In a previous post I wrote about highlighting a subgraph of a larger network graph. In response to this post, I was asked how extract a subgraph from a larger graph while retaining all essential characteristics among the extracted nodes.

Vinay wrote:

Dear Will,
The code is well written and only highlights the members of a subgraph. I need to fetch them out from the main graph as a separate subgraph (including nodes and edges). Any suggestions please.

Thanks.

Extract subgraph
For a given list of subgraph members, we can extract their essential characteristics (i.e., tie structure and attributes) from a larger graph using the iGraph function induced.subgraph(). For instance,

library(igraph)                   # Load R packages

set.seed(654654)                  # Set seed value, for reproducibility
g <- graph.ring(10)               # Generate random graph object
E(g)$label <- runif(10,0,1)       # Add an edge attribute

# Plot graph
png('graph.png')
par(mar=c(0,0,0,0))
plot.igraph(g)
dev.off()

g2 <- induced.subgraph(g, 1:7)    # Extract subgraph

# Plot subgraph
png('subgraph.png')
par(mar=c(0,0,0,0))
plot.igraph(g2)
dev.off()

Graph

Subgraph