#12135: Multiple loops handling in c_graph
----------------------------+-----------------------------------------------
Reporter: brunellus | Owner: jason, ncohen, rlm
Type: defect | Status: needs_review
Priority: major | Milestone: sage-5.0
Component: graph theory | Keywords:
Work_issues: | Upstream: N/A
Reviewer: | Author:
Merged: | Dependencies:
----------------------------+-----------------------------------------------
Changes (by brunellus):
* status: new => needs_review
Old description:
> Sage handles multiple loops on one vertex inconsistently.
>
> {{{
> sage:g=Graph({0:[0,0]}, loops=True, multiedges=True,
> implementation="c_graph")
> sage:g.edges()
> [(0, 0, None), (0, 0, None)]
> sage:g.delete_edge(0,0)
> sage:g.edges()
> []
> }}}
>
> Compare with
>
> {{{
> sage:g=Graph({0:[0,0]}, loops=True, multiedges=True,
> implementation="networkx")
> sage:g.edges()
> [(0, 0, None), (0, 0, None)]
> sage:g.delete_edge(0,0)
> sage:g.edges()
> [(0, 0, None)]
> }}}
>
> and
>
> {{{
> sage:g=Graph({0:[1,1]}, loops=True, multiedges=True,
> implementation="c_graph")
> sage:g.edges()
> [(0, 1, None), (0, 1, None)]
> sage:g.delete_edge(0,1)
> sage:g.edges()
> [(0, 1, None)]
> }}}
>
> Another c_graph backend functions work in this spirit: for example
> add_edges won't add any new loops to the vertex where some already exist.
> Well, that is not precise. Look:
>
> {{{
> sage:g=Graph({0: [0]}, loops=True, multiedges=True,
> implementation="c_graph")
> sage:g.add_edges([(0,0), (0,0)])
> sage:g.edges()
> [(0, 0, None)]
> sage:h=Graph({0: [0,0]}, loops=True, multiedges=True,
> implementation="c_graph")
> sage:h.add_edges([(0,0), (0,0)])
> sage:h.edges()
> [(0, 0, None), (0, 0, None), (0, 0, None), (0, 0, None)]
> }}}
>
> I guess that multiple loops can be handy to have, for example when you
> want to preserve edges in repeated contractions. It is also consistent
> with the meaning of the two options "allow loops" and "allow multiple
> edges" -- it would be weird if "allow multiple edges" stopped work in the
> previously allowed case of (v,v) edge.
>
> But I'm not very experienced mathematician, so I can't decide what
> behavior is right. I just see that the current state is... bizzare... :-)
New description:
Sage handles multiple loops on one vertex inconsistently.
{{{
sage:g=Graph({0:[0,0]}, loops=True, multiedges=True,
implementation="c_graph")
sage:g.edges()
[(0, 0, None), (0, 0, None)]
sage:g.delete_edge(0,0)
sage:g.edges()
[]
}}}
Compare with
{{{
sage:g=Graph({0:[0,0]}, loops=True, multiedges=True,
implementation="networkx")
sage:g.edges()
[(0, 0, None), (0, 0, None)]
sage:g.delete_edge(0,0)
sage:g.edges()
[(0, 0, None)]
}}}
and
{{{
sage:g=Graph({0:[1,1]}, loops=True, multiedges=True,
implementation="c_graph")
sage:g.edges()
[(0, 1, None), (0, 1, None)]
sage:g.delete_edge(0,1)
sage:g.edges()
[(0, 1, None)]
}}}
--
Comment:
OK, this was rather easy.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/12135#comment:2>
Sage <http://www.sagemath.org>
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