[R] Subset with missing argument within a function

Stefano de Pretis ste.depo.bio at gmail.com
Thu Feb 4 16:47:33 CET 2016


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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