[R] nargs() inside "[.myclass"

[Robert Gentleman]

On Tue, Jun 27, 2000 at 12:35:57AM +0200, Peter Dalgaard BSA wrote:
> "Vadim Kutsyy" <vadim at kutsyy.com> writes:
> 
> > > This does seem to work:
> > > 
> > > > "[.myclass"<-function(x,...)
> > > > {n<-nargs();if(n==2&&missing(..1))n<-1;print(n-1)}
> > > >  x[]
> > > [1] 0
> > > >  x[1]
> > > [1] 1
> > > >  x[1,2]
> > > [1] 2
> > 
> > 
> > It does, thank you.  could you explain what ..1 stands for?
> 
> It's the first of the "..." arguments.
> 
> Some of the things that surround the handling of those are barely
> understood by the maintainers. I've certainly been surprised by having
> "..1" pop up in match.call() output once in a while. I'm not convinced
> that the [] case isn't a bug. 
> 
> We seem to have managed to code the entire base library without
> referring to ..n arguments a single time, but your example looks like
> it requires it, unless the current behavior is a bug.

  I don't think that is true. It is needed for the generic functions
  at least at an internal level. Also, I expect that the dispatching
  needs a bit of a tidy-up for to handle this; my guess is that nargs
  is settled on at time of dispatch and possibly not adjusted once the
  method is selected (but I could be wrong and don't have time this week
  to do anything about it).

> 
> 
> Apropos: Here's a little exercise:
> 
> > f<-function(...)nargs()
> > f(1)
> [1] 1
> > f()
> [1] 0
> > f(,)
> [1] 2
> > f(,,)
> [1] 3
> 
> Now, how do you call f with exactly *one* missing argument?
> 
> 
> -- 
>    O__  ---- Peter Dalgaard             Blegdamsvej 3  
>   c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
>  (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
> ~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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