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Re: [igraph] iGraph in Python: Relation between a VertexDendrogram objec
From: |
Tamás Nepusz |
Subject: |
Re: [igraph] iGraph in Python: Relation between a VertexDendrogram object and VertexClustering object |
Date: |
Tue, 2 Jul 2013 20:31:06 +0200 |
Hi there,
I have recently answered your question on Stack Overflow, but I'm copying my
response here as well for the record:
"When an algorithm in igraph produces a `VertexDendrogram`, it may optionally
produce a "hint" as well that tells us where to cut the dendrogram (i.e. after
how many merges) to obtain a `VertexClustering` that is in some sense optimal.
For instance, the `VertexDendrogram` produced by `community_fastgreedy()`
proposes that the dendrogram should be cut at the point where the modularity is
maximized. Running `as_clustering()` on a `VertexDendrogram` simply uses the
hint produced by the clustering algorithm to flatten the dendrogram into a
clustering, but you may override this by specifying the desired number of
clusters as an argument to `as_clustering()`."
--
T.
On 2 Jul 2013, at 18:53, "Durante, Joseph [USA]" <address@hidden> wrote:
> Hi there,
>
> I’m a relatively new user of iGraph and need some help with understanding a
> function in iGraph run in Python.
>
> Using iGraph's community detection function community_fastgreedy(), I got a
> VertexDendrogram object, let's call it V. Then using V.as_clustering() I got
> a VertexClustering object from the dendrogram. I know that the communities
> are clustered so that modularity is maximized, but I think the dendogram
> object keeps merging all the way, so it's tough to see the communities on the
> dendrogram.
>
> My question is: How does the output for the V.as_clustering() correspond to
> distance between the communities in the dendrogram?
>
> In other words, The communities each come with a representative number (
> Community [0], Community [2] for example), so how does the that number
> correspond to position on the dendrogram? Are community 0 and Community 1
> adjacent to each other when the dendrogram is merged up to the level that
> maximizes modularity, and is community 12 farther from community 0 than 3 is?
>
> If not, then how is the order (the numbers for each community) of the output
> decided by the as_clustering function?
>
> Thanks in advance.
>
> -Joe
>
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