[R] Subset with missing argument within a function

[William Dunlap]

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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