R help - Aug 2015 - match.arg: how to prevent users from not specifying a value

Hello, I have a function that looks like

f=function( type=c("dummy,"A","B,"C"), ... ) {
    type<-match.arg(type)
    if (type=="dummy") stop("Please choose a type that is not
dummy.")
    ...
}

I put a "dummy" in the list of choices as a mechanism to prevent users
from not specifying a value for "type" when calling the function. My
question is whether there is a better way to achieve it that does not
need "dummy".

Thanks,
Youyi

If you want to force the user to enter the 'type' argument,
move the vector of choices out of the argument list
and into the call to match.arg():

   f1 <- function(type, ...) {
       match.arg(type, c("A", "B", "C"))
   }
   f1()
   #Error in match.arg(type, c("A", "B", "C")) :
   #  argument "type" is missing, with no default
   f1("X")
   #Error in match.arg(type, c("A", "B", "C")) :
   #  'arg' should be one of ?A?, ?B?, ?C?
   f1("B")
   #[1] "B"


Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Tue, Aug 18, 2015 at 5:42 PM, Youyi Fong <yfong at fhcrc.org> wrote:
> Hello, I have a function that looks like
>
> f=function( type=c("dummy,"A","B,"C"), ... )
{
>     type<-match.arg(type)
>     if (type=="dummy") stop("Please choose a type that is
not dummy.")
>     ...
> }
>
> I put a "dummy" in the list of choices as a mechanism to prevent
users
> from not specifying a value for "type" when calling the function.
My
> question is whether there is a better way to achieve it that does not
> need "dummy".
>
> Thanks,
> Youyi
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
	[[alternative HTML version deleted]]

... and you could also use missing() (?missing for details) if you
wanted to give the user more verbose instructions, e.g.

   f1 <- function(type, ...) {
       if(missing(type)){
       cat("You must enter a 'type' argument that is one of
etc....\n")
       return(invisible())
       }
       match.arg(type, c("A", "B", "C"))
   }
Bert Gunter

"Data is not information. Information is not knowledge. And knowledge
is certainly not wisdom."
   -- Clifford Stoll


On Wed, Aug 19, 2015 at 9:19 AM, William Dunlap <wdunlap at tibco.com>
wrote:
> If you want to force the user to enter the 'type' argument,
> move the vector of choices out of the argument list
> and into the call to match.arg():
>
>    f1 <- function(type, ...) {
>        match.arg(type, c("A", "B", "C"))
>    }
>    f1()
>    #Error in match.arg(type, c("A", "B",
"C")) :
>    #  argument "type" is missing, with no default
>    f1("X")
>    #Error in match.arg(type, c("A", "B",
"C")) :
>    #  'arg' should be one of ?A?, ?B?, ?C?
>    f1("B")
>    #[1] "B"
>
>
> Bill Dunlap
> TIBCO Software
> wdunlap tibco.com
>
> On Tue, Aug 18, 2015 at 5:42 PM, Youyi Fong <yfong at fhcrc.org>
wrote:
>
>> Hello, I have a function that looks like
>>
>> f=function( type=c("dummy,"A","B,"C"),
... ) {
>>     type<-match.arg(type)
>>     if (type=="dummy") stop("Please choose a type that
is not dummy.")
>>     ...
>> }
>>
>> I put a "dummy" in the list of choices as a mechanism to
prevent users
>> from not specifying a value for "type" when calling the
function. My
>> question is whether there is a better way to achieve it that does not
>> need "dummy".
>>
>> Thanks,
>> Youyi
>>
>> ______________________________________________
>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.