extracting simple list of adjacent neighbours

On 13/09/11 18:39, Roger Bivand wrote:

On Tue, 13 Sep 2011, Moritz Lennert wrote:

On 06/09/11 21:41, Roger Bivand wrote:

On Tue, 6 Sep 2011, Moritz Lennert wrote:

Hello,

I have a very simple problem, which I know will have a simple
solution, but I just can't get my head wrapped around it.

I have a shapefile with regions, including an attribut containing a
regional id.

I would like to extract a simple neighbourhood file, with
neighbourhood defined by simple adjacency with symmetry of
relationship in both directions.

Ideally, I would like to get something like this:

id_region_1 id_neighbour_1
id_region_1 id_neighbour_2
id_region_1 id_neighbour_3
id_region_2 id_neighbour_1
id_region_2 id_neighbour_2
id_region_3 id_neighbour_1
etc

What I've done so far:

originalmap<-readShapePoly("/path/to/shapefile.shp")
nb<-poly2nb(originalmap, row.names=originalmap$idcolumn)

Roughly:

lw <- nb2listw(nb, style="B")
sn <- listw2sn(lw)

then the first two columns of the "spatial.neighbour" object sn (used in
the S-PLUS SpatialStats module) are what you need. They can also be
exported for use in Matlab.

This does give me the right format, but I don't get the ids in
originalmap$idcolumn but rather the internal ids of the polygons.

example(columbus)
nb <- poly2nb(columbus, row.names=as.character(columbus$NEIGNO))
lw <- nb2listw(nb, style="B")
sn <- listw2sn(lw)
head(sn)
sn1 <- cbind(attr(sn, "region.id")[sn$from], attr(sn,
"region.id")[sn$to])
head(sn1)

using the region.id attribute.

Hope this helps,

Yep, this gives me exactly what I was looking for. And I learned something 
new: never used attr() before. Is there no other way to access the region.id 
info in sn ?

BTW: Is there any documentation about which attributes are available in these 
objects other than running attributes() ?

Thanks in any case !

Moritz