title: sum columns values in data.table in r using .SDcols
body: <pre><code>DT &lt;- data.table(A = 1:5, B = 2:6, C = 3:7)
  </code></pre>
  
  <p>I want to make summation of 3 columns using column index:</p>
  
  <pre><code>DT[, D := do.call(sum, .SD), .SDcols = 1:3]
  </code></pre>
  
  <p>but code above doesnt work,</p>
  
  <p>also I dont want to use <code>DT[, D := (A+B+C)]</code></p>