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

[Youyi Fong]

Great. missing is what I was looking for, and it seems to also work
with interfaces like

f=function( type=c("A","B,"C"), ... ) {
}

Thanks!
Youyi



On Wed, Aug 19, 2015 at 9:35 AM, Bert Gunter <bgunter.4567 at gmail.com> wrote:
> ... 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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>>
>>         [[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.