Den lördag 11 april 2015 kl. 09:35:22 UTC+2 skrev Steven D'Aprano: > On Sat, 11 Apr 2015 04:08 pm, Paul Rubin wrote: > > > Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> writes: > >> It may be a bit slow for very large numbers. On my computer, this takes > >> 20 seconds: > >> py> pyprimes.factors.factorise(2**111+1) > >> [3, 3, 1777, 3331, 17539, 25781083, 107775231312019L] > > > > This takes about 4 seconds on a Intel(R) Core(TM) i5-3230M CPU @ 2.60GHz > > laptop (64 bit linux): > > Nice for some who have fast machines, but on my old jalopy, your code takes > 110 seconds, more than five times slower than mine. It also returns the > wrong result: > > [3, 3, 3, 3, 7, 19, 73, 87211, 262657, 1.4411518807585587e+17] > > Oops, you have a float in there, how did that happen? > > We can tell your code gives the wrong result. It claims that 7 is a factor > of 2**111+1: > > py> n = 2**111 + 1 > py> itertools.islice(fac(n), 0, 5) > <itertools.islice object at 0xb7c8f914> > py> list(itertools.islice(fac(n), 0, 5)) > [3, 3, 3, 3, 7] > > > but that can't be right: > > py> n % 7 > 2L > > Seven in not a factor of 2**111 + 1. > > The reason your code gives wrong results is that you perform true division / > rather than integer division // which means that you with n a float, which > loses precision: > > py> (n/9) % 3 # nine is a factor > 0.0 > py> (n//9) % 3 # exact, without rounding > 1L > > > > -- > Steven
Love it. -- https://mail.python.org/mailman/listinfo/python-list
