#13239: Contraction of edges in a graph.
------------------------------------------------+---------------------------
Reporter: lkeough | Owner: jason,
ncohen, rlm
Type: enhancement | Status: needs_info
Priority: major | Milestone: sage-5.3
Component: graph theory | Resolution:
Keywords: SD40 | Work issues:
Report Upstream: N/A | Reviewers: Marshall
Hampton, David Coudert
Authors: Lauren Keough, Jeremy Martin | Merged in:
Dependencies: | Stopgaps:
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Comment (by lkeough):
Replying to [comment:4 dcoudert]:
> Hello,
>
> I don't understand this new patch. If the behavior of the contract_edge
function proposed in patch #7304 is not adapted to your purpose, you could
add extra parameters (inplace = False, keep_multiedges, etc.)
#7304 will keep multiedges if you do G.allow_multiedges . However, we also
need to do G.allow_loops (for the tutte polynomial), but if we do that
then the patch leaves the contracted edge (rather than deleting it which
is what we want). I suppose I could use their code and then add a line to
mine that deletes that loop.
I'm not really sure what the best option is here. 7304 already has a lot
of parameters and adding another might be very confusing. And I'm also
not sure what most people mean by contracting an edge - I thought the
definition I have written in this code was "the" definition, but it's
becoming more apparent that it isn't at all! Perhaps I could just include
this code in the Tutte polynomial code and not as a separate function?
I'm very open to advice on this!
> Furthermore, it works only for undirected graphs. So it should at least
be in graph.py and not in generic_graph.py.
Okay, I will definitely move to graph.py unless I can figure out how to
make it work for digraphs.
> In the following example, arc (3,0) becomes arc (0,3)
> {{{
> sage: D = digraphs.RandomDirectedGNP(5,.5)
> sage: h = D.contraction((0,1))
> sage: D.edges(labels=None)
> [(0, 1), (0, 2), (0, 3), (1, 0), (1, 3), (1, 4), (2, 0), (3, 0), (3, 1),
(3, 4), (4, 2)]
> sage: h.edges(labels=None)
> [(0, 0), (0, 2), (0, 2), (0, 3), (0, 3), (0, 3), (0, 3), (0, 4), (2, 4),
(3, 4)]
> }}}
>
> D.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/13239#comment:5>
Sage <http://www.sagemath.org>
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