R help - Jul 2007 - getting the name of variables passed to a function

Folks,

I've entered into an R programming territory I'm not very familiar with,
thus this probably very elementary question concerning the mechanic of a
function call.

I want to know from within a function the name of the variables I pass down. The
function makes use of the "..." to allow for multiple unknown
arguments,

myfun = function(...) { do something }

In the body I put,

{    
nm <- names(list(...))
nm
}

When the function is called with two vectors x, and y

myfun(x, y)

It returns NULL. However, when the call made is,
>myfun(x=x, y=y)
The result is
[1] "x" "y"

Question : how do i get the names of the unknown variables without explicitly
saying x=x...

Thanks in advance.

Horace

On Fri, 2007-07-27 at 09:52 -0700, Horace Tso wrote:
> Folks,
> 
> I've entered into an R programming territory I'm not very familiar
> with, thus this probably very elementary question concerning the
> mechanic of a function call.
> 
> I want to know from within a function the name of the variables I pass
> down. The function makes use of the "..." to allow for multiple
> unknown arguments,
> 
> myfun = function(...) { do something }
> 
> In the body I put,
> 
> {    
> nm <- names(list(...))
> nm
> }
> 
> When the function is called with two vectors x, and y
> 
> myfun(x, y)
> 
> It returns NULL. However, when the call made is,
> 
> >myfun(x=x, y=y)
> 
> The result is
> [1] "x" "y"
> 
> Question : how do i get the names of the unknown variables without
> explicitly saying x=x...
> 
> Thanks in advance.
> 
> Horace

See ?match.call and take note of the 'expand.dots' argument, which
defaults to TRUE.

  DotsFun <- function(...) as.character(match.call())[-1]
  
  x <- 1:10
  y <- 5:6

  > DotsFun(x, y)
  [1] "x" "y"

match.call() returns the full function call. In the above, we take that
result, coerce it to a character vector and remove the first element,
which is the function being called, thus leaving the arguments.

HTH,

Marc Schwartz

I would start from match.call(expand.dots=TRUE) which has done the hard 
work for you.

On Fri, 27 Jul 2007, Horace Tso wrote:
> Folks,
>
> I've entered into an R programming territory I'm not very familiar
with,
> thus this probably very elementary question concerning the mechanic of a 
> function call.
>
> I want to know from within a function the name of the variables I pass 
> down. The function makes use of the "..." to allow for multiple
unknown
> arguments,
>
> myfun = function(...) { do something }
>
> In the body I put,
>
> {
> nm <- names(list(...))
> nm
> }
>
> When the function is called with two vectors x, and y
>
> myfun(x, y)
>
> It returns NULL. However, when the call made is,
>
>> myfun(x=x, y=y)
>
> The result is
> [1] "x" "y"
>
> Question : how do i get the names of the unknown variables without 
> explicitly saying x=x...
>
> Thanks in advance.
>
> Horace
>
> ______________________________________________
> R-help at stat.math.ethz.ch 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.
>
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

Here's one possibility:

R> f <- function(...) { call <- match.call(); sapply(as.list(call[-1]),
deparse) }
R> f(x, y)
[1] "x" "y"
R> f(x=x, y=y)
  x   y 
"x" "y" 

You basically need to know how to manipulate call objects.  The relevant
section in the R Language Definition should help.

Andy

 
From: Horace Tso
> 
> Folks,
> 
> I've entered into an R programming territory I'm not very 
> familiar with, thus this probably very elementary question 
> concerning the mechanic of a function call.
> 
> I want to know from within a function the name of the 
> variables I pass down. The function makes use of the "..." to 
> allow for multiple unknown arguments,
> 
> myfun = function(...) { do something }
> 
> In the body I put,
> 
> {    
> nm <- names(list(...))
> nm
> }
> 
> When the function is called with two vectors x, and y
> 
> myfun(x, y)
> 
> It returns NULL. However, when the call made is,
> 
> >myfun(x=x, y=y)
> 
> The result is
> [1] "x" "y"
> 
> Question : how do i get the names of the unknown variables 
> without explicitly saying x=x...
> 
> Thanks in advance.
> 
> Horace
> 
> ______________________________________________
> R-help at stat.math.ethz.ch 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.
> 
> 
> 

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