Flip node/edge behavior

[apeterson91]

I have a street network sf object. I want to get the adjacency matrix defined by the streets connectivity. I'm trying to see if there is an easy way to do this using sfnetworks, here is what I have tried so far:

## create street network object from stdf street sf object
ntdf <- as_sfnetwork(stdf,directed=F)

A <- igraph::get.adjacency(ntdf)

Here A will not give me what I want, as it will give me the adjacency matrix of the vertices defined at the intersections on the streets, rather than the adjacency of the streets --- defined by line string geometries --- themselves.

Is there an easy way to flip or switch the edges and nodes within the sfnetworks paradigm? I have some sense of how I might go about doing this through either some manual manipulation of the sfnetwork object and/or just applying st_* operations on the original street graph, but my intuition is that there might be an easy fix here somewhere.

I can provide a reprex and further details on my motivation if desired.

[agila5]

Hi @apeterson91! I think you are looking for the to_linegraph morpher to convert your graph into its Line graph. See also our vignette on morphers and ?igraph::line_graph.

For example, we can create a super simple spatial network with 3 vertices and 2 edges:

# packages
library(sf)
#> Linking to GEOS 3.9.0, GDAL 3.2.1, PROJ 7.2.1
library(tidygraph)
#> 
#> Attaching package: 'tidygraph'
#> The following object is masked from 'package:stats':
#> 
#>     filter
library(sfnetworks)

# data
my_sfc <- st_sfc(
  st_linestring(rbind(c(-1, 0), c(0, 0))), 
  st_linestring(rbind(c(0, 0), c(0, 1)))
)

# create sfnetwork
my_sfn <- as_sfnetwork(my_sfc, directed = FALSE)
par(mar = rep(0, 4))
plot(my_sfn)

we can extract the adjacency matrix among the vertices

igraph::as_adjacency_matrix(my_sfn)
#> 3 x 3 sparse Matrix of class "dgCMatrix"
#>           
#> [1,] . 1 .
#> [2,] 1 . 1
#> [3,] . 1 .

and also extract the adjacency matrix among the edges

my_sfn %>% 
  convert(to_linegraph) %>% 
  igraph::as_adjacency_matrix()
#> 2 x 2 sparse Matrix of class "dgCMatrix"
#>         
#> [1,] . 1
#> [2,] 1 .

^{Created on 2021-08-27 by the reprex package (v2.0.0)}

[apeterson91]

This is it! I knew this had to be built-in somewhere!

Thanks for your help!

[apeterson91]

Trying to mark this as the answer but github is currently bugging out - will try again later