[R-sig-Geo] extending sp classes

[Roger Bivand]

On Fri, 6 Oct 2006, Roger Bivand wrote:

> 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.

The missing bit, after providing an as.data.frame method for your class, 
was to either specify the zcol= argument:

spplot(ex, zcol=c("x", "y", "z"))

or provide a names() method for your class:

names.myclass <- function(x) names(x at data)
spplot(ex)

The next > 0.9-2 release of sp will remove the as.data.frame() problem, 
but a names() method will be needed if zcol= cannot be used.

Roger 

> 
> Roger
> 
> > 
> > Thank you in advance.
> > 
> > Katarzyna
> > 
> > _______________________________________________
> > R-sig-Geo mailing list
> > R-sig-Geo at stat.math.ethz.ch
> > https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> > 
> 
> 

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no