C API for sp classes?

Wouter, if you want to use the .C interface, you need to pass
the maps as simple vectors. I would pass one with the map
values, and one with the topology. Below is some example
code how to get them. An R function should be used to do
the coercion to double arrays, and wrap the .C call.
Is the overhead of learning and using the .Call() interface too great? As 
Edzer says, since the AttributeList can contain multiple vectors of 
possibly different types, it may be helpful to get a better grip on them 
that way. The geometry slots are more complicated, and would need more 
thought, though, but would be needed to make a raster, as Edzer shows.

Interesting issue. What would be returned? I guess in fact that a .C() 
interface with a more complicated R function to pick everything apart and 
check types is the way to go for prototyping. What would you use as 
baseline for comparison to see if your output is plausible?

Roger
Please note that a SpatialGridDataFrame can actually hold
more than one attribute (map).
--
Edzer

For getting the values:

 > library(sp)
 > data(meuse.grid)
 > gridded(meuse.grid)=~x+y
 > fullgrid(meuse.grid)=T
 > meuse.grid at data@att[[1]] # first attribute
 [1] NA NA NA NA NA NA NA NA NA NA ...
 >
Getting the topology:

 > meuse.grid at grid
                       x      y
cellcentre.offset 178460 329620
cellsize              40     40
cells.dim             78    104
 > class(meuse.grid at grid)
[1] "GridTopology"
attr(,"package")
[1] "sp"
 > unlist(gridtopology(meuse.grid))
Error in unlist(gridtopology(meuse.grid)) :
        couldn't find function "gridtopology"
 > unlist(gridparameters(meuse.grid))
cellcentre.offset1 cellcentre.offset2          cellsize1          cellsize2
            178460             329620                 40                 40
        cells.dim1         cells.dim2
                78                104
 > as.double(unlist(gridparameters(meuse.grid)))
[1] 178460 329620     40     40     78    104
 >

Wouter Buytaert wrote:

Hi,

I want to write some R functions for calculating hydrological 
catchment characteristics from a DEM (topographic index, overland flow 
delay function etc.).

Looks like using a DEM with SpatialGridDataFrame class as an input is a 
good idea.

Then what is the best way to extract the data matrix from this class to 
pass it on to a .C() function which does the actual calculations?

cheers,

wouter

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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

C API for sp classes?

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