Assigning/Referencing a column name in data.table dynamically (in i, j and by)

Question

A) Instead of this (where cars <- data.table(cars))

cars[ , .(`Totals:`=.N), by=speed]  

I need this

strCo         


        

Answer 1

Edit: Based on your clarifications, here is an approach with setNames and get. The trick here is that .. instructs the evaluation to occur in the calling environment.

library(data.table)
cars <- data.table(cars)
strFactor <- "dist"
strNewVariable <- "Totals x Factor: "
strBy <- "speed"
cars[ get(strFactor)  > 50, 
     setNames(.(.N * get(..strFactor)),strNewVariable),
     by=strBy]