#12587: simplicial complexes lack hash function
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Reporter: vpilaud | Owner: sage-combinat
Type: defect | Status: needs_review
Priority: major | Milestone: sage-5.4
Component: combinatorics | Resolution:
Keywords: | Work issues:
Report Upstream: N/A | Reviewers:
Authors: Travis Scrimshaw | Merged in:
Dependencies: | Stopgaps:
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Comment (by tscrim):
Replying to [comment:20 stumpc5]:
> Since you propose to make the hash only dependent on the facets:
> {{{
> sage: S = SimplicialComplex([1,2,3],[[1,2]])
> sage: S
> Simplicial complex with vertex set (1, 2, 3) and facets {(1, 2)}
> }}}
> ({{{[1,2,3]}}} is actually not the vertex set, but the *ground set*,
while the vertex set is {{{[1,2]}}}.) The proposed implementation only
takes the facets into account for hashing, thus we get the same hash for
> {{{
> sage: S = SimplicialComplex([1,2],[[1,2]])
> sage: S
> Simplicial complex with vertex set (1, 2) and facets {(1, 2)}
> }}}
>
> Is this the desired behaviour - or should the ground set be used as well
for hashing?
>
> (I don't have a strong opinion on this question, but I thought I bring
it up anyway. If you don't want to take care of the vertex set vs. ground
set issue in this ticket here, I will open another and ask for other
peoples opinion there.)
There are a few possibilities:
1. This is a bug, in that `(3,)` should also be listed as a facet.
2. We handle this more like a matroid (I believe this is what you're
suggesting) and add methods like `ground_set()`.
3. Split the difference and make an optional argument.
4. We just do a stopgap here and make the hash depend upon the
vertex/ground set and push this to another ticket.
My thought is we handle this as a bug, but I'm happy to implement
whichever is decided upon. I have no strong opinions either.
Best,[[BR]]
Travis
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/12587#comment:21>
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