adj provides a lightweight adjacency list class for R, built on the vctrs package. Adjacency lists are validated on creation, automatically reindex when subsetted or indexed, and support pretty-printing. Lists can be easily converted to a zero-index basis, which allows for easy passing of objects to low-level languages for processing. Creation of adjacency lists from shapefiles is supported through an optional dependency on geos.

Installation

You can install the development version of adj from GitHub with:

# install.packages("pak")
pak::pak("alarm-redist/adj")

Examples

adj comes loaded with example data for the seven bridges of Königsberg.

library(adj)
data("konigsberg")
konigsberg
#>   area     bridge_to       x      y
#> 1    A B, B, C, C, D 20.5100 54.706
#> 2    B       A, A, D 20.5115 54.709
#> 3    C       A, A, D 20.5110 54.703
#> 4    D       A, B, C 20.5170 54.705

We can build an adjacency graph using the unique identifiers of each area.

a = adj(konigsberg$bridge_to, ids = konigsberg$area, duplicates = "allow")
print(a, n = 5)
#> <adj[4]>
#> [1] {2, 2, 3, 3, 4} {1, 1, 4}       {1, 1, 4}       {1, 2, 3}

Alternatively, we can create an adjacency list from a list of integers. Here, we set duplicates = "remove" to remove any duplicate edges.

adj(c(2, 3, 3), c(1, 3), c(1, 1, 2), duplicates = "remove")
#> <adj[3]>
#> [1] {2, 3} {1, 3} {1, 2}

Once created, adjacency lists can be subsetted using standard R indexing, and the internal indices will be automatically updated.

a[1:2]
#> <adj[2]>
#> [1] {2, 2} {1, 1}
rev(a)
#> <adj[4]>
#> [1] {4, 3, 2}    {4, 4, 1}    {4, 4, 1}    {3, 3, 2, …}

Quotient graphs can be created from adjacency lists and a grouping vector. Here, we create a quotient graph by grouping the two islands (areas A and D) together.

adj_quotient(a, c("AD", "B", "C", "AD"))
#> <adj[3]>
#> [1] {2, 3} {1}    {1}

Finally, adjacency lists can be converted to a matrix or zero-indexed.

as.matrix(a)
#>      [,1] [,2] [,3] [,4]
#> [1,]    0    2    2    1
#> [2,]    2    0    0    1
#> [3,]    2    0    0    1
#> [4,]    1    1    1    0
adj_zero_index(a)
#> [[1]]
#> [1] 1 1 2 2 3
#> 
#> [[2]]
#> [1] 0 0 3
#> 
#> [[3]]
#> [1] 0 0 3
#> 
#> [[4]]
#> [1] 0 1 2