[Haskell-beginners] rational exponents
Daniel Fischer
daniel.is.fischer at googlemail.com
Wed Sep 7 23:27:19 CEST 2011
On Wednesday 07 September 2011, 22:38:21, Brandon Allbery wrote:
> On Wed, Sep 7, 2011 at 16:25, Christopher Howard <
>
> christopher.howard at frigidcode.com> wrote:
> > Hi. I'm working with simple functions involving rational exponents. I
> > noticed that the (**) function seems to do okay with negative powers,
> > but
>
> > that something else is needed for rational exponents:
> Nothing else is needed; you're just seeing the inevitable failure mode
> of floating point math (once you get into exponents that aren't
> integers, you can't escape it).
One could have an exact
(??) :: Rational -> Rational -> Maybe Rational
so that (r % s) ?? (p % q) = Just (n % d) if r^p == n^q && s^p == d^q,
Nothing otherwise (if r < 0, p even, q odd, then n has to be chosen
negative).
But when dealing with floating point numbers, you can only get the
occasional correct result by accident.
> You may want to restrict printing
> precision.
Or round to k bits.
Or write a function
rationalPower :: Floating a => a -> Rational -> a
which calculates the power e.g. by Newton's method. That would still give
incorrect results most of the time, but could produce exact results in
cases where the exact result is representable and all involved numbers are
such that no overflow occurs.
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