[R] Messing with the "..." argument

[Duncan Murdoch]

On 2/12/2009 2:34 PM, Steve Lianoglou wrote:
> Hi all,
> 
> Sorry if this is documented somewhere, but trying to search and google for 
> ways to alter my "..." argument is having me chasing my tail.
> 
> Is there someway that I can manipulate the elements in "..."? 
> Specifically I'd like to use certain vars in "...", then remove them 
> from "..." and pass the rest of "..." to another function.
> 
> Here's a short example: I'm adding more functionality to a generic 
> method by defining how it works on a new class "MyClass":
> 
> setMethod('existingFunction', 'MyClass', function (object, ...) {
>   vars <- list(...)
>   doSomethingWith(vars$myCustomVar)
>   
>   # now I want to pass the rest of the vars down to another function,
>   # let's say some plotting function, so I'd like to remove
>   # `myCustomVar` from "...", keep the rest of the vars and just
>   # pass the valid "..." elements down through the call chain
>   
>   doSomePlot(somethingSane, ...)
> })
> 
> Currently the `doSomePlot` that's called does fine until it passes the
> rest of it's vars to the "real" plot method, at which point I'll get
> a warning like:
> 
> Warning messages:
> 1: In plot.window(...) : "myCustomVar" is not a graphical parameter
> 2: In plot.xy(xy, type, ...) : "myCustomVar" is not a graphical parameter
> 3: In axis(side = side, at = at, labels = labels, ...) :
>   "myCustomVar" is not a graphical parameter
> 4: In axis(side = side, at = at, labels = labels, ...) :
>   "myCustomVar" is not a graphical parameter
> 5: In box(...) : "myCustomVar" is not a graphical parameter
> 6: In title(...) : "myCustomVar" is not a graphical parameter
> 
> Depending on what the name of myCustomVar is, this can work even with
> the warnings, which is passable. It might also bail if myCustomVar
> happens to share a name with a real plotting var, but has an illegal
> value for it.
> 
> Anyway, while there are ways it can work in this case, I'm just wondering
> if I can remove elements from "..." and side-step these warning/error issues
> altogether.
> 
> Since "vars <- list(...)" works to create a list of arguments from "...",
> I'm guessing there must be a way to take an already existing list variable
> and turn its name/value pairs into an appropriate "..." object?

I don't think so, but an alternative is to put together a parameter list 
and call do.call().  For example,

 > g <- function(...) {
+    list(...)    # just return the arg list to see what arrived
+ }
 >
 > f <- function(...) {
+    all <- list(...)
+    all["nothanks"] <- NULL
+    do.call("g", all)
+ }
 >
 > g(nothanks=1, yesplease=2)
$nothanks
[1] 1

$yesplease
[1] 2

 > f(nothanks=1, yesplease=2)
$yesplease
[1] 2

Duncan Murdoch