Many thanks!
I forgot about the row standardisation. I will subset the  nb class and then
create the weights matrix.

Nikhil

On Thu, Feb 18, 2010 at 8:36 AM, Roger Bivand <Roger.Bivand@nhh.no> wrote:

> On Thu, 18 Feb 2010, Nikhil Kaza wrote:
>
>  Hello,
>>
>> I am not sure if this is the right forum to ask as this is mostly a sparse
>> matrix question, nevertheless here it is.
>>
>> I am using R - 2.10.1 and spdep 0.4-54 on Windows Vista
>>
>> I have a large shapefile for which I need to create a neighborhood matrix
>> file, I used the following command to create a neighborhood matrix
>>
>> shp_file <- readShapePoly(file.choose())
>> grid_nb_mat <- as.spam.listw(nb2listw(poly2nb(shp_file,
>> row.names=shp_file$CELL_ID)))
>>
>> Now I would like to subset this matrix based on a subset of the shapefile
>> and I have a vector of CELL_IDs. However, I cannot figure out a way to
>> assign row and column names to grid_nb_mat, so that I can subset this
>> matrix. Could someone point me in the right direction? If it is were a
>> simple matrix row.names and colnames would work.
>>
>
> If you had saved the output of poly2nb as an nb object, you could have used
> the subset method for that class. You are using default row standardisation
> in the nb2listw() step, so only subsetting grid_nb_mat - by using "[" with
> keep vectors:
>
> keep <- which(shp_file$CELL_ID "... to keep")
>
> kgrid_nb_mat <- grid_nb_mat[keep, keep]
>
> will not have correct row sums. str(grid_nb_mat) shows no row or column
> names stored (probably to save space). If possible start again from
> poly2nb() and save the output object. Otherwise:
>
> example(columbus)
> col.listw <- nb2listw(col.gal.nb)
> col.sp <- as.spam.listw(col.listw)
> str(col.sp)
> keep <- c(1:20, 25:40)
> kcol.sp <- col.sp[keep, keep]
> apply(kcol.sp, 1, sum)
> str(kcol.sp)
> kcol.sp@entries <- rep(1, 150)
> apply(kcol.sp, 1, sum)
> rsum <- apply(kcol.sp, 1, sum)
> kkcol.sp <- diag.spam(1/rsum) %*% kcol.sp
> apply(kkcol.sp, 1, sum)
>
> Roger
>
>
>> regards
>> Nikhil
>>
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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@nhh.no
>
>

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