pass a function a vector or undefined number of arguments - r

Question

I want to be able to pass a function an undefined number of arguments via ... but also to be able to pass it a vector. Here is a silly example:
library(tidyverse)
df <- data.frame(gear = as.character(unique(mtcars$gear)),
id = 1:3)
myfun <- function(...) {
ids_lst <- lst(...)
df2 <- bind_rows(map(ids_lst, function(x)
mtcars %>%
filter(gear == x) %>%
select(mpg)), .id = "gear") %>%
left_join(df)
df2
}
#these all work:
myfun(3)
myfun(3, 4)
myfun(3, 4, 5)
Passing it a vector doesn't work though:
myvector <- unique(mtcars$gear)
myfun(myvector)
The problem is because of the way the function collects the arguments and how it returns them:
myfun_lst <- function(...) {
ids_lst <- lst(...)
ids_lst
}
myfun_lst(3, 4, 5)
# $`3`
# [1] 3
# $`4`
# [1] 4
# $`5`
# [1] 5
myfun_lst(myvector)
# $myvector
# [1] 4 3 5
I thought a fix would be to test if the input is a vector, something like:
myfun_final <- function(...) {
if(is.vector(...) & !is.list(...)) {
ids_lst <- as.list(...)
names(ids_lst) <- (...)
} else {
ids_lst <- lst(...)
}
df2 <- bind_rows(map(ids_lst, function(x)
mtcars %>%
filter(gear == x) %>%
select(mpg)), .id = "gear") %>%
left_join(df)
df2
}
Now, passing the function a vector works but collecting the arguments doesn't:
myfun_final(3, 4, 5)
myfun_final(myvector)
What is a good way to solve this?
Thanks

Answer 1

Of course you can change your function so that it will work with both, regular arguments myfun(3, 4, 5) and a vector myfun(myvector), as shown in the answer above.
Another option is that you make use of argument splicing by unquoting with the bang bang bang operator !!!. This operator is only supported in certain {rlang} and {tidyverse} functions. In your example you evaluate the dots ... inside purrr::map which supports argument splicing. Therefore there might not be the need to rewrite your function:
library(tidyverse)
# your original function:
myfun <- function(...) {
ids_lst <- lst(...)
df2 <- bind_rows(map(ids_lst, function(x)
mtcars %>%
filter(gear == x) %>%
select(mpg)), .id = "gear") %>%
left_join(df)
df2
}
myvector <- unique(mtcars$gear)
myfun(!!! myvector) # works
#> Joining, by = "gear"
#> gear mpg id
#> 1 4 21.0 1
#> 2 4 21.0 1
#> 3 4 22.8 1
#> 4 4 24.4 1
#> 5 4 22.8 1
#> 6 4 19.2 1
#> 7 4 17.8 1
#> 8 4 32.4 1
#> 9 4 30.4 1
#> 10 4 33.9 1
#> ...
myfun(3, 4, 5) # works
#> Joining, by = "gear"
#> gear mpg id
#> 1 3 21.4 2
#> 2 3 18.7 2
#> 3 3 18.1 2
#> 4 3 14.3 2
#> 5 3 16.4 2
#> 6 3 17.3 2
#> 7 3 15.2 2
#> 8 3 10.4 2
#> 9 3 10.4 2
#> 10 3 14.7 2
#> ...
Created on 2021-12-30 by the reprex package (v0.3.0)
You can read more about unquoting with the bang bang bang operator here.
Finally, you should think about the users of your function. If you are the only user then choose whatever suits you. In case there are other users you should think about how they expect the function to work. Probably users don't expect a function to work with several arguments and at the same time, alternatively, by providing those arguments in a vector. In the tidyverse argument splicing with !!! is a well established concept. In base R we would usually use do.call("myfun", as.list(myvector)) to achieve something similar.
To add another option:
The purrr package has a family of lift functions which can be used to alter the kind of arguments a function takes. The most prominent is lift_dl which transforms a function that takes dots as argument to a function that takes a list or vector as argument. This could also be used to solve the problem without the need to rewrite the function:
lift_dl(myfun)(myvector)
#> Joining, by = "gear"
#> gear mpg id
#> 1 4 21.0 1
#> 2 4 21.0 1
#> 3 4 22.8 1
#> 4 4 24.4 1
#> 5 4 22.8 1
#> 6 4 19.2 1
#> 7 4 17.8 1
#> 8 4 32.4 1
#> 9 4 30.4 1
#> 10 4 33.9 1
#> ...
Created on 2022-01-01 by the reprex package (v0.3.0)

Answer 2

How about testing if ... is of length 1 and if the only argument passed through is a vector? If not so, then consider ... a list of scalers and capture them with lst(...).
myfun_final <- function(...) {
if (...length() == 1L && is.vector(..1))
ids_lst <- `names<-`(..1, ..1)
else
ids_lst <- lst(...)
df2 <- bind_rows(map(ids_lst, function(x)
mtcars %>%
filter(gear == x) %>%
select(mpg)), .id = "gear") %>%
left_join(df)
df2
}
Test
> myfun_final(3)
Joining, by = "gear"
gear mpg id
1 3 21.4 2
2 3 18.7 2
3 3 18.1 2
4 3 14.3 2
5 3 16.4 2
6 3 17.3 2
7 3 15.2 2
8 3 10.4 2
9 3 10.4 2
10 3 14.7 2
11 3 21.5 2
12 3 15.5 2
13 3 15.2 2
14 3 13.3 2
15 3 19.2 2
> myfun_final(3,4,5)
Joining, by = "gear"
gear mpg id
1 3 21.4 2
2 3 18.7 2
3 3 18.1 2
4 3 14.3 2
5 3 16.4 2
6 3 17.3 2
7 3 15.2 2
8 3 10.4 2
9 3 10.4 2
10 3 14.7 2
11 3 21.5 2
12 3 15.5 2
13 3 15.2 2
14 3 13.3 2
15 3 19.2 2
16 4 21.0 1
17 4 21.0 1
18 4 22.8 1
19 4 24.4 1
20 4 22.8 1
21 4 19.2 1
22 4 17.8 1
23 4 32.4 1
24 4 30.4 1
25 4 33.9 1
26 4 27.3 1
27 4 21.4 1
28 5 26.0 3
29 5 30.4 3
30 5 15.8 3
31 5 19.7 3
32 5 15.0 3
> myfun_final(c(3,4,5))
Joining, by = "gear"
gear mpg id
1 3 21.4 2
2 3 18.7 2
3 3 18.1 2
4 3 14.3 2
5 3 16.4 2
6 3 17.3 2
7 3 15.2 2
8 3 10.4 2
9 3 10.4 2
10 3 14.7 2
11 3 21.5 2
12 3 15.5 2
13 3 15.2 2
14 3 13.3 2
15 3 19.2 2
16 4 21.0 1
17 4 21.0 1
18 4 22.8 1
19 4 24.4 1
20 4 22.8 1
21 4 19.2 1
22 4 17.8 1
23 4 32.4 1
24 4 30.4 1
25 4 33.9 1
26 4 27.3 1
27 4 21.4 1
28 5 26.0 3
29 5 30.4 3
30 5 15.8 3
31 5 19.7 3
32 5 15.0 3