igraph delete_edges is behaving strangely

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I am trying to delete an edge from a graph.

I ran the following code

library(igraph)

A <- c(0,1,0,0,0,0,0,1,0,0,0,0,0,1,0,0,0,0,0,1,1,0,0,0,0)
dim(A) <- c(5,5)
g1 <- graph_from_adjacency_matrix(t(A), mode = c("directed")) 

g1 <- delete_edges(g1, c(1,2))
g1 <- simplify(g1)

coords <- c(0.0, 0.951056, 0.587785, -0.587785, -0.951056,
            1, 0.309017, -0.809017,-0.809017,  0.309017)
dim(coords) <- c(5,2)

plot(g1, 
     vertex.color=c(rgb(0,.5,.7), 
                    rgb(.15,.6,0), 
                    rgb(.5,.5,.5),
                    rgb(.8,.55,0),
                    rgb(.8,0,0)),
     vertex.size = 20,
     layout = coords,
     main = paste("Graph", toString(1)))

and produced the following

enter image description here

I would like the 1->2 edge to be deleted but the 2 ->3 edge to remain. Any ideas on what is happening and how to fix it?

igraph r
2 Answers

the delete_edges function assumes you are passing in an edge index. You can see the edges with

E(g1)
# + 5/5 edges from faf79f6:
# [1] 1->2 2->3 3->4 4->5 5->1

so when you specify c(1,2) you are specifying the first and second edges, not the edge that goes form vertex 1 to 2. If you wanted to do that instead, you could do

g1 <- delete_edges(g1, E(g1)[1 %--% 2])

Another way to do this is

g1 <- g1 - edge("1|2")

If you do have a vertex sqeuence that you would like to extract the edges for, you can use the P= parameter of the E()` function. For example

g1 <- delete_edges(g1, E(g1, P=c(1,2,3,4)))

This will remove the edge that goes from 1 to 2, and 3 to 4.

user_23720640 
delete_edges(g, get.edge.ids(g, c(1,2)))

will delete an edge incident with vertex 1 and 2.

user_36041030
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