extract function "[" and empty index

numeric(0)[]

numeric(0)[TRUE]

-----Original Message-----
------------------------------

Message: 7
Date: Mon, 10 Mar 2008 02:29:32 +0800
From: "Laurent Gautier" <lgautier at gmail.com>
Subject: Re: [Rd] extract function "[" and empty index
To: "Gabor Grothendieck" <ggrothendieck at gmail.com>
Cc: r-devel at r-project.org
Message-ID:
	<27d1e6020803091129h19bbf3ffu1265ecd04b35993 at mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

Thanks, I was forgetting the recycling rule.


L.


2008/3/9, Gabor Grothendieck <ggrothendieck at gmail.com>:

Use TRUE.


 On Sun, Mar 9, 2008 at 5:05 AM, Laurent Gautier 

<lgautier at gmail.com> wrote:

 > Dear list,
 >
 > I am having a question regarding the extract function "[".
 >
 > The man page says that one usage with k-dimensional arrays is to
 > specify k indices to "[", with an empty index indicating that all
 > entries in that dimension are selected.
 >
 > The question is the following: is there an R object 

qualifying as an

 > "empty index" ? I understand that the lazy evaluation of 

parameters

 > allows one to
 > have genuinely missing parameters, but I would like to 

have an object

 > instead. I understand that one can always have an 

if/else workaround,

 > but I thought should ask, just in case.
 >
 > I tried with NULL but with little success, as it appears 

to give the

 > same results
 > as an empty vector.

 > > m = matrix(1, 2, 2)
 > > m[1, NULL]

 > numeric(0)

 > > m[1, integer(0)]

 > numeric(0)
 >
 > Since I was at it, I noted that the result obtained with 

"numeric(0)"

 > definitely makes sense but could as well be seen as 

challenging the

 > concept of an empty index presented in the man page (One 

could somehow

 > expect the presence of an object  meaning "everything" 

rather than

 > "missingness" meaning it).
 >
 >
 > Thanks,
 >
 >
 > Laurent
 >

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