[Rd] [.raster bug {was "str() on raster objects fails .."}

[Martin Maechler]

On Wed, Feb 2, 2011 at 02:16, Paul Murrell <p.murrell at auckland.ac.nz> wrote:
> Hi
>
> On 2/02/2011 2:03 p.m., Henrik Bengtsson wrote:
>>
>> On Tue, Feb 1, 2011 at 4:46 PM, Paul Murrell<p.murrell at auckland.ac.nz>
>>  wrote:
>>>
>>> Hi
>>>
>>> On 1/02/2011 9:22 p.m., Martin Maechler wrote:
>>>>>>>>>
>>>>>>>>> Henrik Bengtsson<hb at biostat.ucsf.edu>
>>>>>>>>>     on Mon, 31 Jan 2011 11:16:59 -0800 writes:
>>>>
>>>>     >    Hi, str() on raster objects fails for certain dimensions.  For
>>>>     >    example:
>>>>
>>>>     >>    str(as.raster(0, nrow=1, ncol=100)) 'raster' chr [1, 1:100]
>>>>     >    "#000000" "#000000" "#000000" "#000000" ...
>>>>
>>>>     >>    str(as.raster(0, nrow=1, ncol=101)) Error in
>>>> `[.raster`(object,
>>>>     >    seq_len(max.len)) : subscript out of bounds
>>>>
>>>>     >    This seems to do with how str() and "[.raster"() is coded; when
>>>>     >    subsetting as a vector, which str() relies on, "[.raster"()
>>>>     >    still returns a matrix-like object, e.g.
>>>>
>>>>     >>    img<- as.raster(1:25, max=25, nrow=5, ncol=5);
>>>>     >>    img[1:2]
>>>>     >    [,1]      [,2]      [,3]      [,4]      [,5]
>>>>     >    [1,] "#0A0A0A" "#3D3D3D" "#707070" "#A3A3A3" "#D6D6D6"
>>>>     >    [2,] "#141414" "#474747" "#7A7A7A" "#ADADAD" "#E0E0E0"
>>>>
>>>>     >    compare with:
>>>>
>>>>     >>    as.matrix(img)[1:2]
>>>>     >    [1] "#0A0A0A" "#3D3D3D"
>>>>
>>>>
>>>>     >    The easy but incomplete fix is to do:
>>>>
>>>>     >    str.raster<- function(object, ...) {
>>>>     >    str(as.matrix(object), ...);
>>>>     >    }
>>>>
>>>>     >    Other suggestions?
>>>>
>>>> The informal "raster" class is behaving ``illogical''
>>>> in the following sense:
>>>>
>>>>  >    r<- as.raster(0, nrow=1, ncol=11)
>>>>  >    r[seq_along(r)]
>>>>  Error in `[.raster`(r, seq_along(r)) : subscript out of bounds
>>>>
>>>> or, here equivalently,
>>>>  >    r[1:length(r)]
>>>>  Error in `[.raster`(r, 1:length(r)) : subscript out of bounds
>>>>
>>>> When classes do behave in such a way, they definitely need their
>>>> own str() method.
>>>>
>>>> However, the bug really is in "[.raster":
>>>> Currently,  r[i] is equivalent to  r[i,]  which is not at all
>>>> matrix-like and its help clearly says that subsetting should
>>>> work as for matrices.
>>>> A recent thread on R-help/R-devel has mentioned the fact that
>>>> "[" methods for matrix-like methods need to use both nargs() and
>>>> missing() and that "[.dataframe" has been the example to follow
>>>> "forever", IIRC already in S and S-plus as of 20 years ago.
>>>
>>> The main motivation for non-standard behaviour here is to make sure that
>>> a
>>> subset of a raster object NEVER produces a vector (because the conversion
>>> back to a raster object then produces a single-column raster and that may
>>> be
>>> a "surprise").  Thanks for making the code more standard and robust.
>>>
>>> The r[i] case is still tricky.  The following behaviour is quite
>>> convenient
>>> ...
>>>
>>> r[r == "black"]<- "white"
>>>
>>> ... but the next behaviour is quite jarring (at least in terms of the
>>> raster
>>> image that results from it) ...
>>>
>>> r2<- r[1:(nrow(r) + 1)]
>>>
>>> So I think there is some justification for further non-standardness to
>>> try
>>> to ensure that the subset of a raster image always produces a sensible
>>> image.  A simple solution would be just to outlaw r[i] for raster objects
>>> and force the user to write r[i, ] or r[, j], depending on what they
>>> want.
>>
>> FYI, I've tried out Martin's updated version at it seems like a
>> one-column raster matrix is now returned for r[i], e.g.
>
> Yes, that's what I've been looking at ...
>
>>> r<- as.raster(1:8, max=8, nrow=2, ncol=4);
>>> r
>>
>>      [,1]      [,2]      [,3]      [,4]
>> [1,] "#202020" "#606060" "#9F9F9F" "#DFDFDF"
>> [2,] "#404040" "#808080" "#BFBFBF" "#FFFFFF"
>>
>>> r[1:length(r)]
>>
>>      [,1]
>> [1,] "#202020"
>> [2,] "#404040"
>> [3,] "#606060"
>> [4,] "#808080"
>> [5,] "#9F9F9F"
>> [6,] "#BFBFBF"
>> [7,] "#DFDFDF"
>> [8,] "#FFFFFF"
>
> ... and the above is exactly the sort of thing that will fry your mind if
> the image that you are subsetting is, for example, a photo.
>
> Paul

I agree.  But the  r[i]  case  (not subassignment, but subsetting) case
has been in the code, and that's why I left it .. but changed it to
behave sensibly in the sense of "compatible" to matrix....
... and with the consequence that   r[1:length(r)] does not give an
error ... and in this sense behaves more regular.
Yet another alternative might be to only raise a warning and perhaps
return a (character) vector as that is very compatible to matrix
behavior.
BTW, you haven't mentioned the  r[xy] indexing where  xy is a
two-column matrix of indices.
There it's even more non-sense to return

>
>>> r[1:5,drop=TRUE]
>>
>>      [,1]
>> [1,] "#202020"
>> [2,] "#404040"
>> [3,] "#606060"
>> [4,] "#808080"
>> [5,] "#9F9F9F"
>> Warning message:
>> In `[.raster`(r, 1:5, drop = TRUE) :
>>   'drop' is always implicitly FALSE in '[.raster'
>>
>> Also,
>>
>>> r[1:5]<- "white"
>>> r
>>
>>      [,1]    [,2]    [,3]      [,4]
>> [1,] "white" "white" "white"   "#DFDFDF"
>> [2,] "white" "white" "#BFBFBF" "#FFFFFF"
>>
>> /Henrik
>>
>>>
>>> Paul
>>>
>>>> Thank you, Henrik, for the bug report.
>>>> Martin
>>>>
>>>> ______________________________________________
>>>> R-devel at r-project.org mailing list
>>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>
>>> --
>>> Dr Paul Murrell
>>> Department of Statistics
>>> The University of Auckland
>>> Private Bag 92019
>>> Auckland
>>> New Zealand
>>> 64 9 3737599 x85392
>>> paul at stat.auckland.ac.nz
>>> http://www.stat.auckland.ac.nz/~paul/
>>>
>>> ______________________________________________
>>> R-devel at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>
>
> --
> Dr Paul Murrell
> Department of Statistics
> The University of Auckland
> Private Bag 92019
> Auckland
> New Zealand
> 64 9 3737599 x85392
> paul at stat.auckland.ac.nz
> http://www.stat.auckland.ac.nz/~paul/
>
>