[R] match.arg: how to prevent users from not specifying a value

[Bert Gunter]

... 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
>>
>> ______________________________________________
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>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
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>
> ______________________________________________
> 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.