[R] Extracting specific arguments from "..."

[Sorkin, John]

Bert and other on this Chain,

The original question asked by Bert Gunter, highlights one of my long standing wishes. Perhaps my wish has already been fulfilled (if it has, please let me know where I can look of fulfill my wish), if it hasn't perhaps someone can grant me my wish.

I have tried to understand how to write a function (beyond a basic function), get values of the parameters, and process the parameters. I have looked for a source that clearly describes how his may be done (with examples), but I have yet to find one. Yes, one can look at a function (e.g. lm) look at the statements that are used and use the help system to research each of the statements (I have done this), but doing this is not the most efficient way to learn the topic. It would be very helpful if someone Knowledgeable would put together a primer on writing functions and processing function arguments. If such a document exist, I would be happy to have someone send me its URL.

Thank you,
John

John David Sorkin M.D., Ph.D.
Professor of Medicine, University of Maryland School of Medicine;
Associate Director for Biostatistics and Informatics, Baltimore VA Medical Center Geriatrics Research, Education, and Clinical Center;
PI Biostatistics and Informatics Core, University of Maryland School of Medicine Claude D. Pepper Older Americans Independence Center;
Senior Statistician University of Maryland Center for Vascular Research;

Division of Gerontology and Paliative Care,
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524
Cell phone 443-418-5382




________________________________________
From: R-help <r-help-bounces using r-project.org> on behalf of Bert Gunter <bgunter.4567 using gmail.com>
Sent: Monday, January 6, 2025 5:22 PM
To: Jorgen Harmse
Cc: r-help using r-project.org
Subject: Re: [R] Extracting specific arguments from "..."

Thanks Jorgen.

I thought your approach to getting the argument expressions was clever, but
somewhat convoluted. I think the usual simple way is to use match.call()
(or sys.call() )to get the unevaluated argument expressions;  e.g. ...

f <- function(...){
   match.call()
}
> f(a = 'red', b = sin(zzz))
f(a = "red", b = sin(zzz))

The return value is an object of class call that can be subscripted as (or
converted by as.list() to) a list to extract the argument expressions:
> f(a = 'red', b = sin(zzz))$b
sin(zzz)

You'll note that the $b component is again of class "call". So you may wish
to convert it to character or expression or whatever for further
processing, depending on context. Obviously, I haven't thought about this
carefully.

You raise an important point about robustness. I believe this approach to
extracting the call expressions should be fairly robust, but I do get
confused about the lay of the land when you add promises with default
arguments that may not yet have been forced before match.call() is called.
You may have to wrestle with sys.call() and it's "wh" argument to make
things work the way you want in that situation. I leave such delights to
wiser heads, as well as any corrections or refinements to anything that
I've said here.

Cheers,
Bert

On Mon, Jan 6, 2025 at 9:55 AM Jorgen Harmse <JHarmse using roku.com> wrote:

> I think Bert Gunter is right, but do you want partial matches (not found
> by match), and how robust do you want the code to be?
>
>
>
> f <- function(…)
>
> { pos <- match('a', ...names())
>
>   if (is.na(pos))
>
>     stop("a is required.")
>
>   …elt(pos)
>
> }
>
>
>
> Incidentally, what is the best way to extract the expression without
> evaluating it?
>
>
>
> g <- function(...)
>
> { pos <- match('a',...names())
>
>   if (is.na(pos))
>
>     stop("a is missing.")
>
>   (function(a,...) substitute(a)) (...)
>
> }
>
>
>
> Regards,
>
> Jorgen Harmse.
>
>
>
> Message: 8
> Date: Sun, 5 Jan 2025 11:17:02 -0800
> From: Bert Gunter <bgunter.4567 using gmail.com>
> To: Iris Simmons <ikwsimmo using gmail.com>
> Cc: R-help <R-help using r-project.org>
> Subject: Re: [R] Extracting specific arguments from "..."
> Message-ID:
>         <
> CAGxFJbROnopt-boDF6sRPP79bWUcPPOO3+ycDGN3y-YTDU5b6Q using mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> Thanks, Iris.
> That is what I suspected, but it wasn't clear to me from the docs.
>
> Best,
> Bert
>
> On Sun, Jan 5, 2025 at 10:16 AM Iris Simmons <ikwsimmo using gmail.com> wrote:
> >
> > I would use two because it does not force the evaluation of the other
> arguments in the ... list.
> >
> >
> >
> > On Sun, Jan 5, 2025, 13:00 Bert Gunter <bgunter.4567 using gmail.com> wrote:
> >>
> >> Consider:
> >>
> >> f1 <- function(...){
> >>   one <- list(...)[['a']]
> >>   two <- ...elt(match('a', ...names()))
> >>   c(one, two)
> >> }
> >> ## Here "..." is an argument list with "a" somewhere in it, but in an
> >> unknown position.
> >>
> >> > f1(b=5, a = 2, c=7)
> >> [1] 2 2
> >>
> >> Which is better for extracting a specific named argument, one<- or
> >> two<- ?  Or a third alternative that is better than both?
> >> Comments and critiques welcome.
> >>
> >> Cheers,
> >> Bert
> >>
> >> ______________________________________________
> >> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide
> https://www.r-project.org/posting-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
>
>
>
>

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