On 10/24/2006 12:58 PM, Paul Gilbert wrote:
> (I'm not sure if this is a request for a feature, or another instance
> where a feature has eluded me for many years.)
>
> Often I have a function which calls other functions, and may often use
> the default arguments to those functions, but needs the capability to
> pass along non-default choices. I usually do this with some variation on
>
> foo <- function(x, foo2Args=NULL or a list(foo2defaults),
> foo3Args=NULL or a list(foo3defaults))
>
> and then have logic to check for NULL, or use the list in combination
> with do.call. It is also possible to do this with ..., but it always
> seems a bit dangerous passing all the unnamed arguments along to all the
> functions being called, especially when I always seem to be calling
> functions that have similar arguments (maxit, eps, start, frequency, etc).
>
> It is a situation I have learned to live with, but one of my
> co-maintainers just pointed out to me that there should be a good way to
> do this in R. Perhaps there is something else I have missed all these
> years? Is there a way to do this cleanly? It would be nice to have
> something like
>
> foo <- function(x, foo2Args=as.missing(), foo3Args=as.missing())
>
> then the call to foo2 and foo3 could specify foo2Args and foo3Args, but
> these would get treated as if they were missing, unless they are given
> other values.
I was going to say I couldn't see the difference between this and just
declaring
foo <- function(x, foo2Args, foo3Args)
with no defaults. However, this little demo illustrates the point, I think:
> g <- function(gnodef, gdef=1) {
+ if (missing(gnodef)) cat('gnodef is missing\n')
+ if (missing(gdef)) cat('gdef is missing\n')
+ cat('gdef is ',gdef,'\n')
+ }
>
> f <- function(fnodef, fdef) {
+ g(fnodef, fdef)
+ }
>
> g()
gnodef is missing
gdef is missing
gdef is 1
> f()
gnodef is missing
gdef is missing
Error in cat("gdef is ", gdef, "\n") : argument "fdef" is missing, with
no default
What would be nice to be able to do is to have a simple way for f() to
act just like g() does.
Duncan Murdoch
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