Question

I have the following problem:
I have the following graph: g6 which does not include isolate vertices. The vertices are mainly grouped into two ethnic groups, given as attribute, namely: mestizo and saraguro.
So I run the subgraphoption for saraguros as follows:

g2saraguro <- subgraph(g6, which(V(g6)$ethnic==”saraguro”))
summary(g2saraguro)

which did work:

Vertices: 166
Edges: 330
Directed: TRUE
No graph attributes.
Vertex attributes: ii, village, parroquia, canton, provinz, kindR, ethnic, situation, researcha.
Edge attributes: question, activity.

However, if I try the same with mestizo, there is a proble, since it just identify:

s2mestizo <- subgraph(g6,V(g6)$ethnic==”mestizo”)
summary(s2mestizo)
Vertices: 2
Edges: 3
Directed: TRUE
No graph attributes.
Vertex attributes: ii, village, parroquia, canton, provinz, kindR, ethnic, situation, researcha.
Edge attributes: question, activity.

but in the reality that is no the case, if I see: V(g6)$ethnic, mestizo are the majority !!
I really don´t know where the problem is, with igraph 0.6 the procedure to form a subgraph should be in that or a similar way, without 1-based indexing.
thanks a lot for any help
regards
Vladimir

Answer 1

You forgot to use the which function in the second case; i.e., instead of

s2mestizo <- subgraph(g6,V(g6)$ethnic=="mestizo")

you should write

s2mestizo <- subgraph(g6, which(V(g6)$ethnic=="mestizo"))

Also note that if you are using igraph 0.5.x or earlier, you must subtract 1 from the result of which because of the 0-based indexing of igraph. This has been fixed in igraph 0.6 so you don’t need to subtract 1 if you are using igraph 0.6 or later.

Edit: based on your recent comment, it looks like you are using igraph 0.5.x or earlier (because the output format of summary is different for 0.6). In this case, you must subtract 1 from the result of which because igraph 0.5.x and earlier versions use zero-based vertex indices. Therefore, the correct line should be:

s2mestizo <- subgraph(g6, which(V(g6)$ethnic=="mestizo")-1)