title: R data.table compute new column, but insert at beginning
body: <p>In R <code>data.table</code>s, I can use this syntax to add a new column:</p>
  
  <pre><code>&gt; dt &lt;- data.table(a=c(1,2), b=c(3,4))
  &gt; dt[, c := a + b]
  &gt; dt
     a b c
  1: 1 3 4
  2: 2 4 6
  </code></pre>
  
  <p>But how would I insert c at the front of the dt like so:</p>
  
  <pre><code>   c a b
  1: 4 1 3
  2: 6 2 4
  </code></pre>
  
  <p>I looked on SO, and found some people suggesting <code>cbind</code> for <code>data.frame</code>s, but it's more convenient for me to use the <code>:=</code> syntax here, so I was wondering if there was a <code>data.table</code> sanctioned way of doing this. My <code>data.table</code> has around 100 columns, so I don't want to list them all out.</p>