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## Re: [igraph] Creating subgraphs on the basis of clusters()

 From: Gabor Csardi Subject: Re: [igraph] Creating subgraphs on the basis of clusters() Date: Fri, 14 Dec 2007 10:35:02 +0100 User-agent: Mutt/1.5.13 (2006-08-11)

```On Fri, Dec 14, 2007 at 11:35:40AM +0900, MATSUDA, Noriyuki wrote:
> Hello:
>
>  I need help in creating subgraphs by selecting clusters of certain sizes.
>  To
> do so, I want to obtain the list of memberships of the vertices to be
> selected.
> -------------------------------------------------------------------------------------------------------------------
> a <- c(0,1)
> b <- c(2,3,3,4)
> c <- c(5,6,5,7,6,7)
> d <- c(8,9,8,10,8,11, 9,10,9,11, 10,11)
> e <- c(12,13,12,15, 13,14, 14,15, 15,16)
> g<- graph(c(a,b,c,d,e),directed=F)
> plot(g, layout= layout.circle) #vertex.label=V(g)\$name
> cls<- clusters(g)
> # \$membership
> #  [1] 0 0 1 1 1 2 2 2 3 3 3 3 4 4 4 4 4
> # \$csize
> # [1] 2 3 3 4 5   #e.g., the membership & the size of the first
> cluster are 0 & 2
>
> clsID<- which(cls\$size == 2) - 1     # resulting in a single value 0
> which(cls\$membership == clsID)  # 1, 2
> clsID<- which(cls\$csize == 3) - 1  # resulting in a vector consisted of 1 &
> 2
> which(cls\$membership == clsID)  # resulting in 0 0 which are hard to
> interpret.

clsID <- which(cls\$size==n) - 1

gives you the cluster ids of the clusters with 'n' vertices, i think that
is clear. Then

which(cls\$membership == clsID)

is not a very good idea, since it uses R's recycling rule and if
clsID is longer than 1 then it is not what you want. Eg. try:

a <- 1:4
a == 1
a == c(1,2)
a == c(2,1)

I think what you want is

which(cls\$membership %in% clsID) - 1

This gives the vertex ids in the clusters clsID.

Gabor

> -------------------------------------------------------------------------------------------------------------------
>
>   Is there a way to specify the condition like cls\$membership in clsID ?
> --
> MATSUDA, Noriyuki　<in Kanji> 松 田 紀 之
[...]

--
Csardi Gabor <address@hidden>    MTA RMKI, ELTE TTK

```