[R] take precisely one named argument

[BXC (Bendix Carstensen)]

specify:

f <- function(...,a=NULL,b=NULL) {...etc

Bendix
----------------------
Bendix Carstensen
Senior Statistician
Steno Diabetes Center
Niels Steensens Vej 2
DK-2820 Gentofte
Denmark
tel: +45 44 43 87 38
mob: +45 30 75 87 38
fax: +45 44 43 07 06
bxc at steno.dk
www.biostat.ku.dk/~bxc
----------------------



> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Robin Hankin
> Sent: Friday, December 17, 2004 1:18 PM
> To: R-help at stat.math.ethz.ch
> Subject: [R] take precisely one named argument
> 
> 
> 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
> 
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