testnb: Test a neighbours list for symmetry
In spdep: Spatial Dependence: Weighting Schemes, Statistics and Models

Description Usage Arguments Value Note Author(s) See Also Examples

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)

`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

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

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.

Roger Bivand Roger.Bivand@nhh.no

read.gal

example(columbus)
coords <- coordinates(columbus)
ind <- sapply(slot(columbus, "polygons"), function(x) slot(x, "ID"))
print(is.symmetric.nb(col.gal.nb, verbose=TRUE, force=TRUE))
k4 <- knn2nb(knearneigh(coords, k=4), row.names=ind)
k4 <- sym.attr.nb(k4)
print(is.symmetric.nb(k4))
k4.sym <- make.sym.nb(k4)
print(is.symmetric.nb(k4.sym))