r - Warnings in `.SD` when selecting columns named in a variable

Question

Assuming I have a data.table as below

DT <- data.table(x = rep(c("b", "a", "c"), each = 3), v = c(1, 1, 1, 2, 2, 1, 1, 2, 2), y = c(1, 3, 6), a = 1:9, b = 9:1)

> DT
   x v y a b
1: b 1 1 1 9
2: b 1 3 2 8
3: b 1 6 3 7
4: a 2 1 4 6
5: a 2 3 5 5
6: a 1 6 6 4
7: c 1 1 7 3
8: c 2 3 8 2
9: c 2 6 9 1

I have a variable sl <- c("a","b") that selects columns to compute rowSums. If I try the code below

DT[, ab := rowSums(.SD[, ..sl])]

I am still able to get the desired output but given a warning message telling

DT[, ab := rowSums(.SD[, ..sl])] Warning message: In [.data.table(.SD, , ..sl) : Both 'sl' and '..sl' exist in calling scope. Please remove the '..sl' variable in calling scope for clarity.

However, no warnings occur when running

DT[, ab := rowSums(.SD[, sl, with = FALSE])]

I am wondering how to fix the warning issue when using .SD[, ..sl]. Thanks in advance!

Answer 1

It may be that the syntax to use is either specify the .SDcols and call the .SD or directly call the ..cols from the original object. According to ?data.table

x[, cols] is equivalent to x[, ..cols] and to x[, cols, with=FALSE] and to x[, .SD, .SDcols=cols]

if we check the source code of data.table, line 248 seems to be the one triggering the warning

as

DT[, exists(..sl, where = DT)]
#[1] TRUE

and

DT[, .SD[, exists(..sl)]]
#[1] TRUE

DT[, .SD[, exists(..sl, where = .SD)]]
#[1] TRUE