#16475: Bug in Gomory-Hu tree algorithm
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Reporter: foosterhof | Owner:
Type: defect | Status: needs_review
Priority: major | Milestone: sage-6.3
Component: graph theory | Resolution:
Keywords: gomory hu tree | Merged in:
gomory-hu gomory_hu_tree | Reviewers:
Authors: | Work issues:
Report Upstream: N/A | Commit:
Branch: | e4604fa9ba9cf8ef5475620ed6b3d156402015d6
u/foosterhof/ticket/16475 | Stopgaps:
Dependencies: #12797 |
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Comment (by ncohen):
Oh. I see ! Sorry, I had not realized that the problem came from
frozensets inside of the graph !
> This can be fixed very easily, but also more efficiently.
> First of all, we can pass g1.vertex_iterator() to frozenset(), which
will probably solve it all together.
Right, but that's going too far I guess.
> Ok, so apparently Graph._backend.iterator_edges() also compares vertices
before yielding them, to make sure the 'lesser' one is reported as the
first vertex. And _ford_fulkerson uses this function through
Graph.edge_iterator().
Yepyep. And g.vertices() sorts the vertices, which is not a good idea in
itself but has some advantages, still ... `:-/`
> I guess there is no way around using a Set?
Ahahaah. As you said there are many ways around, but the best is indeed to
use Set. I don't think that it is a performance issue in this context, I
was just saying aloud that it's better to avoid Set in general. Let's keep
the code as it is `:-)`
Nathann
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Ticket URL: <http://trac.sagemath.org/ticket/16475#comment:11>
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