Error in `[&lt;-.data.table`(`*tmp*`, 2, list(var1, var2), value = list(var1 = -0.394006912428776,  : 
    object 'var1' not found
  &gt; DT[2, c(var1, var2)] &lt;- DT[8, c(var1, var2)]
  Error in `[&lt;-.data.table`(`*tmp*`, 2, c(var1, var2), value = c(-0.394006912428776,  : 
    object 'var1' not found
  </code></pre>