[R] Passing (Optional) Arguments

[Duncan Murdoch]

Jason Q. McClintic wrote:
> Dear List:
>
> In short, I am writing a number of functions as building blocks for 
> other functions and have some questions about scoping and passing arguments.
>
> Suppose I have functions foo1, foo2, and foo3 such that:
>
> 	foo1<-function(a=1,b=TRUE,c=FALSE){#do stuff};
> 	foo2<-function(x=1,y=FALSE,z=c(1,2,3,4)){#do stuff};
> 	foo3<-function(lambda,...){lambda*foo1()*foo2()};
>
> I want to be able to pass a,b,c,x,y,z to the functions foo1 and foo2 
> though foo3 (whether I define default values or not). How do I do this?
> I read a bit in the wiki about problems with partial argument matching 
> and argument matching (lesson: make argument names not match truncated 
> versions of other argument names in the "target" function).
>   
That's tricky, because ... becomes one big list.  You have a few choices:

foo1 and foo2 can gain a ... arg, and ignore it.  Then you pass ... to 
both of them.  Problem:  you can pass typos to them and won't get a 
complaint.

foo3 can gain a,b,c,x,y,z args, and pass those along to foo1 and foo2.  
Problem:  duplication, maintenance problems, etc.

You can use list(...) in foo3, and manually split the args to those that 
belong in foo1 and those that belong in foo2, and then construct calls 
from them.  (This allows you to recognize args that don't go to either 
place, and signal errors.)

You can pass foo1Args and foo2Args as lists to foo3, and construct calls 
from those, e.g.

foo3 <- function(lambda, foo1Args, foo2Args)
  lambda*do.call(foo1, foo1Args)*do.call(foo2,foo2Args)

There are other options (e.g. using local versions of foo1 and foo2 
within foo3) that  are variations on the ones above.

It's hard to give good advice on this, because whatever you do will be a 
tradeoff.  You'll see variations on all of the above in the base packages.

Duncan Murdoch
> To get a better feel for things I've been playing with examples such as:
>
> 	b<-c(0.25,0.25);
> 	fun<-function(a=1,...){a*b};
>
> fun() returns 0.25 0.25 as expected.
> fun(a=2) returns 0.5 0.5 as expected.
> However, fun(b=1) returns 0.25 0.25 when I want to overwrite b with the 
> value 1 and have it return 1.
>
> Likewise with
>
> 	fun<-function(a=1,...){a*return(b)};
>
> any argument I supply for b seems to be ignored.
>
> I understand as b is not defined within the function when I enter
>
> 	fun()
>
> lexical scoping means R looks for b up one level and, finding b, uses it.
>
> Thanks for any/all help.
>
> Sincerely,
>
> Jason Q. McClintic
> --
> Jason Q McClintic
> UST MB 1945
> 2115 Summit Avenue
> St. Paul, MN 55105
> jqmcclintic at stthomas.edu
> mccl0219 at tc.umn.edu
>
> "It is insufficient to protect ourselves with laws, we must protect 
> ourselves with mathematics."--Bruce Schneier
>
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