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.

Answer 2

Using ifelse and get:

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

Or using substitute to create the expression like this :

expr <- substitute(a4 := ifelse (a1 == "A",a2 , a3),
                   list(a1=as.name(Col1),
                        a2=as.name(Col2),
                        a3=as.name(Col3),
                        a4=as.name(Col4)))

DT[, eval(expr)]