On Mon, May 4, 2009 at 11:01 AM, Robert Bradshaw <rober...@math.washington.edu> wrote: > > On May 4, 2009, at 10:53 AM, William Stein wrote: > >> On Mon, May 4, 2009 at 10:45 AM, kcrisman <kcris...@gmail.com> wrote: >>> >>> Dear support, >>> >>> I assume this is known, but I am wondering whether it should be >>> treated as a bug, >> >> This is not a bug. It's a stupid design decision in Python, which we >> have to live with until we switch to Python 3.0 or switch to doing >> "from __future__ import division": >> >> sage: from __future__ import division >> sage: len([2,2])/len([2,3,4]) >> 0.66666666666666663 > > And I'm not a fan of this behavior either, but in many ways it's less > surprising than 0.
I also don't like it either, but it is *massively* better than getting 0. > >> >> >>> or whether someone using len() on lists should be >>> assumed to know it might then be operated on with Python /, not >>> Sage /, as opposed to the preparser catching this sort of thing. >>> >>> sage: len([2,2])/len([2,3,4]) >>> 0 >>> >>> Thanks for any suggestions on what to do with this - right now I have >>> to do >>> >>> sage: Integer(len([2,2]))/Integer(len([2,3,4])) >>> 2/3 >> >> Trust me, I understand that Python's int floor division sucks. I'm >> teaching undergrads about stats using Sage now, and the most obvious >> line of code to compute the mean of a list gets the answer totally >> wrong because of this problem. This already caused a lot of >> confusion. >> >> This is definitely not something that should be addressed by the >> preparser. It could be addressed by rewriting len, but I'm very >> hesitant to do that, because it will introduce subtle bugs when moving >> code from preparsed to the library (.py files). >> The way one might rewrite len would be: >> >> sage: import __builtin__ >> sage: len = lambda x: Integer(__builtin__.len(x)) >> sage: len([2,2])/len([2,3,4]) >> 2/3 > > Good point, I hadn't though about that. We could introduce a size() > or cardinality() method that returns an Integer, or possibly infinity. We could also redefine len. Can you think of any problems this will cause *besides* when moving code from .sage preparsed files to Python in the Sage library? I can't think of any. -- William --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to sage-support@googlegroups.com To unsubscribe from this group, send email to sage-support-unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sage-support URLs: http://www.sagemath.org -~----------~----~----~----~------~----~------~--~---
