R devel - Dec 2005 - 0/1 vector for indexing leads to funny behaviour (PR#8389)

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]

rasche at molgen.mpg.de writes:
> 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.

Yes, that is completely as intended. Zeros in a numerical index vector
produce nothing and ones produce the first element. The documentation
could arguably be better on this point though.

 
> 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
> 
-- 
   O__  ---- Peter Dalgaard             ?ster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907

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
>

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

It is peculiar behaviour on your part, but the developers do not find it 
at all `funny'.

On Tue, 13 Dec 2005 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.
They are, and in your example they work as documented.  You have not even 
told us what you think the bug is.

What happens in clearly stated in the R Language Definition and in all 
good reference books on R.
> 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.
As it should.
>
> 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]

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595