On 13-10-18 1:54 PM, Bert Gunter wrote:
Yes, similar, but better, as match.call() will get unwanted named
arguments, too.
However, I do not understand the
substitute(...())
idiom. Would you care to explain it? (No is an acceptable answer!).
I suspect it's a bug, though I can't see it's one that's likely to need
fixing. The general idea is that a function call like ...() is parsed
into a pairlist containing just the name "...", then substitute replaces
it with the content of that variable, which is a pairlist containing the
unevaluated argument list. So by that analysis, you might expect to get
the same result using
pairlist(...)
However, you don't, because the latter expression evaluates all the
arguments to the function, while Bill's idiom leaves them unevaluated.
I can't think of any documented reason why that should be, but on the
other hand, I can't think of any reason it would cause problems. So I'd
say it's unlikely to be deliberately changed, but it might change as a
result of some internal change to R.
Duncan Murdoch
I would have expressed it as:
as.list(substitute(list(...)))[-1]
to convert the parse tree to a list. (which is again better than using
match.call() ).
Best,
Bert
On Fri, Oct 18, 2013 at 10:27 AM, William Dunlap <wdun...@tibco.com> wrote:
I am using the ... argument to parmeterize a user define fn to accept
multiple input objects. I subsquently save all these data as a list.
Question: what is the best way to recover or extract the original object
names that were fed to the fn?
The following function, ellipsisInfo, returns character strings representing the
actual arguments to the function. If the function was called with tags on the
arguments, as in ellipsisInfo(tag=argument), it makes those tags the names
on the returned character vector. It does not evaluate the ... arguments, so
you don't run into problems with evaluating arguments too soon or evaluating
ones that should not be evaluated most of the time.
ellipsisInfo <- function(...) {
# get the unevaluated expressions given as arguments
unevaluatedArgs <- substitute(...())
# convert those expressions to text (truncate to single line)
unevaluatedArgsAsText <- vapply(unevaluatedArgs, function(a)deparse(a)[1],
"")
unevaluatedArgsAsText
}
E.g.,
i <- ellipsisInfo(x, log(10), e=exp(1), onProblem=stop("there was a problem"))
i
"x"
"log(10)"
e
"exp(1)"
onProblem
"stop(\"there was a problem\")"
ifelse(names(i)=="", i, names(i)) # use tag if supplied, otherwise argument
itself
[1] "x" "log(10)" "e"
[4] "onProblem"
Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
-----Original Message-----
From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On
Behalf
Of Dan Abner
Sent: Friday, October 18, 2013 9:06 AM
To: r-help@r-project.org
Subject: [R] Recovering object names when using the ... argument in a fn XXXX
Hi all,
I am using the ... argument to parmeterize a user define fn to accept
multiple input objects. I subsquently save all these data as a list.
Question: what is the best way to recover or extract the original object
names that were fed to the fn?
Thanks,
Dan
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