Apply a function to a subset of data.table columns, by column-indices instead of name

Question

I\'m trying to apply a function to a group of columns in a large data.table without referring to each one individually.

a <- data.table(
  a=as.character(         


        

Answer 1

The idiomatic approach is to use .SD and .SDcols

You can force the RHS to be evaluated in the parent frame by wrapping in ()

a[, (b) := lapply(.SD, as.numeric), .SDcols = b]

For columns 2:3

a[, 2:3 := lapply(.SD, as.numeric), .SDcols = 2:3]

or

mysubset <- 2:3
a[, (mysubset) := lapply(.SD, as.numeric), .SDcols = mysubset]