PS the slow behaviour also happens with mat.change_ring() since the key line there is mat = M(self.list(), coerce=True, copy=False) where M is the newly created matrix space. Though here, self.list() is a flat list of entries.
On Thursday, December 6, 2012 9:08:40 AM UTC, John Cremona wrote: > > > > On Wednesday, December 5, 2012 11:24:44 PM UTC, Nils Bruin wrote: >> >> >> >> On Wednesday, December 5, 2012 2:59:59 PM UTC-8, Maarten Derickx wrote: >>> >>> >>> Maybe we should overwrite the sum() function such that it behaves >>>> different for lists, since the command sum(entries,[]) looks much more >>>> clear and intuitive then the for loop. >>>> >>> >>> It seems like the top level sum in sage is already optimized by doing >>> some binary tree balances sum stuff, so maybe just importing that sum in >>> the cyclotomic field code should also fix this performance issue. >>> >> >> No, list concatenation does not benefit from balanced trees. >> sum(entries,[]) should simply be spelled >> L=[] >> L.extend(itertools.chain(*entries)) >> >> You could put that as a special case in sum, but I'd hesitate to do that. >> Most python docs warn against using '+' on lists. >> >> Sometimes it sucks that python has an aversion to being a properly >> functional language. >> >> However, since at this point the expected lengths of all objects involved >> are known anyway, I don't think it should be necessary to construct any of >> these intermediate data structures. Just create the matrices to be >> initialized, loop through the list and write the entries into the right >> spots. >> > > > This has been a very interesting discussion. In my code I have many lists > of lists and often use sum(...,[]) to concatenate them. Now I know not to > do that (at least when the lists are long ot complicated). > > In fact most of the time I first do sum(list-of-lists) and then get the > error message about not being able to add a list to 0, at which point I > insert ",[]", but in future that will serve as a reminder to me to use > something else (I like Nils's itertools solution!) > > John > > -- You received this message because you are subscribed to the Google Groups "sage-support" group. 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. Visit this group at http://groups.google.com/group/sage-support?hl=en.
