[R] can a function alter the "..." argument and passed the result to another function?

Uwe Ligges ligges at statistik.tu-dortmund.de
Fri Dec 19 11:28:59 CET 2008

Dan Kelley wrote:
> Is there a way a function ('parent', say) can manipulate the ... 
> argument, and then pass the manipulated value into another function 
> ('child', say) that it calls?
> What I'm trying to do is to use some plotting defaults within 'parent' 
> that are not part of parent's argument list, but to let the use 
> over-ride these defaults if they choose.  A sketch is below.
> parent <- function(stuff, ...) {
>   dots <- list(...)
>   if (!"type" in names(list)) dots$type='p'
>   plot(stuff, unlist(dots))  # or alist()? or what?
> }

do.call() is your friend here, because you can easily pass a *list* of 
arguments to the function when calling it:

parent <- function(stuff, ...) {
     dots <- list(...)
     if (!"type" %in% names(list)) dots$type <- 'l'
     do.call("plot", c(list(x=stuff), dots))

Uwe Ligges

> I know, I can just make "type" be a formal argument of 'parent', and I'm 
> happy to do that, but I'm still in the learning stages with R, and so 
> I'm trying to become more familiar with as much R syntax as I can, even 
> if I end up doing things differently later on.
> D. Kelley
> Oceanography Department, Dalhousie University
> Halifax, Nova Scotia, Canada
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