[Rd] rownames() problem with 0-extent arrays (PR#9136)

[Kjetil Brinchmann Halvorsen]

Raubertas, Richard wrote:
> Prof Ripley,
> Thanks for your reply.  I've inserted a couple of inline 
> comments below.
> 
> Rich Raubertas
> 
>> -----Original Message-----
>> From: Prof Brian Ripley [mailto:ripley at stats.ox.ac.uk] 
>> Sent: Wednesday, August 16, 2006 7:21 AM
>> To: Raubertas, Richard
>> Cc: r-devel at stat.math.ethz.ch
>> Subject: Re: [Rd] rownames() problem with 0-extent arrays (PR#9136)
>>
>>
>> R-bugs is not the place to ask questions, see the FAQ ....
> 
> Yes, I should have sent a separate message.
> 
>> On Fri, 11 Aug 2006, richard_raubertas at merck.com wrote:
>>
>> [...]
>>
>>> A separate but related question (not a bug) has to do with 
>> the handling
>>> of dimnames for 0-extent arrays.  If an array starts out 
>> with non-NULL
>>> dimnames and is subscripted down to have 0-extent, the 
>> corresponding 
>>> component is set to NULL, rather than character(0):
>> and that dimension does not have any names, so this is appropriate.
> 
> I think the question of whether a dimension has names or not 
> should be independent of the extent of that dimension.  (My reason 
> for raising the issue was that some code I was writing would 
> be cleaner if "dimension has names" and "extent of dimension" were 
> orthogonal concepts.)  Under that premise, there is a difference 
> between NULL (no names) and character(0) (names).  
> 
> It appears you don't agree with my premise, and I can see how a 
> discussion about naming things that don't exist could get rather 
> metaphysical. 

Yes. There has been quite a lot discussion about 'god' during history!

Kjetil

  So I have worked around the issue in my code.
> 
>>> dimnames(tmp[0,,drop=FALSE])   # first component is NULL
>>>
>>> Why is that?  Note that this is different from how data frames are 
>>> handled:
>>>
>>> tmp.df <- data.frame(tmp)
>>> dimnames(tmp.df[0,])  # first component is character(0)
>> Data frames do not have 'dimnames', and what is reported by 
>> the accessor 
>> function is list(row.names(x), names(x)), both of which are 
>> required to be 
>> character vectors.  Indeed, as the help file says
>>
>>   For the \code{"data.frame"} method both dimnames must be 
>> non-null, and
>>   the rownames must contain no duplicates nor missing values.
>>
>>> The description in ?dimnames seems to allow character(0) components 
>>> of dimnames, 
>>>
>>>      "The dimnames of a matrix or array can be 'NULL' or a 
>> list of the
>>>      same length as 'dim(x)'.  If a list, its components are either
>>>      'NULL' or a character vector the length of the appropriate
>>>      dimension of 'x'."
>> That is accurate.
>>
>>> yet the implementation seems to resist them:
>>>
>>> tmp2 <- tmp[0,,drop=FALSE]
>>> dimnames(tmp2) <- list(character(0), LETTERS[1:3])
>>> dimnames(tmp2)  #  still NULL
>>>
>>> So my question is whether it would be reasonable to change 
>> 'dimnames'
>>> and/or "dimnames<-" to use (or at least allow) character(0) 
>> rather than 
>>> NULL for array dimensions of 0 extent.
>> Why would it be desirable to have two equivalent 
>> representations of the 
>> same thing (no names)?  As you could easily have found out, 
>> the C code 
>> replaces a zero-length vector by NULL.
>>
>> -- 
>> 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
>>
>>
> 
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