On 29/07/2013 17:40, Ian Kelly wrote:
On Mon, Jul 29, 2013 at 10:20 AM, Chris Angelico <ros...@gmail.com> wrote:
On Mon, Jul 29, 2013 at 5:09 PM, MRAB <pyt...@mrabarnett.plus.com> wrote:
I'm surprised that Fraction(1/3) != Fraction(1, 3); after all, floats
are approximate anyway, and the float value 1/3 is more likely to be
Fraction(1, 3) than Fraction(6004799503160661, 18014398509481984).
At what point should it become Fraction(1, 3)?
At the point where the float is exactly equal to the value you get
from the floating-point division 1/3. If it's some other float then
the user didn't get there by entering 1/3, so it's not worth trying to
pretend that they did.
I thought that you're not meant to check for equality when using floats.
We do a similar rounding when formatting floats to strings, but in
that case one only has to worry about divisors that are powers of 10.
I imagine it's going to take more time to find the correct fraction
when any pair of relatively prime integers can be a candidate
numerator and denominator. Additionally, the string rounding only
occurs when the float is being formatted for display; we certainly
don't do it as the result of numeric operations where it could result
in loss of precision.
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