[R] Randomly remove condition-selected rows from a matrix

[Duncan Murdoch]

On 02/01/2009 10:07 AM, Wacek Kusnierczyk wrote:
> Stavros Macrakis wrote:
>> On Wed, Dec 31, 2008 at 12:44 PM, Guillaume Chapron
>> <carnivorescience at gmail.com> wrote:
>>   
>>>> m[-sample(which(m[,1]<8 & m[,2]>12),2),]
>>>>       
>>> Supposing I sample only one row among the ones matching my criteria. Then
>>> consider the case where there is just one row matching this criteria. Sure,
>>> there is no need to sample, but the instruction would still be executed.
>>> Then if this row index is 15, my instruction becomes which(15,1), and this
>>> can gives me any row from 1 to 15, which is not correct. I have to make a
>>> condition in case there is only one row matching the criteria.
>>>     
>> Yes, this is a (documented!) design flaw in 'sample' -- see the man page.
>>
>> For some reason, the designers of R have chosen to document the flaw
>> and leave it up to individual users to work around it rather than fix
>> it definitively.  A related case is sample(c(),0), which gives an
>> error rather than giving an empty vector, though in general R deals
>> with empty vectors correctly (e.g. sum(c()) => 0).
>>
>>   
> 
> interestingly, ?sample says:
> 
> "
>      'sample' takes a sample of the specified size from the elements of
>      'x' using either with or without replacement.
> 
>        x: Either a (numeric, complex, character or logical) vector of
>           more than one element from which to choose, or a positive
>           integer.
> 
>     If 'x' has length 1, is numeric (in the sense of 'is.numeric') and
>      'x >= 1', sampling takes place from '1:x'.  _Note_ that this
>      convenience feature may lead to undesired behaviour when 'x' is of
>      varying length 'sample(x)'.  See the 'resample()' example below.
> 
> "
> 
> yet the following works, even though x has length 1 and is *not* numeric:
> 
> x = "foolme"
> is.numeric(x)
> sample(x, 1)
> sample(x)
> 
> x = NA
> is.numeric(NA)
> sample(x, 1)
> sample(x)
> 
> is this a bug in the code, or a bug in the documentation?
> 
> 
> 
>> To my mind, it is bizarre to have an important basic function which
>> works for some argument lengths but not others.  The convenience of
>> being able to write sample(5,2) for sample(1:5,2) hardly seems worth
>> inflicting inconsistency on all users -- but perhaps one of the
>> designers of R/S can enlighten us on the design rationale here.
>>
>>   
> 
> hopefully.

This is more of an R-devel sort of question.  My guess is that this is 
in the S blue book, but I don't have a copy here to check.

Duncan Murdoch