Chris Angelico wrote: > On Sun, Sep 25, 2011 at 3:01 AM, Steven D'Aprano > <steve+comp.lang.pyt...@pearwood.info> wrote: >> Mark Dickinson wrote: >> >>> Using Fraction for intermediate calculations actually works perfectly >>> for this, since conversions from float to Fraction are exact, while >>> conversions from Fraction to float are correctly rounded. So if >>> you're using Python, you're not too bothered about efficiency, and you >>> want provably correctly-rounded results, why not use Fraction? >>> >> Ah, I knew it was too easy! > > Try using Fraction for the start and stop too:
If you look again at the code I used, I did. I just did it inside the list comp. >>>> from fractions import Fraction as F >>>> start,stop,n = F(0),F(21,10),7 >>>> [float(start+i*(stop-start)/n) for i in range(n+1)] > [0.0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1] >>>> [float(start+i*(stop-start)/n) for i in range(n+1)] > [-1.0, -0.7, -0.4, -0.1, 0.2, 0.5, 0.8, 1.1] Something looks a tad suspicious here... the two list comps are identical, but give completely different results, even though you don't re-assign start and stop between calls. You're not editing your results are you? <wink> But seriously, you were freaking me out there for a bit. I couldn't see why pulling the conversion to fraction outside of the list comp was giving different results. And then it hit me... >>> 2.1 == F(21, 10) False You're testing different numbers from me. Try again with F(2.1) as the stop value. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
