[igraph] Connected Components

[Michael Knudsen]

Hello,

I have a graph, and I want to plot only the biggest connected
component, but something appears not to be working as it is supposed
to do. First I read in my data:

> g <- graph(scan("protein_data.txt"),directed=F)
Read 8960 items
> g <- simplify(g)

I then use the clusters function to get the clusters:

> g_clusters <- clusters(g)
> which(g_clusters$csize == max(g_clusters$csize))
[1] 3

Hence cluster number three is the biggest. Since counting starts at 0
in igraph, this means that the connected component consists of the
vertices:

selected_vertices <- V(g)[which(g_clusters$membership == 2)]

But when I construct the corresponding subgraph, it isn't connected:

> h <- subgraph(g,selected_vertices)
> is.connected(h)
[1] FALSE

It really puzzles me; I can't see what is wrong with the approach
above. I'm absolutely sure that the index shifting is correct -- but
it shouldn't matter anyway: All components are, by definition,
connected, so even if I picked a wrong component it should still be
connected.

Any thoughts?

Thanks!
Michael Knudsen

-- 
Michael Knudsen
address@hidden
http://sites.google.com/site/micknudsen/