get connected components using igraph in R

Question

I would like to find all the connected components of a graph where the components have more than one element.

using the clusters gives the membership to different clusters and using cliques does not give connected components.

This is a follow up from

multiple intersection of lists in R

My main goal was to find all the groups of lists which have elements in common with each other.

Thanks in advance!

Display

do you need clusters? – Atheling

@Atheling yes. clusters with more than one element. sorry I meant clusters instead of connectedComp. Edited the question – Display

The ?clusters function returns cluster membership and size - should be easy enough to subset it to only include clusters with more than one node – Atheling

@Atheling Is there any efficient way to do this instead of manually going through the results of clusters? – Display

There is probably a function to extract this... but you can just use the results from clusters. One way: cl <- clusters(g); lapply(seq_along(cl$csize)[cl$csize > 1], function(x) V(g)$name[cl$membership %in% x]) – Atheling

@Atheling I guess you do not need the $name and why not you post the answer? Also we need to convert vertex type to numeric – Display

Answer 1

You can use the results from components to subset your nodes according to the component size.

library(igraph)

# example graph
set.seed(1)
g <- erdos.renyi.game(20, 1/20)
V(g)$name <- letters[1:20]
par(mar=rep(0,4))
plot(g)

# get components
cl <- components(g)
cl
# $membership
# [1]  1  2  3  4  5  4  5  5  6  7  8  9 10  3  5 11  5  3 12  5
# 
# $csize
# [1] 1 1 3 2 6 1 1 1 1 1 1 1
# 
# $no
# [1] 12


# loop through to extract common vertices
lapply(seq_along(cl$csize)[cl$csize > 1], function(x) 
                                         V(g)$name[cl$membership %in% x])
# [[1]]
# [1] "c" "n" "r"
# 
# [[2]]
# [1] "d" "f"
# 
# [[3]]
# [1] "e" "g" "h" "o" "q" "t"

Atheling

Just for fun, here's another approach once you have the clusters: grps <- with(cl, duplicated(membership)|duplicated(membership, fromLast=TRUE)) followed by unname(split(V(g)$name[grps], cl$membership[grps])) – Stupa