Working with dot-dot-dot has been a challenge.
I would not have understood this post by Gabor Grothenieck in r-help on 2011-10-9, but I do now! And I'm so happy!
"
Suppose we wish to call f with the control.list components plus
those in the default.args not already specified in the control.list.
If any such arg is not an arg of f exclude it:
# test data - f, default.args and control.list
f < - function(a, b, c = 0, d = 1) print(match.call())
default.args <- list(a = 2, b = 1)
control.list <- list(a = 1, d = 2, e = 3)
# override default.args with control.list
use.args <- modifyList(default.args, control.list)
# exclude components of use.args that are not args of f
sel <- names(use.args) %in% names(as.list(args(f)))
final.args <- use.args[sel]
# run f
do.call("f", final.args)
The last line gives:
> do.call("f", final.args)
f(a = 1, b = 1, d = 2)
"
Wow. That touches all the bases. Understand that, you understand "it".
The problem I fought first was the "double specification" of options. The ... may add arguments that are already set. The magic bullet for that is the "modifyList" function, which can take a default list of settings and selectively replace them with new settings. Awesome.
Then you have the problem that the dot arguments may not apply to a function in which they are to be called. That's where the args function retrieves the legal arguments from a function and then picks the ones that match. Awesome.
I used "formals" for same purpose in my code, before I saw Gabor's post.
See also
Richie Cotton:
f <- function(x, ...)
{
dots <- list(...) #1
if(length(dots) == 0) return(NULL)
cat("The arguments in ... are\n")
print(dots)
f(...) #2
}
f(1,2,3,"a", list("monkey"))
The function, f, stores all but the first input argument in the ellipsis variable. For accessing its contents, it is easiest to convert it to a list (1). The main use however is for passing arguments to subfunctions, which requires no conversion (2).
########################
Why sometimes
input_list <- list(...)
and other times:
object <- as.list(substitute(list(...)))[-1L]
http://stackoverflow.com/questions/3057341/how-to-use-rs-ellipsis-featur...
