Re: [igraph] degrees of separation

[Gábor Csárdi]

Hi, please see below.

On Tue, May 21, 2013 at 12:11 PM, Marie Lalanne <address@hidden> wrote:

Hello,

I am a new R and igraph user and I am wondering whether you could give me some hints to do the following:

Assuming 'g' is the graph, 's' the subset and 'v' the vertex:

library(igraph)

data <- "1 2

1 3

2 3

2 5

3 4

3 5

4 5

"

g <- graph.data.frame(read.table(textConnection(data)), 

       directed=FALSE, vertices=data.frame(1:5))

s <- c(1,2)

v <- 4

I have an edgelist (undirected). I would like to compute the following four things:

- for a subset of vertices, the number of edges (without counting twice a same vertex linked with two different vertices of the subset)

So this is effectively the number of vertices connected to s? 

sum(colSums(g[s,-s]) != 0)

 

- for a subset of vertices, if at least one of the vertices from this subset is linked with a particular vertex (from the whole set)

any(g[s,v] != 0)

 

- for a subset of vertices, the minimal degree of separation required to reach a particular vertex (from the whole set)

min(shortest.paths(g, v=4, to=s))

 

- for all the vertices (in the whole set), if they are linked with at least one vertex from the subset

colSums(g[s,]) != 0

Please see the manual, in particular ?'[.igraph' for details.

Best,

Gabor

ps. be critical with my answers, and check them, I haven't checked them too extensively. 

For example:

1-2

1-3

2-3

2-5

3-4

3-5

4-5

and consider the subset of vertices being 1 and 2 and the particular vertex being 4

Answer to the first question: 3 (edges 1-3 and 2-3 are count only for one)

Answer to the second question: 0 (neither 1, neither 2 is directly linked to 4)

Answer to the third question: 2 (edge 1-3 and then edge 3-4 or edge 2-3 and then edge 3-4)

Answer to the fourth question: [1] - [2] - [3] 1 [4] 0 [5] 1

Many thanks for your help!

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Gabor Csardi <address@hidden>     MTA KFKI RMKI