[R] dot-dot-dot as an actual argument

[Paul Hiemstra]

Jyotirmoy Bhattacharya wrote:
> I could not find any documentation of how dot-dot-dot works when used
> as an argument in a function call (rather than as a formal argument in
> a definition). I would appreciate some references to the rules
> governing situations like:
>
> f1<-function(x,y,...){
>   print(x)
> }
>   
it would print(x), probably complain that y is missing
> f2<-function(...){
>   f1(...)
> }
>
> f2(1,2,3)
>   
Hi!

print the .. and see what happens:

f2<-function(...){
  f1(...)
  print(list(...))
}

f2(1,2,3)

> In the call above how are the three numbers bound to the individual
> formal arguments x and y of f1 rather than f1 being called with a
> single pairlist, which is what the documentation says ... is.
>
> And while the example above succeeds, why does the following fail,
>
> library(lattice)
> f.barchart <- function(...) {
>     barchart(...)
> }
>
> x <- data.frame(a = c(1,1,2,2), b = c(1,2,3,4), d = c(1,2,2,1))
> print(f.barchart(a ~ b, data = x, groups = d))
>
> This gives the error:
> Error in eval(expr, envir, enclos) :
>   ..3 used in an incorrect context, no ... to look in
>   
The problem is that d is a column in x and not a seperate R object. This 
is solved in barchart because the function knows that it needs to look 
in x for d. The problem only is that when the third (group = d) is taken 
from the ... (..3) it doesn't find any R object called d. So it crashes 
with the above error.

cheers,
Paul
> Jyotirmoy Bhattacharya
>
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>   


-- 
Drs. Paul Hiemstra
Department of Physical Geography
Faculty of Geosciences
University of Utrecht
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