How to use R's ellipsis feature when writing your own function?
I read answers and comments and I see that few things weren't mentioned:
data.frame
useslist(...)
version. Fragment of the code:object <- as.list(substitute(list(...)))[-1L] mrn <- is.null(row.names) x <- list(...)
object
is used to do some magic with column names, butx
is used to create finaldata.frame
.
For use of unevaluated...
argument look atwrite.csv
code wherematch.call
is used.As you write in comment result in Dirk answer is not a list of lists. Is a list of length 4, which elements are
language
type. First object is asymbol
-list
, second is expression1:10
and so on. That explain why[-1L]
is needed: it removes expectedsymbol
from provided arguments in...
(cause it is always a list).
As Dirk statessubstitute
returns "parse tree the unevaluated expression".
When you callmy_ellipsis_function(a=1:10,b=11:20,c=21:30)
then...
"creates" a list of arguments:list(a=1:10,b=11:20,c=21:30)
andsubstitute
make it a list of four elements:List of 4 $ : symbol list $ a: language 1:10 $ b: language 11:20 $ c: language 21:30
First element doesn't have a name and this is
[[1]]
in Dirk answer. I achieve this results using:my_ellipsis_function <- function(...) { input_list <- as.list(substitute(list(...))) str(input_list) NULL } my_ellipsis_function(a=1:10,b=11:20,c=21:30)
As above we can use
str
to check what objects are in a function.my_ellipsis_function <- function(...) { input_list <- list(...) output_list <- lapply(X=input_list, function(x) {str(x);summary(x)}) return(output_list) } my_ellipsis_function(a=1:10,b=11:20,c=21:30) int [1:10] 1 2 3 4 5 6 7 8 9 10 int [1:10] 11 12 13 14 15 16 17 18 19 20 int [1:10] 21 22 23 24 25 26 27 28 29 30 $a Min. 1st Qu. Median Mean 3rd Qu. Max. 1.00 3.25 5.50 5.50 7.75 10.00 $b Min. 1st Qu. Median Mean 3rd Qu. Max. 11.0 13.2 15.5 15.5 17.8 20.0 $c Min. 1st Qu. Median Mean 3rd Qu. Max. 21.0 23.2 25.5 25.5 27.8 30.0
It's ok. Lets see
substitute
version:my_ellipsis_function <- function(...) { input_list <- as.list(substitute(list(...))) output_list <- lapply(X=input_list, function(x) {str(x);summary(x)}) return(output_list) } my_ellipsis_function(a=1:10,b=11:20,c=21:30) symbol list language 1:10 language 11:20 language 21:30 [[1]] Length Class Mode 1 name name $a Length Class Mode 3 call call $b Length Class Mode 3 call call $c Length Class Mode 3 call call
Isn't what we needed. You will need additional tricks to deal with these kind of objects (as in
write.csv
).
If you want use ...
then you should use it as in Shane answer, by list(...)
.
You can convert the ellipsis into a list with list()
, and then perform your operations on it:
> test.func <- function(...) { lapply(list(...), class) }
> test.func(a="b", b=1)
$a
[1] "character"
$b
[1] "numeric"
So your get_list_from_ellipsis
function is nothing more than list
.
A valid use case for this is in cases where you want to pass in an unknown number of objects for operation (as in your example of c()
or data.frame()
). It's not a good idea to use the ...
when you know each parameter in advance, however, as it adds some ambiguity and further complication to the argument string (and makes the function signature unclear to any other user). The argument list is an important piece of documentation for function users.
Otherwise, it is also useful for cases when you want to pass through parameters to a subfunction without exposing them all in your own function arguments. This can be noted in the function documentation.
Just to add to Shane and Dirk's responses: it is interesting to compare
get_list_from_ellipsis1 <- function(...)
{
list(...)
}
get_list_from_ellipsis1(a = 1:10, b = 2:20) # returns a list of integer vectors
$a
[1] 1 2 3 4 5 6 7 8 9 10
$b
[1] 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
with
get_list_from_ellipsis2 <- function(...)
{
as.list(substitute(list(...)))[-1L]
}
get_list_from_ellipsis2(a = 1:10, b = 2:20) # returns a list of calls
$a
1:10
$b
2:20
As it stands, either version appears suitable for your purposes in my_ellipsis_function
, though the first is clearly simpler.