I'm looking at Trac #6823 which implements the Kneser graph. The
vertices of this graph are the k-element subsets of an n-element set,
built by the call graphs.KneserGraph(n,k). By and large the patch
seems fine, but I'm seeing the behavior below with regard to sorted
lists of sets, which I do not think is limited to anything in
particular that the patch is doing.
For any graph, vertices() is suppose to return its results sorted.
I get
sage: g = Graph()
sage: g.add_vertices(Subsets(3,2))
sage: g.vertices()
[{2, 3}, {1, 2}, {1, 3}]
sage: sorted(g.vertices())
[{1, 3}, {1, 2}, {2, 3}]
sage: Subsets(3,2).list()
[{1, 2}, {1, 3}, {2, 3}]
sage: sorted(Subsets(3,2).list())
[{2, 3}, {1, 3}, {1, 2}]
Either of the last two lists I'd say are sorted. I'd expect the first
two lists to be equal to each other, and then also equal to one or the
other of the last two. Is this all to be expected, or are the
vertices of the graph getting mangled (hashed?) somewhere? In the
creation of the Kneser graph using the patch, the vertices are created
via calls to Subsets() and I get identical results for the first two
lists when I use graphs.KneserGraph(3,2) instead of the more generic
g demonstrated above.
Rob
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