[R] Subset with missing argument within a function

Fri Feb 5 16:39:32 CET 2016

R's subscripting operators do not "guess" the value of a missing
argument: a missing k'th subscript means seq_len(dim(x)[k]).
I bet that you use syntax like x[,1] (the entire first column of x)
all the time and that you don't want this syntax to go away.

Some languages use a placeholder like '.' or '*' to do this.  Perhaps
S should have, but it is now late to make such a change.

Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Thu, Feb 4, 2016 at 11:23 PM, Stefano de Pretis <ste.depo.bio at gmail.com>
wrote:

> 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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>>
>>
>>
>

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