Assigning/Referencing a column name in data.table

Question

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

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

I need this

strColumnName <- "Totals:"
cars [ , strColumnName = .N, by=speed]  

How to do it?

B) Similarly (more general case) - instead of this:

cars[ dist > 50, .(`Totals:`=.N, x=dist*100), by=speed] 

I need this:

strFactor <- "dist"
cars[ strFactor > 50, .(`Totals:`=.N, x=strFactor*100), by=speed] 

This question is about GENERAL way of assigning/referencing column name variables in data.table, i.e. in 'j' (both RHS and LHS), as well as in 'i' and 'by' - dynamically. This is needed when are chosen elsewhere in the code (e.g. a user my enter them in shiny app)

C) General case involving i,j and by - Instead of this:

 cars[ dist > 50, .(`Totals x Factor: ` = .N * dist), by=speed] 

I need this:

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

Answer 1

We can use := and wrap the variable with () to evaluate it instead of assinging it literally

library(data.table)
cars[ , (strColumnName) := .N, by=speed]  

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If we need a summarised column,

setnames(cars[, .N, by = speed], 'N',  strColumnName)[]

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With the updated code

cars[eval(as.name(strFactor)) > 50, .(`Totals:`=.N, x=eval(as.name(strFactor))*100), by=speed]

Answer 2

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]