Antonio,
You need to compare the names of list(...) with the arguments you wish to
check. Here's one way to do so (note that I replaced c with d, because c
is a function):
f <- function(a, ...) {
argnames <- names(list(...))
# check whether b is an argument
if(!("b" %in% argnames)) {
b <- 1
}
# check whether d is an argument
if(!("d" %in% argnames)) {
d <- 1
}
# return NA for b and d if specified, but don't set a value
list(a=a, b=ifelse(exists("b"), b, NA), d=ifelse(exists("d"), d, NA),
args=list(...))
}
> f(a=1)
$a
[1] 1
$b
[1] 1
$d
[1] 1
$args
list()
> f(a=1, b=2)
$a
[1] 1
$b
[1] NA
$d
[1] 1
$args
$args$b
[1] 2
> f(a=1, b=2, d=3)
$a
[1] 1
$b
[1] NA
$d
[1] NA
$args
$args$b
[1] 2
$args$d
[1] 3
Sarah
On Wed, Dec 1, 2010 at 2:34 PM, <antonio.gasparr...@lshtm.ac.uk> wrote:
> Dear R-users,
>
> I'm trying to work out a way to set default values for arguments in a
> function which includes the optional argument '...'.
> In practice, I have a 'plot' method for a function which specifies different
> types of plots. Every different plot should have different default arguments
> (for example for 'col', 'ylim' etc), plus the argument '...' to leave further
> optional choices to the user.
> My problem is that I need to specify a large number of arguments in the usage
> (losing clarity), or alternatively leaving to the user the specification of
> an high number of argument (adding complexity).
>
> An example.
> Suppose I want to create a function f with arguments 'a', and '...'.
> Two more arguments 'b' and 'c' should be both set to 1 by default if the user
> don't include them in '...'.
>
> f <- function(a,...) {
> # set b <- 1 if b is not included in '...'
> # set c <- 1 if c is not included in '...'
> somefunction(a,b,c,...)
> }
>
> I found that internally the optional arguments are stored in list(...), but I
> haven't found a way to manage them, so the final call to somefunction() will
> include 'b' and 'c' twice if the user have included them in '...', and
> returns an error.
>
> Does anyone have some suggestion?
> Thanks for your time
>
> Antonio Gasparrini
> London School of Hygiene & Tropical Medicine
> Department of Social and Environmental Health Research
> 15-17 Tavistock Place, London WC1H 9SH, UK
>
> _______
--
Sarah Goslee
http://www.functionaldiversity.org
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