[R] Subset with missing argument within a function

[Stefano de Pretis]

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]]
> >
> > ______________________________________________
> > 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.
>
> ________________________________
> Tento e-mail a jakékoliv k němu připojené dokumenty jsou důvěrné a jsou
> určeny pouze jeho adresátům.
> Jestliže jste obdržel(a) tento e-mail omylem, informujte laskavě
> neprodleně jeho odesílatele. Obsah tohoto emailu i s přílohami a jeho kopie
> vymažte ze svého systému.
> Nejste-li zamýšleným adresátem tohoto emailu, nejste oprávněni tento email
> jakkoliv užívat, rozšiřovat, kopírovat či zveřejňovat.
> Odesílatel e-mailu neodpovídá za eventuální škodu způsobenou modifikacemi
> či zpožděním přenosu e-mailu.
>
> V případě, že je tento e-mail součástí obchodního jednání:
> - vyhrazuje si odesílatel právo ukončit kdykoliv jednání o uzavření
> smlouvy, a to z jakéhokoliv důvodu i bez uvedení důvodu.
> - a obsahuje-li nabídku, je adresát oprávněn nabídku bezodkladně přijmout;
> Odesílatel tohoto e-mailu (nabídky) vylučuje přijetí nabídky ze strany
> příjemce s dodatkem či odchylkou.
> - trvá odesílatel na tom, že příslušná smlouva je uzavřena teprve
> výslovným dosažením shody na všech jejích náležitostech.
> - odesílatel tohoto emailu informuje, že není oprávněn uzavírat za
> společnost žádné smlouvy s výjimkou případů, kdy k tomu byl písemně zmocněn
> nebo písemně pověřen a takové pověření nebo plná moc byly adresátovi tohoto
> emailu případně osobě, kterou adresát zastupuje, předloženy nebo jejich
> existence je adresátovi či osobě jím zastoupené známá.
>
> This e-mail and any documents attached to it may be confidential and are
> intended only for its intended recipients.
> If you received this e-mail by mistake, please immediately inform its
> sender. Delete the contents of this e-mail with all attachments and its
> copies from your system.
> If you are not the intended recipient of this e-mail, you are not
> authorized to use, disseminate, copy or disclose this e-mail in any manner.
> The sender of this e-mail shall not be liable for any possible damage
> caused by modifications of the e-mail or by delay with transfer of the
> email.
>
> In case that this e-mail forms part of business dealings:
> - the sender reserves the right to end negotiations about entering into a
> contract in any time, for any reason, and without stating any reasoning.
> - if the e-mail contains an offer, the recipient is entitled to
> immediately accept such offer; The sender of this e-mail (offer) excludes
> any acceptance of the offer on the part of the recipient containing any
> amendment or variation.
> - the sender insists on that the respective contract is concluded only
> upon an express mutual agreement on all its aspects.
> - the sender of this e-mail informs that he/she is not authorized to enter
> into any contracts on behalf of the company except for cases in which
> he/she is expressly authorized to do so in writing, and such authorization
> or power of attorney is submitted to the recipient or the person
> represented by the recipient, or the existence of such authorization is
> known to the recipient of the person represented by the recipient.
>

	[[alternative HTML version deleted]]