How to union graphs and save directed and undirected components in new graph?

Question

I have read the can-an-igraph-object-have-directed-and-undirected-edges.

I have two graphs gUed and gDed.

The gUed is the undirected graph and the gDed is the directed one. I need to union these graph into one. I made this union with the edge list.

library(igraph)
gDed <- graph.formula(1-+2, 2-+3, 3-+4, 4-+5)

gUed <- graph.formula(6--1, 7--2, 8--4, 9--5)

EL  = as_edgelist(gDed, names = TRUE)
EL1 = as_edgelist(gUed, names = TRUE)
ELU = rbind(EL, EL1)
ELU = ELU[!duplicated(ELU),]
GU = graph_from_edgelist(ELU)

## To check the result
par(mfrow=c(1,3))
plot(gDed)
plot(gUed)
plot(GU)

But I have a problem: in new graph GU all edges are directed.

Question. How to union two graphs and save directed and undirected components in new graph?

desval desval · Accepted Answer · 2020-12-09T14:53:14

A network has to be either directed or undirected. That said, you can work around that, a bit.

Similar to the answer you found, you create a directed network, and pass in an argument that keeps track of whether the edge is directed or not. You can use this when you plot, or to subset your network:

E(gDed)$type <- "dir"
E(gUed)$type <- "undir"


g <- graph_from_data_frame(
  rbind(
    as_data_frame(gDed, what = "edges"),
    as_data_frame(gUed, what = "edges")
  )
)


plot(g,
     edge.arrow.mode = ifelse(E(g)$type=="undir", "-", ">"))


g_sub1 <- subgraph.edges(g, eids = E(g)[E(g)$type=="undir"] )
plot(g_sub1,
     edge.arrow.mode = ifelse(E(g_sub1)$type=="undir", "-", ">"))


g_sub2 <- subgraph.edges(g, eids = E(g)[E(g)$type!="undir"] )
plot(g_sub2,
     edge.arrow.mode = ifelse(E(g_sub2)$type=="undir", "-", ">"))

If there are double edges, one directed and one undirected, they will both be in g, leading to weird plots. You could fix this by creating a third type of edge, which will be either directed or undirected, depending on what you want to do.

How to union graphs and save directed and undirected components in new graph?

1 Answers