From rasche at molgen.mpg.de  Tue Dec 13 19:09:23 2005
From: rasche at molgen.mpg.de (rasche@molgen.mpg.de)
Date: Tue, 13 Dec 2005 19:09:23 +0100 (CET)
Subject: [Rd] 0/1 vector for indexing leads to funny behaviour (PR#8389)
Message-ID: <20051213180923.C804CA21B@slim.kubism.ku.dk>

Hi,

OK, no bug. I got stuck into to much indexing. Now I understand this 
point definitely better.

Sorry for disturbing you,
Axel

Tony Plate wrote:
> Yes, 0/1 (numeric) are intended to be used as index vectors -- and they 
> have the semantics of numeric indices, which is that 0 elements in the 
> index are omitted from the result.  This can be a very useful mode of 
> operation in many situations.
> 
> I was going to write "This is described in both the introduction to R, 
> and in the documentation for '['", except that I checked before I wrote 
> and was surprised to be unable to any discussion of zeros in indexing in 
> any of the first three places I looked:
> 
> (1) help page for '[' (There is discussion of zero indices here, but 
> only in the context of using matrices to index matrices, not in the 
> context of ordinary vector indices).
> 
> (2) Section 2.7 "Index vectors: selecting and modifying subsets of a 
> data set" in "An Introduction to R", which does say this about numeric 
> indices:
>     2. A vector of positive integral quantities. In
>        this case the values in the index vector must
>        lie in the set {1, 2, . . . , length(x)}
> (This seems to commit the sin of not telling the whole truth.)
> 
> (3) Section 5.5 "Array Indexing.  Subsections of an array" (In "An 
> Introduction to R")
> 
> Question for others: did I miss something obvious, or is this a 
> documentation deficiency that zeros in indices are not discussed in 3 of 
> some obvious first places to look?
> 
> If indeed this is a documentation deficiency, I'm happy to contribute 
> documentation patch, but I await other opinions before spending any time 
> on that.
> 
> -- Tony Plate
> 
> rasche at molgen.mpg.de wrote:
> 
>> Full_Name: Axel Rasche
>> Version: 2.2.0
>> OS: Linux
>> Submission from: (NULL) (141.14.21.81)
>>
>>
>> Dear Debuggers,
>>
>> This is not a serious problem. Are 0/1 vectors intended to be used as 
>> index
>> vectors? If yes, there is a bug. If not, it leads just to some funny 
>> behaviour
>> rather than an error message.
>>
>> In the appendix is some simple code to reproduce the problem. A 
>> logical vector
>> as.logic(a) helps by indexing the vector b. The 0/1 vector a just 
>> returns the
>> first value "a". But as many times as there is a 1 in a.
>>
>> Best regards,
>> Axel
>>
>>
>> Appendix:
>>
>> b = c("a","b","c","d")
>> a = c(0,1,1,0)
>> b[as.logical(a)]
>> b[a]
>> a = c(1,0,1,0)
>> b[as.logical(a)]
>> b[a]
>> a = c(0,1,1,1)
>> b[as.logical(a)]
>> b[a]
>>
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>
> 

-- 
*******************************************
Dipl. Math. ETH Axel Rasche
Max-Planck-Institute for Molecular Genetics
Department Lehrach (Vertebrate Genomics)
Ihnestrasse 63-73
D-14195 Berlin-Dahlem
GERMANY

Tel. ++49-30-8413-1289
Fax  ++49-30-8413-1380



[Rd] 0/1 vector for indexing leads to funny behaviour (PR#8389)
rasche@molgen.mpg.de