Programming with `{data.table}`: how to name a new column? - r

Question

The following question seems very basic in programming with data.table, so my apologies if it's a duplicate. I spent time researching but could not find an answer.
I want to create a "user-defined function" that wraps around a data.table wrangling procedure. In this procedure, a new column is created, and I want to let the user set the name of that new column.
Example
Consider the following code that works as-is. I want to wrap it inside a function.
library(data.table)
library(magrittr)
library(tibble)
mtcars %>%
as.data.table() %>%
.[, .(max_mpg = max(mpg)), by = cyl] %>%
as_tibble()
#> # A tibble: 3 x 2
#> cyl max_mpg
#> <dbl> <dbl>
#> 1 6 21.4
#> 2 4 33.9
#> 3 8 19.2
Created on 2021-10-13 by the reprex package (v0.3.0)
All I want my function to do is let the user set the name of new_colname_of_choice:
my_wrapper <- function(new_colname_of_choice) {
mtcars %>%
as.data.table() %>%
.[, .(new_colname_of_choice = max(mpg)), by = cyl] %>%
as_tibble()
}
my_wrapper(new_colname_of_choice = "my_lovely_colname")
#> # A tibble: 3 x 2
#> cyl new_colname_of_choice <---------- why this isn't called "my_lovely_colname"?
#> <dbl> <dbl>
#> 1 6 21.4
#> 2 4 33.9
#> 3 8 19.2
I've tried using curly braces which didn't work either (actually threw an error):
my_wrapper_2 <- function(new_colname_of_choice) {
mtcars %>%
as.data.table() %>%
.[, .({new_colname_of_choice} = max(mpg)), by = cyl] %>%
as_tibble()
}
Error: unexpected '=' in:
" as.data.table() %>%
.[, .({new_colname_of_choice} ="
Which is surprising because curly braces do promote the desired naming ability, but in a different (yet similar) kind of code:
my_wrapper_3 <- function(new_colname_of_choice) {
mtcars %>%
as.data.table() %>%
.[, {new_colname_of_choice} := max(mpg), by = cyl] %>%
as_tibble()
}
my_wrapper_3(new_colname_of_choice = "my_lovely_colname")
## # A tibble: 32 x 12
## mpg cyl disp hp drat wt qsec vs am gear carb my_lovely_colname <---- SUCCESS!
## <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
## 1 21 6 160 110 3.9 2.62 16.5 0 1 4 4 21.4
## 2 21 6 160 110 3.9 2.88 17.0 0 1 4 4 21.4
## 3 22.8 4 108 93 3.85 2.32 18.6 1 1 4 1 33.9
## 4 21.4 6 258 110 3.08 3.22 19.4 1 0 3 1 21.4
## 5 18.7 8 360 175 3.15 3.44 17.0 0 0 3 2 19.2
## 6 18.1 6 225 105 2.76 3.46 20.2 1 0 3 1 21.4
## 7 14.3 8 360 245 3.21 3.57 15.8 0 0 3 4 19.2
## 8 24.4 4 147. 62 3.69 3.19 20 1 0 4 2 33.9
## 9 22.8 4 141. 95 3.92 3.15 22.9 1 0 4 2 33.9
## 10 19.2 6 168. 123 3.92 3.44 18.3 1 0 4 4 21.4
## # ... with 22 more rows
Bottom line
My conclusion is that the = operator is sensitive to {...} on the LHS. How can I otherwise pass a name (from argument) to the LHS in the initial my_wrapper() example?
EDIT
I'd like to add the dplyr solution for the same problem, taken from the programming with dplyr vignette:
library(dplyr)
my_wrapper_dplyr <- function(new_colname_of_choice) {
mtcars %>%
group_by(cyl) %>%
summarise("{new_colname_of_choice}" := max(mpg))
}
my_wrapper_dplyr("another_lovely_colname")
Which is pretty robust and works in all naming situations I've encountered. Is there a built-in/canonical practice in data.table similar to {dplyr}'s?

Answer 1

With the upcoming data.table version 1.14.3, you'll be able to use the new env parameter:
A new interface for programming on data.table has been added, closing #2655 and many other linked issues. It is built using base R's substitute-like interface via a new env argument to [.data.table. For details see the new vignette programming on data.table, and the new ?substitute2 manual page. Thanks to numerous users for filing requests, and Jan Gorecki for implementing.
# install dev version
install.packages("https://github.com/Rdatatable/data.table/archive/master.tar.gz", repo = NULL, type = "source")
library(tibble)
library(data.table)
my_wrapper_new <- function(new_colname_of_choice) {
mtcars %>%
as.data.table() %>%
.[, .(new_colname_of_choice = max(mpg)), by = cyl,
env=list(new_colname_of_choice = new_colname_of_choice)] %>%
as_tibble()
}
my_wrapper_new('test')
# A tibble: 3 x 2
cyl test
<dbl> <dbl>
1 6 21.4
2 4 33.9
3 8 19.2

Answer 2

One thing you can do is separate the creation of the column and the naming of the column like so:
my_wrapper <- function(new_colname_of_choice) {
mtcars %>%
as.data.table() %>%
.[, .(tempcol = max(mpg)), by = cyl] %>%
setnames(., "tempcol", new_colname_of_choice) %>%
as.tibble()
}
my_wrapper("my_lovely_colname")
Using this method you can use either .(tempcol = max(mpg)) or tempcol := max(mpg)

Answer 3

Using setNames from stats:
my_wrapper <- function(new_colname_of_choice) {
mtcars %>%
as.data.table() %>%
.[, setNames(list(max(mpg)), new_colname_of_choice), by = cyl] %>%
as_tibble()
}
my_wrapper(new_colname_of_choice = "my_lovely_colname")