title: data.table recode in selected columns
body: <p>So I'm struggling with data.table. How do I make v1 and v3 numeric?</p>
  
  <pre><code>dt = data.table(v1 = c('1','2','3'), v2 = c(1,2,3), v3 = c('1','2','3'))
  dt[,c(1,3), with = F] = lapply(dt[,c(1,3), with = F], as.numeric)
  </code></pre>