IMHO the real bug is that == is group-comparison instead of presentation-comparison. This obviously breaks hashes, and therefor makes associative containers useless. Getting rid of the hashes just means that associative containers are now useless because the word problem is not decidable. At least you just run out of memory instead of getting the wrong result, yay.
On Wednesday, August 12, 2015 at 1:19:06 PM UTC+2, Nathann Cohen wrote: > > Hello everybody, > > I have been playing with groups recently, and fought very hard with my code > before I noticed the usual bug: > > sage: a == b > True > sage: hash(a) == hash(b) > False > > Which has (among others) the following consequence: > > sage: G = groups.presentation.Cyclic(4) > sage: G.cayley_graph().vertices() > [1, a, a^2, a^-2, a^3, a^-1] > > To me, the problem comes from the "default" implementation of __hash__ > that can > be found in sage.structure.element: > > def __hash__(self): > return hash(str(self)) > > Of course, with such a definition it is very likely that "__hash__" and > "__eq__" > will never agree. As I expect that removing this dangerous default > implementation may break a *lot* of things, I created a ticket > (needs_review) > that replaces it with "return 0". > > http://trac.sagemath.org/ticket/19016 > > Nathann > -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-devel+unsubscr...@googlegroups.com. To post to this group, send email to sage-devel@googlegroups.com. Visit this group at http://groups.google.com/group/sage-devel. For more options, visit https://groups.google.com/d/optout.
