[R] help for cell2nb and queencell in spdep package

[Roger Bivand]

On Wed, 3 Aug 2005, ecoinfo wrote:

> Dear Dr. Bivand and R-users,
>  I have a 5 by 5 grid, say, location[1:5,1:5], and I want to know the 
> indices of 8 neighbours of each cell. For example, for location[2,2], its 
> neighbour coordinates are [1,1:3], [2,1], [2,3] and [3,1:3]. Sometimes I 
> also need to remove edge effects (torus = TRUE).
>  I have tried "cell2nb" function in your spdep package. Here's my example:
>  > neigh <- cell2nb(5,5,type="queen",torus=T)
> > neigh
> Neighbour list object:
> Number of regions: 25 
> Number of nonzero links: 200 
> Percentage nonzero weights: 32 
> Average number of links: 8 
> > neigh[1]
> [[1]]
> [1] 2 5 6 7 10 21 22 25
>  Is there way to index each element of neigh[1], i.e., the first is 2, the 
> second is 5, ... ? 

neigh is a list of integer vectors, so you can use standard indexing:

> neigh[[1]][1]
[1] 2

using double square brackets for accessing the list components, and single 
for the vector elements. I think this is what you are asking for, if not, 
please clarify. To find out which (row, column) address this corresponds 
to, look it up in the "region.id" attribute:

> attr(neigh, "region.id")[neigh[[1]][1]]
[1] "2:1"

>  Could you also give me an example of the function "queencell(rowcol, nrow, 
> ncol, torus=FALSE, rmin=1, cmin=1)"? What's a rowcol? 

>From the help page: 

	"rowcol: matrix with two columns of row, column indices"

The queencell function is used internally, but takes a single row matrix 
of (row, column) indices, and returns a matrix of (row, column) indices 
for the contiguous neighbours of that cell:

> queencell(matrix(c(1,1),1,2), 8, 8, TRUE)
     row col
[1,]   2   8
[2,]   1   8
[3,]   8   8
[4,]   2   1
[5,]   8   1
[6,]   2   2
[7,]   1   2
[8,]   8   2
attr(,"coords")
[1] 1 1

for your case on a torus.

Hope this helps,

Roger

>  Thanks,
> Xiaohua
> 
> 

-- 
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