Thank you Bill (and the R-Core Team), this is even more than what I
thought of. I somehow missed this in the NEWS.
BTW, substitue(...()) is beautiful.
On 05/03/2018 05:01 PM, William Dunlap 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 <http://tibco.com>
On Thu, May 3, 2018 at 7:29 AM, Dénes Tóth <toth.de...@kogentum.hu
<mailto: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
______________________________________________
R-devel@r-project.org <mailto:R-devel@r-project.org> mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
<https://stat.ethz.ch/mailman/listinfo/r-devel>
--
Dr. Tóth Dénes ügyvezető
Kogentum Kft.
Tel.: 06-30-2583723
Web: www.kogentum.hu
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
