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

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.

r <- as.raster(1:8, max=8, nrow=2, ncol=4);
r

r[1:length(r)]

r[1:5,drop=TRUE]

r[1:5] <- "white"
r

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/