#18910: Boost minimum spanning tree
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Reporter: borassi | Owner:
Type: enhancement | Status: needs_review
Priority: major | Milestone: sage-6.8
Component: graph theory | Resolution:
Keywords: Boost, minimum | Merged in:
spanning tree | Reviewers:
Authors: Michele Borassi | Work issues:
Report Upstream: N/A | Commit:
Branch: | ceda13f0b83784adce78b527099e08e7e003c6ac
u/borassi/boost_minimum_spanning_tree| Stopgaps:
Dependencies: #18876, #18906 |
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Comment (by borassi):
Hello!
Yeah, I have some problems, too: it is difficult to decide which is the
correct behavior. In your examples, I have trouble understanding which
improvement you are suggesting: could you be a bit more specific?
In particular, in the first two cases, you provide a weight function which
is not correct, so a TypeError is raised (if you provide a function, the
algorithm raises an error if it is not correct). In the third case, your
graph is not weighted:
{{{
sage: g = Graph([(0,1,1), (1,2,'b')])
sage: g.weighted()
False
}}}
Hence, the algorithm simply says "Since the graph is not weighted, I
ignore the label and set weight 1 to each edge".
Maybe a problem occurs if the graph is weighted and no function is
provided: in this case, the algorithm tries to use labels even if they are
not numbers. In Boost algorithms, non-numeric weights are assumed to be 1,
while in other algorithms labels are simply compared, and Python decides
which is the biggest.
{{{
sage: g = Graph([(0,1,1), (1,2,'b')], weighted=True)
sage: g.min_spanning_tree(algorithm="Prim_Boost")
[(0, 1, 1), (1, 2, 'b')]
sage: g.min_spanning_tree(algorithm="Prim_fringe")
[(0, 1, 1), (1, 2, 'b')]
sage: g.min_spanning_tree(algorithm="Prim_edge")
[(0, 1, 1), (1, 2, 'b')]
sage: g.min_spanning_tree(algorithm="Kruskal")
[(0, 1, 1), (1, 2, 'b')]
sage: g.min_spanning_tree(algorithm="Kruskal_Boost")
[(0, 1, 1), (1, 2, 'b')]
sage: g.min_spanning_tree(algorithm="NetworkX")
[(0, 1, 1), (1, 2, 'b')]
}}}
In the latter example, do you think I should only improve the explanation,
which is in the INPUT section of the documentation and in the examples? Or
do you think I should raise an exception if a weight is not a number?
Thank you very much!
Michele
--
Ticket URL: <http://trac.sagemath.org/ticket/18910#comment:9>
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