title: Operation on new data.table affects old data.table
body: <p>I'm having trouble with the data.table package in R (v1.9.6). I'm not sure what I'm doing wrong. Any help is much appreciated.</p>
  
  <p>I create a data.table and then assign this to a new variable. So now I have two data.tables. I then perform an operation on the second data.table. However this operation is also performed on the original data.table as well. 
  Is this a bug or a feature of the package? or am I doing something horribly wrong?</p>
  
  <pre><code>a&lt;-data.table(x=1:100,y=1:100)
  
  b&lt;-a
  
  b[,x:=2]
  
  head(a)
  
  head(b)
  </code></pre>