[graph-tool] weird in deleting vertices and edges

20 Apr 2015

      Hi all,

I found something weird in deleting vertices and edges of a graph. Say I
simply create a graph like:

    g = Graph(directed=False)
    g.add_vertex(7)
    g.add_edge(g.vertex(0), g.vertex(1))
    g.add_edge(g.vertex(1), g.vertex(2))
    g.add_edge(g.vertex(2), g.vertex(3))
    g.add_edge(g.vertex(3), g.vertex(4))
    g.add_edge(g.vertex(2), g.vertex(5))
    g.add_edge(g.vertex(5), g.vertex(6))

   del_list = [g.vertex(1), g.vertex(3)]

   for v in reversed(sorted(del_list)):
        print 'deleting vertex', int(v)
    ##    for ve in v.out_edges():
    ##       print 'deleting edges', ve.source(), ve.target()
    ##        g.remove_edge(ve)
        g.remove_vertex(v, fast=True)

The above code will delete vertex 3 first, then vertex 1 as expected.
However, in order to keep correct topology, we should delete some related
edges first, like the three commented lines (start with "##").

But if you uncomment the three lines, weird thing happens:  the program
will delete vertex 1 first, then vertex 3.

Of course I can sort the vertex index instead of vertex descriptor to avoid
the problem. But I am wondering where is wrong.  Could anyone explain this?

-- 
Cheers,
Bo Wu