How to use R's ellipsis feature when writing your own function?

Question

The R language has a nifty feature for defining functions that can take a variable number of arguments. For example, the function data.frame takes any number of arguments, and each argument becomes the data for a column in the resulting data table. Example usage:

> data.frame(letters=c("a", "b", "c"), numbers=c(1,2,3), notes=c("do", "re", "mi"))
  letters numbers notes
1       a       1    do
2       b       2    re
3       c       3    mi

The function's signature includes an ellipsis, like this:

function (..., row.names = NULL, check.rows = FALSE, check.names = TRUE, 
    stringsAsFactors = default.stringsAsFactors()) 
{
    [FUNCTION DEFINITION HERE]
}

I would like to write a function that does something similar, taking multiple values and consolidating them into a single return value (as well as doing some other processing). In order to do this, I need to figure out how to "unpack" the ... from the function's arguments within the function. I don't know how to do this. The relevant line in the function definition of data.frame is object <- as.list(substitute(list(...)))[-1L], which I can't make any sense of.

So how can I convert the ellipsis from the function's signature into, for example, a list?

To be more specific, how can I write get_list_from_ellipsis in the code below?

my_ellipsis_function(...) {
    input_list <- get_list_from_ellipsis(...)
    output_list <- lapply(X=input_list, FUN=do_something_interesting)
    return(output_list)
}

my_ellipsis_function(a=1:10,b=11:20,c=21:30)

---

Edit

It seems there are two possible ways to do this. They are as.list(substitute(list(...)))[-1L] and list(...). However, these two do not do exactly the same thing. (For differences, see examples in the answers.) Can anyone tell me what the practical difference between them is, and which one I should use?

Answer 1

You gave half the answer already. Consider

R> my_ellipsis_function <- function(...) {
+   input_list <- as.list(substitute(list(...)))
+ }
R> print(my_ellipsis_function(a=1:10, b=2:20))
[[1]]
list

$a
1:10

$b
11:20

R> 

So this took two arguments a and b from the call and converted it to a list. Wasn't that what you asked for?

Answer 2

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.

Answer 3

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)

$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)

$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.

Answer 4

I read answers and comments and I see that few things wasn't mentioned.

1. data.frame uses list(...) 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, but x is used to create final data.frame.
   For use of unevaluated ... argument look at write.csv code where match.call is used.

2. 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 a symbol - list, second is expression 1:10 and so on. That explain why [-1L] is needed: it removes expected symbol from provided arguments in ... (cause it is always a list).
   As Dirk states substitute returns "parse tree the unevaluated expression".
   When you call my_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) and substitute 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)
   

3. 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(...).