extending sp classes

On Fri, 6 Oct 2006, Katarzyna Adamczyk wrote:

Dear members of r-sig-geo,

I discovered "sp" library and I found
it very usefull  for switching
between different classes of R spatial
libraries.

I  defined my own class, extending
SpatialPolygonsDataFrame.
When I try to apply "spplot" function to
the extended class, it doesn't work :
I have a coercion problem error message,
but I don't know what it means.

I proceeded as follows:

I considered SpatialPolygonsDataFrame help example :

grd <- GridTopology(c(1,1), c(1,1), c(10,10))
polys <- as.SpatialPolygons.GridTopology(grd)
centroids <- getSpPPolygonsLabptSlots(polys)
x <- centroids[,1]
y <- centroids[,2]
z <- 1.4 + 0.1*x + 0.2*y + 0.002*x*x
ex_1.7 <- SpatialPolygonsDataFrame(polys, data=data.frame(x=x, y=y, z=z, row.names=getSpPPolygonsIDSlots(polys)))
spplot(ex_1.7)


I defined my own class :

 > setClass("myclass",contains="SpatialPolygonsDataFrame")

[1] "myclass"

 > ex<-ex_1.7
 > class(ex) = "myclass"
 > is(ex)

[1] "myclass"                  "SpatialPolygonsDataFrame"
[3] "SpatialPolygons"          "Spatial"

 > spplot(ex)

Erreur dans as.data.frame.default(obj) : cannot coerce class "myclass" 
into a data.frame

Did anyone of you encounter a similar problem and can help me?

Well, writing new-style classes is quite advanced. If you are not on 
Windows, please download the source package, and try editing line 158 of 
R/spplot.R from:

	sdf = as.data.frame(obj)

to

	sdf = as(obj, "data.frame")

to see if that way of doing method dispatch works:

t1 <- as.data.frame(ex)

Error in as.data.frame.default(ex) : cannot coerce class "myclass" into a data.frame

t2 <- as(ex, "data.frame")
class(t2)

[1] "data.frame"

suggests that it might. I guess that "myclass" actually is richer, please 
let us know how you get on. 

If you are on Windows, you will need to write a small wrapper function 
like:

as.data.frame.myclass <- function(obj) obj at data
t1 <- as.data.frame(ex)
class(t1)

[1] "data.frame"

This doesn't yet work, but you get the idea - provide an old-style method 
to match the one that cannot be found.

Roger

Thank you in advance.

Katarzyna