Re: [R] empty array

From: Robin Hankin <r.hankin_at_noc.soton.ac.uk>
Date: Tue, 18 Mar 2008 09:31:20 +0000

Hello everyone

I know other, more knowledgeable, people have replied to Christophe's question, but perhaps the List would be interested to know that zero-extent arrays are useful (to me at least)   because although such an array has no content, the dimname are nevertheless retained:

 > a <- array(0,dim=c(0,3))
 > dimnames(a) <- list(size=c() , fish=c("cod","skate","crab"))
 > b <- array(0,dim=c(2,0))
 > dimnames(b) <- list(size=c("huge","small"),depth=c())
 >


We can attach these arrays ---both of which are of length 0---using adiag():

 > library(magic)
 > adiag(a,b)

        fish
size cod skate crab

   huge    0     0    0
   small   0     0    0

 >

Note how the dimnames of "a" and "b" are retained in the output. The contents of this array are the default "pad" value of adiag().

This is terribly useful in the humble workaday world of high-dimensional magic hypercubes.

rksh

On 15 Mar 2008, at 15:33, Christophe Genolini wrote:

> Hi the list
>
> Is it possible to create an empty matrix ? I do not mean an matrix
> with
> a single value that is NA (which is not empty) but a real empty one,
> with length=0.
>
> I do not understand why we have length(numeric()), length(factor())
> and
> length(character()) to zero, and length(array()) to one... Any rason
> for
> that ?
>
> Thanks
>
> Christophe
>
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--
Robin Hankin
Uncertainty Analyst and Neutral Theorist,
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743

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Received on Tue 18 Mar 2008 - 09:36:43 GMT

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