How do I pass names for new summary columns to data.table in a function?

Question

Say I want to create a function that calculates a summary dataset from a data.table in R, and I want to be able to pass the name of the new calculated variable in programmat

Answer 1

Courtesy of docendo discimus, you can use a named list created with setNames, like this:

groupMeans <- function(out.var, by.var, dat = dt) {
  return(dat[, setNames(list(mean(x)), out.var), by = by.var])  
}

groupMeans("group.means", "by.var")
#    by.var group.means
# 1:      a  -0.1159832
# 2:      b   0.2910531

Answer 2

You could consider changing the column names inside your function:

groupMeans <- function(out.var, by.var, dat = dt) {
  res <- dat[, list(mean(x)), by=by.var]
  setnames(res, "V1", out.var)
  res
}

Answer 3

We could use setnames to name the summarised column with the 'out.var' vector.

groupMeans <- function(out.var, by.var, dat = dt) {
  setnames(dat[, list(mean(x)), by = by.var], 
                 length(by.var)+1L, out.var)
}

groupMeans("group.var","by.var", dt)[]
#    by.var  group.var
#1:      a -0.1159832
#2:      b  0.2910531

EDIT: Based on @Frank's suggestion.