Unexpected results comparing float to Fraction
Steven D'Aprano
steve+comp.lang.python at pearwood.info
Mon Jul 29 15:32:38 EDT 2013
On Mon, 29 Jul 2013 17:20:27 +0100, Chris Angelico wrote:
>>>> Fraction(0.3)
> Fraction(5404319552844595, 18014398509481984)
>>>> Fraction(0.33)
> Fraction(5944751508129055, 18014398509481984)
[...]
>>>> Fraction(0.33333333333333333)
> Fraction(6004799503160661, 18014398509481984)
>
> Rounding off like that is a job for a cool library function (one of
> which was mentioned on this list a little while ago, I believe), but not
> IMO for the Fraction constructor.
Or:
py> Fraction(1/3).limit_denominator(100)
Fraction(1, 3)
I think it would be useful for Fraction.from_float() to accept an
optional second argument, the maximum denominator:
Fraction.from_float(x, den=None)
=> nearest fraction to float 1/3, with denominator no greater than den
--
Steven
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