How to check if any arguments were passed via "..." (ellipsis) in R? Is missing(...) valid?

Question

I'd like to check whether an R function's "..." (ellipsis) parameter has been fed with some values/arguments.

Currently I'm using something like:

test1 <- function(...) {
   if (missing(...)) TRUE
   else FALSE
}

test1()
## [1] TRUE
test1(something)
## [2] FALSE

It works, but ?missing doesn't indicate if that way is proper/valid.

If the above is not correct, what is THE way to do so? Or maybe there are other, faster ways? PS. I need this kind of verification for this issue.

Dhahran

#9877771 – Redcap

Answer 1

Here's an alternative that will throw an error if you try to pass in a non-existent object.

test2 <- function(...) if(length(list(...))) FALSE else TRUE

test2()
#[1] TRUE
test2(something)
#Error in test2(something) : object 'something' not found
test2(1)
#[1] FALSE

Redcap

Sure, but I really wonder also if missing(...) is valid. :) BTW, for the sole purpose of testing if there's something under ..., length(list(...))==0 is a little bit slower. Anyway, thanks! – Dhahran

@Dhahran it should be slower because it's forcing evaluation. It's hard to say what's better without knowing why you're checking or what you're doing with that information. Maybe it's better to throw here, maybe it's better to throw later. Presumably, if you find out that there are arguments, you'll do something with them, which will force evaluation and the error. length(list(...)) is also one more function call than missing(...) – Redcap

BTW, you said if (length(list(...))==0), and you used if (length(list(...)) > 0) in your benchmark. I think if (length(list(...))) is marginally faster (i.e. the implicity conversion to logical is faster than comparing to a numeric) – Redcap

Answer 2

I think match.call is what you need:

test <- function(...) {match.call(expand.dots = FALSE)}

> test()
test()

> test(x=3,y=2,z=5)
test(... = list(x = 3, y = 2, z = 5))

It will give you every time the values passed in the ellipsis, or it will be blank if you won't pass any.

Hope that helps!

Tagmeme

If there were more parameters in the arg list (other than "..."), I should test for any(names(match.call(expand.dots=FALSE)[-1]) == "..."), don't I? :) – Dhahran

If you necessarily want it to give you TRUE or FALSE then yeah the above works great. If you don't then the above function works with more parameters other than '...' too. As you already know test <- function(k=7,...) {match.call(expand.dots = FALSE)} for test(x=3,y=2,z=5,k=5) returns test(k = 5, ... = list(x = 3, y = 2, z = 5)) – Tagmeme

Answer 3

If it helps anyone I ended up using the following function to get the ellipsis parameters for every function (returns an empty list or a list of arguments):

get.params <- function (...) {
  params <- list()

  if (length(list(...)) && !is.null(...)) 
    params <- unlist(...)

  return(params)
}

for:

f <- function(t, ...) {
 params <- get.params(...)
 print(paste(params))
}

Vintage