[Rd] Attributes of 1st argument in ...

[Prof Brian Ripley]

I think you have missed the use of ..1 etc: see e.g. cBind() in 
package Matrix.

So x <- attr(list(...)[[1L]], "foo") can be x <- attr(..1, "foo")

As for 'extra copying', it all depends on exactly what you are doing, 
but compare

> foo1 <- function(...) length(..1)
> foo2 <- function(...) length(list(...)[[1L]])
> tracemem(x <- runif(1000))
[1] "<0x1b27800>"
> foo1(x)
[1] 1000
> tracemem(x <- runif(1000))
[1] "<0x1b29800>"
> foo2(x)
tracemem[0x1b29800 -> 0x10a2200]: foo2
[1] 1000


On Sat, 3 Jul 2010, Daniel Murphy wrote:

> Hi Hadley,
>
> My actual goal is to have a cbind method in the mondate package that behaves
> just like the base cbind function: class and shape of the result, names,
> etc. Perhaps it's due to the fact that 'cbind' uses its own internal
> dispatching, but I have not found a way to implement a "true" S3-style cbind
> method. (This is probably ancient news to the development team.) An S4 cbind
> method will utilize callNextMethod with just setGeneric("cbind"), which has
> no 'x' in the formal arguments. With no 'x', there's no "first argument" on
> which to dispatch a "mondate" method. I can make the cbind of mondates also
> be a mondate with an all-encompassing setMethod("cbind","ANY", etc) method,
> but that wrests dispatch control from cbind which makes no sense whatsoever.
> So, to make a long story even longer, I settled for a "cbindmondate
> function" that utilizes the speed of base::cbind and (with one exception)
> gives me the hoped-for "base cbind behavior."
>
> I can send examples of my trial-and-error attempts under separate email if
> you're interested.
>
> Best regards,
> Dan
>
> On Sat, Jul 3, 2010 at 9:17 AM, Hadley Wickham <hadley at rice.edu> wrote:
>
>> Hi Dan,
>>
>> Is there a reason you can't change the function to
>>
>> f <- function(x, ...) {}
>>
>> ?
>>
>> Hadley
>>
>> On Fri, Jul 2, 2010 at 4:26 PM, Daniel Murphy <chiefmurphy at gmail.com>
>> wrote:
>>> R-Devel:
>>>
>>> I am trying to get an attribute of the first argument in a call to a
>>> function whose formal arguments consist of dots only and do something,
>> e.g.,
>>> call 'cbind', based on the attribute
>>> f<- function(...) {get first attribute; maybe or maybe not call 'cbind'}
>>>
>>> I thought of (ignoring "deparse.level" for the moment)
>>>
>>> f<-function(...) {x <- attr(list(...)[[1L]], "foo"); if (x=="bar")
>>> cbind(...) else x}
>>>
>>> but I feared my solution might do some extra copying, with a performance
>>> penalty if the dotted objects in the actual call to "f' are very large.
>>>
>>> I thought the following alternative might avoid a potential performance
>> hit
>>> by evaluating the attribute in the parent.frame (and therefore avoid
>> extra
>>> copying?):
>>>
>>> f<-function(...)
>>> {
>>>   L<-match.call(expand.dots=FALSE)[[2L]]
>>>   x <- eval(substitute(attr(x,"foo"), list(x=L[[1L]])))
>>>   if (x=="bar") cbind(...) else x
>>> }
>>>
>>> system.time tests showed this second form to be only marginally faster.
>>>
>>> Is my fear about extra copying unwarranted? If not, is there a better way
>> to
>>> get the "foo" attribute of the first argument other than my two
>>> alternatives?
>>>
>>> Thanks,
>>> Dan Murphy
>>>
>>>        [[alternative HTML version deleted]]
>>>
>>> ______________________________________________
>>> R-devel at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>
>>
>>
>>
>> --
>> Assistant Professor / Dobelman Family Junior Chair
>> Department of Statistics / Rice University
>> http://had.co.nz/
>>
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595