Checks a neighbours list for symmetry/transitivity (if i is a neighbour of j, then j is a neighbour of i). This holds for distance and contiguity based neighbours, but not for k-nearest neighbours. The helper function sym.attr.nb() calls is.symmetric.nb() to set the sym attribute if needed, and make.sym.nb makes a non-symmetric list symmetric by adding neighbors. is.symmetric.glist checks a list of general weights corresponding to neighbours for symmetry for symmetric neighbours.

is.symmetric.nb(nb, verbose = NULL, force = FALSE)
sym.attr.nb(nb)
make.sym.nb(nb)
old.make.sym.nb(nb)
is.symmetric.glist(nb, glist)

## Arguments

nb

an object of class nb with a list of integer vectors containing neighbour region number ids.

verbose

default NULL, use global option value; if TRUE prints non-matching pairs

force

do not respect a neighbours list sym attribute and test anyway

glist

list of general weights corresponding to neighbours

## Value

TRUE if symmetric, FALSE if not; is.symmetric.glist returns a value with an attribute, "d", indicating for failed symmetry the largest failing value.

## Note

A new version of make.sym.nb by Bjarke Christensen is now included. The older version has been renamed old.make.sym.nb, and their comparison constitutes a nice demonstration of vectorising speedup using sapply and lapply rather than loops. When any no-neighbour observations are present, old.make.sym.nb is used.

## Author

Roger Bivand Roger.Bivand@nhh.no

read.gal

## Examples

columbus <- st_read(system.file("shapes/columbus.shp", package="spData")[1], quiet=TRUE)