On Tue, Aug 3, 2010 at 10:33 AM, Robert Miller <r...@rlmiller.org> wrote: > On Sun, Jul 25, 2010 at 6:50 PM, Robert Miller <r...@rlmiller.org> wrote: >> On Sun, Jul 25, 2010 at 8:10 PM, Carl Witty <carl.wi...@gmail.com> wrote: >>> You seem to want to make the vertex dictionary respect the equivalence >>> relation defined by Sage equality. If so, you're going to be in >>> trouble, since Sage equality actually is not an equivalence relation: > > It gets worse: > > sage: a = Set([1,2,3]) > sage: b = Set([2,3,4]) > sage: a < b > True > sage: b > a > False > > sage: b < a > True > sage: a > b > False > > Let's take a look at Sage's implementation of comparison: > > sage: a.__cmp__?? > ... > if isinstance(other, Set_object_enumerated): > if self.set() == other.set(): > return 0 > return -1 > else: > return Set_object.__cmp__(self, other) > > Aduuhhhh! If it is not true that a == b, then a < b. Not good. > > I would like to ask, since obviously nobody cares about doing > comparisons with Sage sets at the moment... We obviously need to > define what it means for a < b and a > b for Sage Sets. Does anyone > mind if we do this so that instead of < meaning subset, and getting a > bad ordering with respect to sorting lists, can we make < give a > lexicographic ordering? I will gladly do all the work, including > implementing an is_subset method.
+1 It makes no sense for < to mean "subset" because < should be a total order. If you want to check for subsets we should use a method like in python: sage: a = set([1,2,3]) sage: b = set([2,3,4]) sage: a.issubset(b) False sage: b.issubset(a) False sage: a.issubset(b.union(a)) True I don't like "issubset" as the method name, and would prefer "is_subset", personally. > > That way, if you like working with sets themselves and you want < to > mean subset, simply use Python sets. Yep. > Or if you are Chris Godsil, e.g., > and like to be able to consistently sort sets because you use them as > vertices for graphs, use Sage Sets. > > I think this is the best solution we can get to this problem... William -- To post to this group, send an email to sage-devel@googlegroups.com To unsubscribe from this group, send an email to sage-devel+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sage-devel URL: http://www.sagemath.org
