[R] Subset with missing argument within a function

Fri Feb 5 08:23:14 CET 2016

Thanks Bill,

This is more clear.

In any case, I find very inappropriate that a programming language tries to
guess the value of a missing argument. It is unfair towards code developers
and it promotes the production of bugged piece of software.

I hope R will revise its policies sooner or later.

Thanks for the discussion,

Stefano

2016-02-04 18:19 GMT+01:00 William Dunlap <wdunlap at tibco.com>:

> The "missingness" of an argument gets passed down through nested function
> calls.  E.g.,
>   fOuter <- function(x) c(outerMissing=missing(x), innerMissing=fInner(x))
>   fInner <- function(x) missing(x)
>   fInner()
>   #[1] TRUE
>   fOuter()
>   #outerMissing innerMissing
>   #      TRUE         TRUE
> It is only when a function evaluates an argument that you get a message
> like 'argument is missing, with no default'.  ('[' checks for missingness
> before
> evaluating a subscript argument so it will not give that error.)
>
>
> Bill Dunlap
> TIBCO Software
> wdunlap tibco.com
>
> On Thu, Feb 4, 2016 at 7:47 AM, Stefano de Pretis <ste.depo.bio at gmail.com>
> wrote:
>
>> Hi Petr,
>>
>> Thank you for your answer.
>>
>> I'm not sure how the empty index reflects what I'm showing in my example.
>> If my function was
>>
>> emptySubset <- function(vec) vec[]
>>
>> I would then agree that this was the case. But I think it's different: I'm
>> specifically telling my function that it should have two arguments ("vec"
>> and "ix")
>>
>> subsettingFun <- function(vec, ix) vec[ix]
>>
>> and I guess why, within the function, it does not happen what happens on
>> the command line:
>>
>> > ix
>> Error: object 'ix' not found
>> > letters[ix]
>> Error: object 'ix' not found
>>
>> My "expectation" came from a matter of coherence, but probably I'm still
>> missing something.
>>
>> Regards,
>>
>> Stefano
>>
>>
>>
>> 2016-02-04 15:39 GMT+01:00 PIKAL Petr <petr.pikal at precheza.cz>:
>>
>> > Hi
>> >
>> > Help page for ?"[" says
>> >
>> > An empty index selects all values: this is most often used to replace
>> all
>> > the entries but keep the attributes.
>> >
>> > and actually you function construction works with empty index
>> >
>> > > x<-c(1,2,5)
>> > > letters[x]
>> > [1] "a" "b" "e"
>> > > letters[]
>> >  [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q"
>> > "r" "s"
>> > [20] "t" "u" "v" "w" "x" "y" "z"
>> >
>> > It is sometimes useful not "expect" the program behavior but "inspect"
>> why
>> > it behaves differently.
>> >
>> > If you want your function to throw error when some arguments are missing
>> > you need to do the check yourself and not rely on programming language.
>> >
>> > And BTW I did not know an answer before I inspected docs.
>> >
>> > Cheers
>> > Petr
>> >
>> >
>> > > -----Original Message-----
>> > > From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of
>> Stefano
>> > > de Pretis
>> > > Sent: Thursday, February 04, 2016 11:00 AM
>> > > To: r-help at r-project.org
>> > > Subject: [R] Subset with missing argument within a function
>> > >
>> > > Hi all,
>> > >
>> > > I'm guessing what's the rationale behind this:
>> > >
>> > > > subsettingFun <- function(vec, ix) vec[ix]
>> > > > subsettingFun(letters, c(1,2,5))
>> > > [1] "a" "b" "e"
>> > > > subsettingFun(letters)
>> > >  [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p"
>> > > "q"
>> > > "r" "s"
>> > > [20] "t" "u" "v" "w" "x" "y" "z"
>> > >
>> > > If the argument "ix" is missing, I'm expecting an error not to return
>> > > the
>> > > variable "vec" as it is.
>> > >
>> > > I think this is VERY dangerous and does not help the development of
>> > > reliable code and the debugging.
>> > >
>> > > Cheers,
>> > >
>> > > Stefano
>> > >
>> > > *Center for Genomic Science of IIT at SEMM*
>> > >
>> > > Stefano de Pretis, PhD
>> > >
>> > > *Postdoctoral fellow *
>> > >
>> > >       [[alternative HTML version deleted]]
>> > >
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