In article <02204669$0$25303$c3e8...@news.astraweb.com>, Steven D'Aprano <st...@remove-this-cybersource.com.au> wrote: >On Sun, 17 May 2009 18:24:34 +0200, Diez B. Roggisch wrote: > >>> But reduce()? I can't see how you can parallelize reduce(). By its >>> nature, it has to run sequentially: it can't operate on the nth item >>> until it is operated on the (n-1)th item. >> >> That depends on the operation in question. Addition for example would >> work. > >You'd think so, but you'd be wrong. You can't assume addition is always >commutative. > >>>> reduce(operator.add, (1.0, 1e57, -1e57)) >0.0 >>>> reduce(operator.add, (1e57, -1e57, 1.0)) >1.0 > > > >> My math-skills are a bit too rusty to qualify the exact nature of >> the operation, commutativity springs to my mind. > >And how is reduce() supposed to know whether or not some arbitrary >function is commutative?
Why would it or need it? A Python that understands the ``par'' keyword is supposed to know it can play some tricks with optimizing reduce() if the specific function is commutative. >-- >Steven Groetjes Albert -- -- Albert van der Horst, UTRECHT,THE NETHERLANDS Economic growth -- like all pyramid schemes -- ultimately falters. alb...@spe&ar&c.xs4all.nl &=n http://home.hccnet.nl/a.w.m.van.der.horst -- http://mail.python.org/mailman/listinfo/python-list
