[R] take precisely one named argument

[Robin Hankin]

Hi

I want a function that takes precisely one named argument and no
unnamed arguments. The named argument must be one of "a" or "b".

If "a" is supplied, return "a".  If "b" is supplied, return 2*b.
That is, the desired behaviour is:

R> f(a=4)   #return 4
R> f(b=33)  #return 66
R> f(5)      #error
R> f(a=3,b=5)   #error
R> f(a=3,q=3)   #error
R> f(q=3)       #error

The following function is intended to implement this:

f <- function(a=NULL, b=NULL){
   if(!xor(is.null(a), is.null(b))){stop("specify exactly one of a and 
b")}
   if(is.null(a)){return(2*b)}else{return(a)}
}


It almost works, but  f(6) returns 6 (and should be an error).

What is the best way to accomplish my desired behaviour?

--
Robin Hankin
Uncertainty Analyst
Southampton Oceanography Centre
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743