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

On Wed, Feb 2, 2011 at 23:30, Simon Urbanek <simon.urbanek at r-project.org> wrote:

On Feb 1, 2011, at 8:16 PM, Paul Murrell 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.

Why doesn't raster behave consistently like any matrix object? I would expect simply

r[1:length(r)]

[1] "#202020" "#404040" "#606060" "#808080" "#9F9F9F" "#BFBFBF" "#DFDFDF"
[8] "#FFFFFF"

Where it's obvious what happened. I saw the comment about the vector but I'm not sure I get it - why don't you want a vector? The raster is no different than matrices - you still need to define the dimensions when going back anyway, moreover what you get now is not consistent at all since there raster never had that dimension anyway ...

Cheers,
Simon

I agree that this would be the most "logical" and notably least
surprising behavior,
which I find the most important argument
(I'm sorry my last message was cut off as it was sent accidentally
before being finished completely).

Martin

Paul

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