Passing data variable to function environmental variable as object or string - r

Question

Take this toy function - just a wrapper around dplyr::select:
select_col <- function(df, chosen_col) {
if(is.character(enquo(chosen_col))){
df %>%
select(all_of(chosen_col))
} else {
df %>%
select({{ chosen_col }})
}
}
What it does is allows the selection of a column passed to chosen_col, regardless of whether that column is presented as a data variable masked as an environmental variable or a string masked as an environmental variable.
Both of these return the same thing:
mtcars %>%
select_col(chosen_col = mpg) %>%
head(2)
mpg
Mazda RX4 21
Mazda RX4 Wag 21
mtcars %>%
select_col(chosen_col = "mpg") %>%
head(2)
mpg
Mazda RX4 21
Mazda RX4 Wag 21
While select_col works, what I want is something more like this doesn't work:
select_col_desired <- function(df, chosen_col) {
if(is.character(enquo(chosen_col))){
chosen_col <- convert_to_env_variable(chosen_col)
}
df %>%
select({{ chosen_col }})
}
What can I use in place of the non-existent function convert_to_env_variablesuch that select_col_desired returns the same things as select_col?
I am aware that select does this already outside a function.

Answer 1

Solution 1: Single Column
If you truly want something like convert_to_env_variable(), you can just use rlang::enquo() to begin with, and skip the conditional.
select_col_desired <- function(df, chosen_col) {
chosen_col <- enquo(chosen_col)
df %>%
select(!!chosen_col)
}
Heck, you could just skip the enquo() and !! altogether:
select_col_desired <- function(df, chosen_col) {
df %>%
select({{ chosen_col }})
}
This is because the "curly-curly" operator {{ first captures the argument passed to chosen_col and immediately defuses it as an expression; before substituting the result in its own place, here to be evaluated as a data-variable in the context of df.
Result
Either way, you'll get this:
# With column as unquoted symbol.
mtcars %>%
select_col_desired(chosen_col = mpg) %>%
head(2)
#> mpg
#> Mazda RX4 21
#> Mazda RX4 Wag 21
# With column as string.
mtcars %>%
select_col_desired(chosen_col = "mpg") %>%
head(2)
#> mpg
#> Mazda RX4 21
#> Mazda RX4 Wag 21
Solution 2: Arbitrary Selection
For a more general solution, you could simply pass your columns as ... to select():
select_any_desired <- function(df, ...) {
df %>% select(...)
}
Result
# With column as unquoted symbol.
mtcars %>%
select_col_desired(mpg) %>%
head(2)
#> mpg
#> Mazda RX4 21
#> Mazda RX4 Wag 21
# With column as string.
mtcars %>%
select_col_desired("mpg") %>%
head(2)
#> mpg
#> Mazda RX4 21
#> Mazda RX4 Wag 21
Note
If you're writing a package, you'll naturally need to #import or qualify functions like rlang::enquo() and magrittr:`%>%`() and dplyr::select(); rather than using library().

Answer 2

Step 1: get the name for the variable
This is done via as.character(substitute(chosen_col)) and we also want to save it for step 2.
varname <- as.character(substitute(chosen_col))
Step 2: assign to a different environment
Assignment can be done using assign(x, value, envir=parent.frame()). Here it is essential to use parent.frame() as assigning environment since we want to assign outside of the function and default behavior is to assign to current environment. Another option here would be to assign to the global environment with envir=globalenv.
This leads us to
assign(varname, x, envir=parent.frame())
where x is containing everything we want to assign. Read this as varname <- x.
Putting both steps
select_col <- function(df, chosen_col) {
if(is.character(enquo(chosen_col))){
x <- df %>%
select(all_of(chosen_col))
} else {
x <- df %>%
select({{ chosen_col }})
}
varname <- as.character(substitute(chosen_col))
assign(varname, x, envir=parent.frame())
x
}
select_col(mtcars, "mpg") %>% head(2)
#> mpg
#> Mazda RX4 21
#> Mazda RX4 Wag 21
head(mpg, 2)
#> mpg
#> Mazda RX4 21
#> Mazda RX4 Wag 21
rm(mpg)
select_col(mtcars, mpg) %>% head(2)
#> mpg
#> Mazda RX4 21
#> Mazda RX4 Wag 21
head(mpg, 2)
#> mpg
#> Mazda RX4 21
#> Mazda RX4 Wag 21