title: How to create new column in a data.table when the name of column must be a string
body: <p>How to create new column in a data.table when the name of column must be a string or character? </p>
  
  <p>For example:</p>
  
  <pre><code>library(data.table)
  DT = data.table(v1=c(1,2,3), v2=2:4)
  new_var &lt;- "v3"
  DT[, new_var:=v2+5]
  </code></pre>
  
  <p>I Get</p>
  
  <pre><code>DT
  #&gt;    v1 v2 new_var
  #&gt; 1:  1  2       7
  #&gt; 2:  2  3       8
  #&gt; 3:  3  4       9
  </code></pre>
  
  <p>But, I want</p>
  
  <pre><code>#&gt;    v1 v2      v3
  #&gt; 1:  1  2       7
  #&gt; 2:  2  3       8
  #&gt; 3:  3  4       9
  </code></pre>