Extracting an edge list with conditions from an igraph object in R

Question

I am working with an undirected igraph object composed of nodes of different types (e.g. males M in yellow and females F in orange):

g <- graph.atlas(711)
V(g)$name <- 1:7
V(g)$gender <- c("M","F","M","M","M","F","F")
V(g)$color <- ifelse(V(g)$gender=="F", "orange","yellow")
g<-delete.edges(g, E(g, P=c(1,2,2,3,2,7,7,6,7,3,3,4,3,5,4,5,5,6,6,1))) 
g<-add.edges(g,c(1,4,4,5,5,1,4,7,7,3,3,5,5,7,2,7,7,6,6,2,6,4))
plot(g)

I would like to extract an edge list composed of the edges connecting nodes of different types (males and females):

edgelist <- rbind(c(3,7),
               c(4,6),
               c(4,7),
               c(5,7))

assortativity uses the fraction of edges connecting vertices of type M and F, but I do not know to extract explicity these edges.

get.edgelist only returns the whole edge list with no possibility of setting conditions.

Answer 1

You can use the %--% selector to find edges that connect male nodes to female nodes. For example

E(g)[V(g)[gender=="M"] %--% V(g)[gender=="F"]]

The V(g)[gender=="M"] finds all the "male" nodes and V(g)[gender=="F"] finds all the female node and %--% finds all the edges between the two sets.

Answer 2

edges = get.edgelist(g)
edges[rowSums(apply(edges, 2, function(x) get.vertex.attribute(g, "gender", x)) == "M") == 1,]
#     [,1] [,2]
#[1,]    4    7
#[2,]    3    7
#[3,]    5    7
#[4,]    4    6