Re: [igraph] fastgreedy.community in R, iterative function to divide huge communities

[Gábor Csárdi]

Janos,

you can certainly do this, but I am not sure that it is a good
solution. If you divide the communities recursively, without taking
the rest of the graph into account then you can get some suboptimal
results.

I mean if you get six clusters with the recursive division, and you
also cut the original dendrogram outputted by the greedy algorithm,
then they will be different and nothing ensures that the recursive
division will be better in terms of modularity. From the top of my
head I would guess the opposite way, at least most of the time.

So I would just cut the dendrogram into the desired number of
clusters. Or at least compare the modularity of the recursive division
and the 'simple cut' approach.

Merging the membership vectors is easy, e.g. assuming you have two of
them: 'memb1' is longer, and 'memb2' is the replacement of the
'to.rep' elements in 'memb2', you can do something like:

## we need this to make unique values for the clusters
memb2 <- memb2 + max(memb1)
## and then merge them
memb1[ memb1==to.rep ] <- memb2

No they are merged into 'memb1', but there might be gaps in the 'ids'
in 'memb1', these can be eliminated by treating 'memb1' as a factor
temporarily:

memb1 <- as.integer(as.factor(memb1))-1

Now they start at 0, and they are consecutive.

Best,
Gabor

2009/4/1 Janos Moldvay <address@hidden>:
> Hello,
>
> while trying out the fastgreedy.community implementation in R I wondered if
> anyone tried to build a function around it which would further divide huge
> communities into smaller ones. It would maybe look something like this:
>
>
> g <- read.graph("c:\\data\\sample.ncol", format="ncol")
> com <- fastgreedy.community(g)
> memb <- community.to.membership(g, com$merges,
> steps=(which.max(com$modularity)-1))
>
> Now I extract nodes belonging to the biggest cluster/communities, to further
> devide them into smaller communities:
>
> tmp <- which(memb$membership == which.max(memb$csize)-1)-1
> g2 <- subgraph(g,tmp)
> memb2 <- community.to.membership(g2, com2$merges,
> steps=(which.max(com2$modularity)-1))
> Since I am not really familiar with R I am having trouble merging the two,
> (respectively n) community_membership vectors (memb and memb2) so that I
> have one community membership per node. Furthermore would I continue to
> iteratively divide large communities into smaller ones till I either reached
> a modularity-threshold or all communities are smaller than a defined
> maximum-clustersize-threshold.
> I'll appreciate any kind of help very much!
>
> Thanks in advance
> János Moldvay
>
>
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>



-- 
Gabor Csardi <address@hidden>     UNIL DGM