How to get the value of list(...) in R and S functions

Question

I am confused by how list(...) works in R and S-plus. For following code

pp <- function(x, ...) { print( length( list(...)))}
pp(1,,,1)

In S-Plus, it works, but in R, it gives "Error in print(length(list(...))) : argument is missing, with no default"

I am more interest in how this works in R and how to get the value of list(...) in R functions.

Answer 1

I'm not sure why that syntax is allowed in S-plus but not in R.

Here, though, is some R code that will have essentially the same effect:

pp <- function(x, ...) {
    print(length(as.list(match.call(expand.dots=FALSE))[["..."]]))
}
pp(1,,,1)
# [1] 3

Alternatively, using a trick from here:

ppp <- function(x, ...) {
    print(length(substitute(...())))
}
ppp(1,,,1)
# [1] 3