title: How to create an empty datatable with columns names and then append datatables to it?
body: <p>First I want to create an empty datatable with column names but it fails:</p>
  
  <pre><code>data &lt;- data.table(va, vb, vc)
  
  &gt; Error in data.table(va, vb, vc) : object 'va' not found
  </code></pre>
  
  <p>Second I want to append datatable to it but it fails too :</p>
  
  <pre><code>data2 &lt;- data.table(va=c(-1,0,1), vb=c(-1,0,1), vc=c(-1,0,1))
  data2
     va vb vc
  1: -1 -1 -1
  2:  0  0  0
  3:  1  1  1
  merge(data2,data2)
  
  &gt; Error in merge.data.table(data2, data2) : 
        Can not match keys in x and y to automatically determine appropriate `by` parameter. Please set `by` value explicitly.
  </code></pre>
  
  <p>Apparently the function can't identify the <code>by</code> parameters with two identical datatables. Any idea?</p>