On Sun, Jul 25, 2010 at 2:01 PM, Nathann Cohen <nathann.co...@gmail.com> wrote:
> Nonononoooo, I understood why there are two copies of what appears to
> be a "zero", and I think it's fine like that !
This is definitely *not* fine, since we have
sage: int(0) == Mod(0, 20)
True
As input, the underlying C graph ends up corrupted (see below for
details). Since they are equal, Sage's graphs should treat them as
equal and they do not. Bug.
To belabor the point for clarity:
When you call g.vertices(), it calls iterator_verts in c_graph.pyx:
{{{
cdef int i
cdef object v
if verts is None:
S = set(self.vertex_ints.iterkeys())
for i from 0 <= i < (<CGraph>self._cg).active_vertices.size:
if (i not in self.vertex_labels and
bitset_in((<CGraph>self._cg).active_vertices, i)):
S.add(i)
return iter(S)
}}}
At this point we have:
{{{
active_vertices == 1111111111111111111110000000000000000000
vertex_ints == {0: 20, 1: 1, 2: 2, 3: 3, 4: 4, 5: 5, 6: 6, 7: 7, 8: 8,
9: 9, 10: 10, 11: 11, 12: 12, 13: 13, 14: 14, 15: 15, 16: 16, 17: 17,
18: 18, 19: 19}
vertex_labels == {1: 1, 2: 2, 3: 3, 4: 4, 5: 5, 6: 6, 7: 7, 8: 8, 9:
9, 10: 10, 11: 11, 12: 12, 13: 13, 14: 14, 15: 15, 16: 16, 17: 17, 18:
18, 19: 19, 20: 0}
}}}
Note that there are 21 bits in active_vertices, so C int 0 and C int
20 are both vertices according to that. That first zero in vertex_ints
is an IntegerMod, and the 20 it points to is a C int. Vice versa for
the 20: 0 entry in vertex_labels.
At first, the line {{{ S = set(self.vertex_ints.iterkeys()) }}}
creates a set of size twenty, containing twenty IntegerMod's, the set
{0, ..., 19}.
When i == 0 in the loop {{{ for i from 0 <= i <
(<CGraph>self._cg).active_vertices.size }}}, the inner branch
discovers that 0 should be added to S, but since S is a set, and
Mod(0, 20) is already in there, adding int(0) to it does nothing.
So as I said before, the problem is created by line 1084 (in
sage-4.5/4.5.1) of c_graph.pyx, namely:
{{{ if (not isinstance(u, (int, long, Integer)) or }}}
In fact this should check for any Sage object X for which {{{
Integer(Y) == X }}} is true for any Y.
I hope this better illustrates the problem, and I hope I've convinced
you that int(0) and Mod(0, 20) should *not* be considered distinct by
Sage graphs.
>
> Sorry for this misunderstanding :-)
>
> Nathann
>
A: "Stop apologizing!"
B: "I'm sorry!"
--
Robert L. Miller
http://www.rlmiller.org/
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