Hi folks,
I noticed the following inconsistent behaviour while working on ticket
#8395 [1]. Given a multigraph G with one self-loop at vertex v,
disable loops in G and query the degree of v. This should report the
correct degree:
sage: version()
'Sage Version 4.6, Release Date: 2010-10-30'
sage: G = Graph({1:[2,2]}); G
Multi-graph on 2 vertices
sage: G.add_edge(1, 2)
sage: G.allow_loops(True); G
Looped multi-graph on 2 vertices
sage: G.add_edge(1, 1)
sage: G.edges(labels=False)
[(1, 1), (1, 2), (1, 2), (1, 2)]
sage: G.allow_loops(False); G
Multi-graph on 2 vertices
sage: G.edges(labels=False)
[(1, 2), (1, 2), (1, 2)]
sage: G.degree(1)
3
But now say we start with G having multiple self-loops at the same
vertex v. Disable loops in G and query the degree of v. The reported
degree of v is greater than the true degree of v:
sage: G = Graph({1:[2,2]}); G
Multi-graph on 2 vertices
sage: G.add_edge(1, 2)
sage: G.allow_loops(True); G
Looped multi-graph on 2 vertices
sage: G.add_edge(1, 1)
sage: G.add_edge(1, 1)
sage: G.edges(labels=False)
[(1, 1), (1, 1), (1, 2), (1, 2), (1, 2)]
sage: G.allow_loops(False); G
Multi-graph on 2 vertices
sage: G.edges(labels=False)
[(1, 2), (1, 2), (1, 2)]
sage: G.degree(1)
4
I just spent an hour looking into this issue and suspect that the
problem may lie in the method SparseGraph.del_arc_label_unsafe() of
the module
sage/graphs/base/sparse_graph.pyx
But I can't pinpoint where the exact problem is. Any help is appreciated.
[1] http://trac.sagemath.org/sage_trac/ticket/8395
--
Regards
Minh Van Nguyen
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