Subset a spatial weights list

The function subsets a spatial weights list, retaining objects for which the subset argument vector is TRUE. At present it will only subset non-general weights lists (that is those created by nb2listw with glist=NULL).

Usage

# S3 method for class 'listw'
subset(x, subset, zero.policy = attr(x, "zero.policy"), ...)

Arguments

x

an object of class listw

subset

logical expression

zero.policy

default attr(x, "zero.policy") as set when x was created, if attribute not set, use global option value; if FALSE stop with error for any empty neighbour sets, if TRUE permit the weights list to be formed with zero-length weights vectors - passed through to nb2listw

...

generic function pass-through

Value

The function returns an object of class listw with component style the same as the input object, component neighbours a list of integer vectors containing neighbour region number ids (compacted to run from 1:number of regions in subset), and component weights as the weights computed for neighbours using style. If no-neighbour observations are created by subsetting and zero.policy in the input weights object was FALSE, it will be set to TRUE and a warning issued.

Author

Roger Bivand Roger.Bivand@nhh.no

See also

nb2listw, subset.nb

Examples

col.gal.nb <- read.gal(system.file("weights/columbus.gal", package="spData")[1])
to.be.dropped <- c(31, 34, 36, 39, 42, 46)
pre <- nb2listw(col.gal.nb)
print(pre)
#> Characteristics of weights list object:
#> Neighbour list object:
#> Number of regions: 49 
#> Number of nonzero links: 230 
#> Percentage nonzero weights: 9.579342 
#> Average number of links: 4.693878 
#> 
#> Weights style: W 
#> Weights constants summary:
#>    n   nn S0       S1       S2
#> W 49 2401 49 23.48489 204.6687
post <- subset(pre, !(1:length(col.gal.nb) %in% to.be.dropped))
print(post)
#> Characteristics of weights list object:
#> Neighbour list object:
#> Number of regions: 43 
#> Number of nonzero links: 212 
#> Percentage nonzero weights: 11.46566 
#> Average number of links: 4.930233 
#> 
#> Weights style: W 
#> Weights constants summary:
#>    n   nn S0       S1       S2
#> W 43 1849 43 19.26584 178.4604
columbus <- st_read(system.file("shapes/columbus.gpkg", package="spData")[1], quiet=TRUE)
nb <- poly2nb(columbus)
lw <- nb2listw(nb, style="W")
attr(lw, "zero.policy")
#> [1] FALSE
(lwa <- subset(lw, 1:nrow(columbus) != c(21)))
#> Warning: subsetting caused increase in subgraph count
#> Characteristics of weights list object:
#> Neighbour list object:
#> Number of regions: 48 
#> Number of nonzero links: 230 
#> Percentage nonzero weights: 9.982639 
#> Average number of links: 4.791667 
#> 2 disjoint connected subgraphs
#> 
#> Weights style: W 
#> Weights constants summary:
#>    n   nn S0       S1       S2
#> W 48 2304 48 22.46811 199.4398
attr(lwa, "zero.policy")
#> [1] FALSE
(lwb <- subset(lw, !(1:nrow(columbus) %in% c(21, 36, 39))))
#> Warning: subsetting caused increase in subgraph count
#> Warning: subsetting created no-neighbour observations, zero.policy set TRUE
#> Characteristics of weights list object:
#> Neighbour list object:
#> Number of regions: 46 
#> Number of nonzero links: 216 
#> Percentage nonzero weights: 10.20794 
#> Average number of links: 4.695652 
#> 1 region with no links:
#> 46
#> 3 disjoint connected subgraphs
#> 
#> Weights style: W 
#> Weights constants summary:
#>    n   nn S0     S1       S2
#> W 45 2025 45 22.857 187.4843
attr(lwb, "zero.policy")
#> [1] TRUE