R help - Feb 2014 - Checking for and adding "..." arguments to a function...

R-helpers:

I'm guessing this is an easy one for some of you, but I'm a bit
stumped.  Given some arbitrary function (doesn't matter what it does):

myfunction <- function(a,b,c)
{
return(a+b+c)
}

I want to test this function for the presence of the ellipses ("...")
and, if they are missing, create a new function that has them:

myfunction <- function(a,b,c,...)
{
return(a+b+c)
}

So, 1) how do I test for whether a function has an ellipses argument
and, 2) how do I "append" the ellipses to the argument list if they do
exist?

Note that the test/modification should be done without invoking the
function, e.g. I'm not asking how to test for this WITHIN the
function, I'm asking how to test "myfunction" directly as an R
object.

Thanks!

--j


-- 
Jonathan A. Greenberg, PhD
Assistant Professor
Global Environmental Analysis and Remote Sensing (GEARS) Laboratory
Department of Geography and Geographic Information Science
University of Illinois at Urbana-Champaign
259 Computing Applications Building, MC-150
605 East Springfield Avenue
Champaign, IL  61820-6371
Phone: 217-300-1924
http://www.geog.illinois.edu/~jgrn/
AIM: jgrn307, MSN: jgrn307 at hotmail.com, Gchat: jgrn307, Skype: jgrn3007

Hello,

Use ?formals.

 > formals(myfunction)
$a


$b


$c


$...


Hope this helps,

Rui Barradas

Em 17-02-2014 21:22, Jonathan Greenberg escreveu:
> R-helpers:
>
> I'm guessing this is an easy one for some of you, but I'm a bit
> stumped.  Given some arbitrary function (doesn't matter what it does):
>
> myfunction <- function(a,b,c)
> {
> return(a+b+c)
> }
>
> I want to test this function for the presence of the ellipses
("...")
> and, if they are missing, create a new function that has them:
>
> myfunction <- function(a,b,c,...)
> {
> return(a+b+c)
> }
>
> So, 1) how do I test for whether a function has an ellipses argument
> and, 2) how do I "append" the ellipses to the argument list if
they do
> exist?
>
> Note that the test/modification should be done without invoking the
> function, e.g. I'm not asking how to test for this WITHIN the
> function, I'm asking how to test "myfunction" directly as an
R object.
>
> Thanks!
>
> --j
>
>

On Feb 17, 2014, at 1:22 PM, Jonathan Greenberg wrote:
> R-helpers:
> 
> I'm guessing this is an easy one for some of you, but I'm a bit
> stumped.  Given some arbitrary function (doesn't matter what it does):
> 
> myfunction <- function(a,b,c)
> {
> return(a+b+c)
> }
> 
> I want to test this function for the presence of the ellipses
("...")
> and, if they are missing, create a new function that has them:
> 
> myfunction <- function(a,b,c,...)
> {
> return(a+b+c)
> }
> 
> So, 1) how do I test for whether a function has an ellipses argument
> and,
One way:
> myfunction <- function(a,b,c,...)
+ {
+ return(a+b+c)
+ }
> formals(myfunction)
$a


$b


$c


$...

> length(formals(myfunction)$...)
[1] 1
> myfunction <- function(a,b,c)
+ {
+ return(a+b+c)
+ }
> length(formals(myfunction)$...)
[1] 0

> 2) how do I "append" the ellipses to the argument list if they do
> exist?
Not exactly sure but the usual way to capture the objects passed in the three
dots mechanism is with list(...)
> myfunction <- function(a,b,c,...)
+ { others <- list(...); for(i in others) {cat(i);cat("\n")}
+ return(a+b+c)
+ }
> myfunction(a=1,b=2,c=3,other1 =4, other2=5)
4
5
[1] 6

> 
> Note that the test/modification should be done without invoking the
> function, e.g. I'm not asking how to test for this WITHIN the
> function, I'm asking how to test "myfunction" directly as an
R object.
> 
> Thanks!
> 
> --j
> 
> 
> -- 
> Jonathan A. Greenberg, PhD
> Assistant Professor
> Global Environmental Analysis and Remote Sensing (GEARS) Laboratory
> Department of Geography and Geographic Information Science
> University of Illinois at Urbana-Champaign
> 259 Computing Applications Building, MC-150
> 605 East Springfield Avenue
> Champaign, IL  61820-6371
> Phone: 217-300-1924
> http://www.geog.illinois.edu/~jgrn/
> AIM: jgrn307, MSN: jgrn307 at hotmail.com, Gchat: jgrn307, Skype: jgrn3007
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
David Winsemius
Alameda, CA, USA

Here are two ways:

## construct formals adding ...
f <- c(formals(myfunction), unlist(alist(... = )))
## replace the formals, excluding the extra ... if it previously existed
formals(myfunction) <- f[!duplicated(names(f))]


## 2nd way, searching for ... and doing the replacement only if it is not found
if(!any(grepl("...", names(formals(myfunction))))) {
    formals(myfunction) <- c(formals(myfunction), unlist(alist(... = )))
}

HTH,
Ista

On Mon, Feb 17, 2014 at 4:22 PM, Jonathan Greenberg <jgrn at illinois.edu>
wrote:
> R-helpers:
>
> I'm guessing this is an easy one for some of you, but I'm a bit
> stumped.  Given some arbitrary function (doesn't matter what it does):
>
> myfunction <- function(a,b,c)
> {
> return(a+b+c)
> }
>
> I want to test this function for the presence of the ellipses
("...")
> and, if they are missing, create a new function that has them:
>
> myfunction <- function(a,b,c,...)
> {
> return(a+b+c)
> }
>
> So, 1) how do I test for whether a function has an ellipses argument
> and, 2) how do I "append" the ellipses to the argument list if
they do
> exist?
>
> Note that the test/modification should be done without invoking the
> function, e.g. I'm not asking how to test for this WITHIN the
> function, I'm asking how to test "myfunction" directly as an
R object.
>
> Thanks!
>
> --j
>
>
> --
> Jonathan A. Greenberg, PhD
> Assistant Professor
> Global Environmental Analysis and Remote Sensing (GEARS) Laboratory
> Department of Geography and Geographic Information Science
> University of Illinois at Urbana-Champaign
> 259 Computing Applications Building, MC-150
> 605 East Springfield Avenue
> Champaign, IL  61820-6371
> Phone: 217-300-1924
> http://www.geog.illinois.edu/~jgrn/
> AIM: jgrn307, MSN: jgrn307 at hotmail.com, Gchat: jgrn307, Skype: jgrn3007
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.