data.table: Proper way to do create a conditional variable when column names are not known?

Question

My question relates to the creation of a variable which depends upon other columns within a data.table when none of the variable names are known in advance.

Below is

Answer 1

1. get/if Try using get :

DT[, (Col4) := if (get(Col1) == "A") get(Col2) else get(Col3), by = 1:nrow(DT)]

2. get/join or try this approach:

setkeyv(DT, Col1)
DT[, (Col4):=get(Col3)]["A", (Col4):=get(Col2)]

3. .SDCols or this:

setkeyv(DT, Col1)
DT[, (Col4):=.SD, .SDcols = Col3]["A", (Col4):=.SD, .SDcols = Col2]

UPDATE: Added some additional approaches.