From: Benoit Daloze <eregontp@...>
Date: 2009-12-18T17:00:21+09:00
Subject: Re: Math errors

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IN 1.9.2 I got:

~ $> ruby -ve 'p v=Math.cbrt(-27); p v**3'
ruby 1.9.2dev (2009-12-11 trunk 26067) [x86_64-darwin10.2.0]
-3.0
-27.0

~ $> ruby -ve 'p v=(-27)**(1/3.0); p v**3'
(1.5000000000000004+2.598076211353316i)
(-27.000000000000007+1.2434497875801753e-14i)

~ $> ruby -ve 'p v=(-27)**(Rational(1,3)); p v**3'
(1.5+2.5980762113533156i)
(-26.99999999999999+3.552713678800501e-15i)

Got easy results, but with all different values ...
So, the first is right, of course.
The second is a good approximation, for floats ...
And the last is almost good, while I would expect to be exact (I think it
should use Math.cbrt in this case)

I would probably interested to use BigDecimal, but it's impossible I think
to use it for Complex. (And actually it accept only Fixnum exponent)

Another error in the exception:
TypeError: wrong argument type BigDecimal (expected Fixnum)

But it accept Float, really ;)

2009/12/18 Tanaka Akira <akr@fsij.org>

> 2009/12/18 jzakiya <jzakiya@mail.com>:
> > (-3)**3 => -27
> >
> > (-27)**(1/3.0)  give NaN instead of -3
>
> Ruby 1.9 has Math.cbrt.
>
> % ruby -e 'p Math.cbrt(-27)'
> -3.0
> --
> Tanaka Akira
>
>

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