nargs() provides the number of arguments without evaluating them
> f = function(x, ..., y) nargs()
> f()
[1] 0
> f(a=1, b=2)
[1] 2
> f(1, a=1, b=2)
[1] 3
> f(x=1, a=1, b=2)
[1] 3
> f(stop())
[1] 1
On 05/03/2018 11:01 AM, William Dunlap via R-devel wrote:
In R-3.5.0 you can use ...length():
> f <- function(..., n) ...length()
> f(stop("one"), stop("two"), stop("three"), n=7)
[1] 3
Prior to that substitute() is the way to go
> g <- function(..., n) length(substitute(...()))
> g(stop("one"), stop("two"), stop("three"), n=7)
[1] 3
R-3.5.0 also has the ...elt(n) function, which returns
the evaluated n'th entry in ... , without evaluating the
other ... entries.
> fn <- function(..., n) ...elt(n)
> fn(stop("one"), 3*5, stop("three"), n=2)
[1] 15
Prior to 3.5.0, eval the appropriate component of the output
of substitute() in the appropriate environment:
> gn <- function(..., n) {
+ nthExpr <- substitute(...())[[n]]
+ eval(nthExpr, envir=parent.frame())
+ }
> gn(stop("one"), environment(), stop("two"), n=2)
<environment: R_GlobalEnv>
Bill Dunlap
TIBCO Software
wdunlap tibco.com
On Thu, May 3, 2018 at 7:29 AM, Dénes Tóth <toth.de...@kogentum.hu> wrote:
Hi,
In some cases the number of arguments passed as ... must be determined
inside a function, without evaluating the arguments themselves. I use the
following construct:
dotlength <- function(...) length(substitute(expression(...))) - 1L
# Usage (returns 3):
dotlength(1, 4, something = undefined)
How can I define a method for length() which could be called directly on
`...`? Or is it an intention to extend the base length() function to accept
ellipses?
Regards,
Denes
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