从有向图网络x上删除可逆边

Question

Is there a way of removing reversible edges in a graph. For instance, let's say the following graph

import networkx as nx
G=nx.DiGraph()
G.add_edge(1,2)
G.add_edge(2,3)
G.add_edge(2,1)
G.add_edge(3,1)
print (G.edges())

[(1, 2), (2, 3), (2,1), (3,1)]

I want to remove (2,1) and (3,1), since I want the graph to be directed in just one direction. I know you can remove self-loops by using G.remove_edges_from(G.selfloop_edges()) but that's not the case here. The output I am looking for would be [(1, 2), (2, 3)] . Is there a way to remove this edges once the graph is created either by networkx or by other graph tool such as cytoscape?.

Answer 1

Method 1:

remove duplicate entries in edgelist -> remove everything from graph -> add back single edges => graph with single edges

Edges are stored as tuples. You could lose index information via temporary conversion to sets. You could then lose duplicate tuples, again, through temporary conversion to a set. After conversion back to a list, you have your list of edges, with duplicate entries removed, like so:

stripped_list = list(set([tuple(set(edge)) for edge in G.edges()]))

Then remove all edges from the graph that are currently there, and add back those that are in the list just created:

G.remove_edges_from([e for e in G.edges()])
G.add_edges_from(stripped_list)

Method 2:

find duplicate edges -> remove only those from graph => graph with single edges

again, losing positional information via conversion to sets:

set_list = [set(a) for a in G.edges()] # collect all edges, lose positional information
remove_list = [] # initialise

for i in range(len(set_list)):
    edge = set_list.pop(0) # look at zeroth element in list:

    # if there is still an edge like the current one in the list, 
    # add the current edge to the remove list:
    if set_list.count(edge) > 0:
        u,v = edge 

        # add the reversed edge
        remove_list.append((v, u))

        # alternatively, add the original edge: 
        # remove_list.append((u, v))

G.remove_edges_from(remove_list) # remove all edges collected above

As far as I know, networkx does not store the order that edges were added in, so unless you want to write further logic, you either remove all duplicate edges going from nodes with lower number to nodes with higher number, or the other way round.