Andrej Andrej - 1 month ago 6
R Question

Extract edges between community nodes and other nodes

Suppose we have a simple weighted network on which we perform some sort of community detection. Next we extract particular community and the final task is to extract all edges between nodes of this community and all other nodes.

Below I pasted the toy code.

# Create toy graph
library(igraph)
set.seed(12345)
g <- make_graph("Zachary")
# Add weights to edges
E(g)$weight <- sample(x = 1:10, size = ecount(g), replace = TRUE)
# Run community detection
cl <- cluster_louvain(g)


There are 5 nodes which belong to community
#1
, 12 nodes which belong to community
#2
, etc.

> table(membership(cl))
1 2 3 4
5 12 2 15


Now we extract community
#1
:

g1 <- induced_subgraph(g, which(cl$membership == 1))


Question: how to find edges which connect nodes in community #1 with all other nodes (excluding edges which define community
#1
)?

Answer

Start by getting all edges based in your community:

all_edges <- E(g)[inc(V(g)[membership(cl) == 1])]
all_edges
+ 10/78 edges:
 [1] 1-- 5 1-- 6 1-- 7 1--11 5-- 7 5--11 6-- 7 6--11 6--17 7--17

Then, filter out the ones that are completely internal (both vertices are in the community):

all_edges_m <- get.edges(g, all_edges) #matrix representation

all_edges[!(
 all_edges_m[, 1] %in% V(g)[membership(cl) == 1] & 
   all_edges_m[, 2] %in% V(g)[membership(cl) == 1]
 )] # filter where in col1 and col2
+ 4/78 edges:
[1] 1-- 5 1-- 6 1-- 7 1--11
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