Re: [Rd] 0/1 vector for indexing leads to funny behaviour (PR#8389)

From: <rasche_at_molgen.mpg.de>
Date: Tue 13 Dec 2005 - 18:09:23 GMT


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@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@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

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Received on Wed Dec 14 07:06:50 2005

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