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

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.

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"

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

--
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/