[R] Recovering object names when using the ... argument in a fn XXXX

[peter dalgaard]

On Oct 19, 2013, at 19:03 , Duncan Murdoch wrote:

> 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
> 

Just curious, does substitute(...()) buy you anything that you don't get from the straightforward

match.call(expand.dots=FALSE)$...

???
-pd


> 
>> 
>> 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 <wdunlap at 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-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
>>>> Of Dan Abner
>>>> Sent: Friday, October 18, 2013 9:06 AM
>>>> To: r-help at 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
>>>> 
>>>>       [[alternative HTML version deleted]]
>>>> 
>>>> ______________________________________________
>>>> R-help at r-project.org mailing list
>>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>>> and provide commented, minimal, self-contained, reproducible code.
>>> 
>>> ______________________________________________
>>> R-help at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>> 
>> 
>> 
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com