[R] take precisely one named argument

[Liaw, Andy]

> From: Liaw, Andy
> Here's my attempt:
> 
> > f <- function(...) {
> +     argList <- list(...)
> +     if (length(argList) != 1) stop("Wrong number of arguments!")
> +     if (length(names(argList)) != 1 || ! names(argList) 
> %in% c("a", "b"))
> +         stop("Wrong name for argument!")
> +     x <- argList[[1]] * if (names(argList) == "a") 1 else 2
> +     x
> + }
> > f(a=6)
> Error in f(6) : Wrong name for argument!
> > f(b=6)
> [1] 6
> > f(c=6)
> [1] 12

This obviously _looks_ wrong, but that's only because I was running R under
ESS, and up-arrowed to edit the commands.  The correct transcript for this
part ought to look like:

> f(a=6)
[1] 6
> f(b=12)
[1] 24
> f(b=6)
[1] 12
> f(c=6)
Error in f(c = 6) : Wrong name for argument!

Andy

> > f(c=6)
> Error in f(c = 6) : Wrong name for argument!
> 
> HTH,
> Andy
> 
> 
> > From: 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
> > 
> > ______________________________________________
> > 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
> > 
> >
> 
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