[R] Checking for and adding "..." arguments to a function...

[David Winsemius]

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
> 
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David Winsemius
Alameda, CA, USA