igraph：基于无向图中源/目标的选择边中的意外行为

Question

I am using igraph in python. I want to find an edge in an undirected graph using source and target vertex. I am using the following code:

g = Graph() # or g = Graph(directed=True)
g.add_vertices(4)
g.add_edges([(0,1),(1,2),(2,3)])
print len(g.es.select(_source=0, _target=1))
print len(g.es.select(_source=1, _target=0))

In case of directed graph, it works as I expected and output is 1 and 0. But in case of undirected graph, I expected that both print statements will return 1, but instead it is same as previous case.

What is the correct way of finding an edge between source and target (that works both for directed and undirected graph)?

Answer 1

The canonical way to get an edge from source and target vertices is to use the get_eid method.

get_eid(v1, v2, directed=True, error=True)

Directed is ignored for undirected graphs. The error parameter controls whether an exception is raised is the edge does not exists. If error=False it returns -1.

g = igraph.Graph(directed=False)
g.add_vertices(4)
g.add_edges([(0,1),(1,2),(2,3)])
print 'Edge id: {0}'.format(g.get_eid(1,0))
print 'Edge id: {0}'.format(g.get_eid(0,1))

Output:

Edge id: 0
Edge id: 0

The full documentation is here.