[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
__
[EMAIL PROTECTED] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
RE: [R] take precisely one named argument
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
__
[EMAIL PROTECTED] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
__
[EMAIL PROTECTED] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
--
Notice: This e-mail message, together with any attachments,
contains information of Merck Co., Inc. (One Merck Drive,
Whitehouse Station, New Jersey, USA 08889), and/or its
affiliates (which may be known outside the United States as
Merck Frosst, Merck Sharp Dohme or MSD and in Japan, as
Banyu) that may be confidential, proprietary copyrighted
and/or legally privileged. It is intended solely for the use
of the individual or entity named on this message. If you
are not the intended recipient, and have received this
message in error, please notify us immediately by reply
e-mail and then delete it from your system.
--
__
[EMAIL PROTECTED] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
RE: [R] take precisely one named argument
On 17-Dec-04 Ted Harding wrote:
I don't know the *best* way (expert R anatomists will know ... )
but the following dirty handed modification seems to do what
you want:
f - function(z=NULL, a=NULL, b=NULL){
if(!is.null(z)){
stop(usage: f(a=...) or f(b=...))
}
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)}
}
(This traps attempts to use f() with an un-named argument).
Ted.
Playing around with the above shows that not only does it
give the correct response for correct usage, e.g. f(a=3) or f(b=3),
but, serendipitously, it gives appropriate responses for just
about any way you could think of using it wrongly:
f(3)
Error in f(3) : usage: f(a=...) or f(b=...)
f(a=2,b=3)
Error in f(a = 2, b = 3) : specify exactly one of a and b
f(d=3)
Error in f(d = 3) : unused argument(s) (d ...)
f(a=2,d=3)
Error in f(a = 2, d = 3) : unused argument(s) (d ...)
etc.
Ted.
E-Mail: (Ted Harding) [EMAIL PROTECTED]
Fax-to-email: +44 (0)870 094 0861 [NB: New number!]
Date: 17-Dec-04 Time: 13:47:34
-- XFMail --
__
[EMAIL PROTECTED] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] take precisely one named argument
BXC (Bendix Carstensen) [EMAIL PROTECTED] writes:
specify:
f - function(...,a=NULL,b=NULL) {...etc
Or (variant of same)
f - function(..., a, b) {
if (nargs() != 1 || length(list(...)))
stop(precisely one of a=x or b=y must be given)
else
if (missing(a)) 2*b else a
}
--
O__ Peter Dalgaard Blegdamsvej 3
c/ /'_ --- Dept. of Biostatistics 2200 Cph. N
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907
__
[EMAIL PROTECTED] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
Re: [R] take precisely one named argument
Ted.Harding at nessie.mcc.ac.uk writes:
:
: On 17-Dec-04 Ted Harding wrote:
: I don't know the *best* way (expert R anatomists will know ... )
: but the following dirty handed modification seems to do what
: you want:
:
:f - function(z=NULL, a=NULL, b=NULL){
: if(!is.null(z)){
: stop(usage: f(a=...) or f(b=...))
: }
: 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)}
:}
:
: (This traps attempts to use f() with an un-named argument).
:
: Ted.
:
: Playing around with the above shows that not only does it
: give the correct response for correct usage, e.g. f(a=3) or f(b=3),
: but, serendipitously, it gives appropriate responses for just
: about any way you could think of using it wrongly:
:
: f(3)
: Error in f(3) : usage: f(a=...) or f(b=...)
:
: f(a=2,b=3)
: Error in f(a = 2, b = 3) : specify exactly one of a and b
:
: f(d=3)
: Error in f(d = 3) : unused argument(s) (d ...)
:
: f(a=2,d=3)
: Error in f(a = 2, d = 3) : unused argument(s) (d ...)
:
Here is a minor reduction of the above:
ff - function(z, a = 0, b = 0) {
stopifnot(missing(z), xor(missing(a), missing(b)))
a+2*b
}
__
[EMAIL PROTECTED] mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
