title: How to assign dynamic column names in data.table under `:=`?
body: <p>Consider the below code :</p>
  
  <pre><code>library(data.table)
  carsDT &lt;- data.table(cars)
  carsDT[speed &lt; 15, `:=`(paste0("col", 1)=1 paste0("col", 2)=2)]
  </code></pre>
  
  <p>This code gives the error :</p>
  
  <pre><code>Error: unexpected '=' in "carsDT[speed &lt; 15, `:=`(paste0("col", 1)="
  </code></pre>
  
  <p>Is there any way, that inside <code>:=</code>, I can use dynamic column names? I know that I can do above like this :</p>
  
  <pre><code>carsDT[speed &lt; 15, (paste0("col", 1)) := 1]
  carsDT[speed &lt; 15, (paste0("col", 2)) := 2]
  </code></pre>
  
  <p>This solution will involve many lines of code(number of columns to be assigned is in 100s). It will be convenient if I can do all the assignments under single <code>:=</code>. </p>
  
  <p>Any help is appreciated. </p>