Add neighbors to adjacency information

Modifies adjacency to indicate that neighs they should be treated as neighbors.

Usage

add_neighbors(adjacency, neighs)

Arguments

adjacency

Adjacency information generated by spdep::poly2nb().

neighs

A vector of regions to mark as adjacent. Accepts a vector of indices or names assigned to adjacency.

Value

A modified adjacency list.

Details

add_neighbors() is useful when adjacency information generated by spdep::poly2nb() indicates lone regions without links/neighbors, particularly in island counties such as the Hawaiian islands, Nantucket in Massachusetts, or San Juan in Washington. Note that add_neighbors() marks all listed counties as adjacent, so if you have a set of chaining counties where the first may not be connected to the last, several instances of add_neighbors() will be needed.

Examples

mamap <- sf::st_as_sf(mamap[order(mamap$GEOID), ])
ma_adj <- spdep::poly2nb(mamap)
#> Warning: some observations have no neighbours;
#> if this seems unexpected, try increasing the snap argument.
#> Warning: neighbour object has 3 sub-graphs;
#> if this sub-graph count seems unexpected, try increasing the snap argument.
new_neighs <- c(1, 4, 10) # attach regions 1, 4, and 10
ma_adj <- add_neighbors(ma_adj, new_neighs)

# Add neighbors by FIPS code instead of index
ma_adj <- suppressWarnings(spdep::poly2nb(mamap))
names(ma_adj) <- mamap$GEOID
ma_adj <- add_neighbors(ma_adj, neighs = c("25001", "25007", "25019"))

ma_adj <- suppressWarnings(spdep::poly2nb(mamap))
ma_adj <- add_neighbors(ma_adj, c(1, 4)) # only attach 1 and 4
ma_adj <- add_neighbors(ma_adj, c(4, 10)) # only attach 4 and 10