Colin Law wrote in post #970496:
> On 24 December 2010 14:25, Marnen Laibow-Koser <li...@ruby-forum.com>
> wrote:
>>>>>
>>> => #<BigDecimal:ad2b270,'0.99999999E0',8(20)>
>>> ruby-1.9.2-p0 > x*BigDecimal('3') == 1
>>> => false
>>> ruby-1.9.2-p0 > x*BigDecimal('3') - 1
>>> => #<BigDecimal:aa4fdf8,'-0.1E-7',4(20)>
>>>
>>
>>
>> Interesting. However, the BigDecimal constructor allows specification
>> of the minimum number of significant figures, so in this case we could
>> have done BigDecimal.new('1', 16).
>
> ruby-1.9.2-p0 > x = BigDecimal.new('1', 16)/BigDecimal.new('3',16)
> => #<BigDecimal:a146bbc,'0.33333333E0',8(16)>
> ruby-1.9.2-p0 > x*BigDecimal('3',16) - 1
> => #<BigDecimal:a114acc,'-0.1E-7',4(20)>
>
> It doesn't make any difference how many digits you have, 1/3 as a
> decimal number multiplied by 3 will never be equal to 1.
I'm aware of that, of course. In this particular case, we could use a
Rational to fix that. But that won't necessarily work with arbitrary
floating-point numbers...
...unless we recast them as Rationals and do all math that way. Hmm.
>
> I am not sure why the above still gives only 8 digits accuracy
> however, I would have thought it aught to be 16.
Yeah, that's peculiar. I'll have to run some tests.
(I can almost guarantee that Rubinius would get that one right, since I
wrote most of the BigDecimal library at one time. Don't know if they're
still using it.)
>
> Colin
Best,
--
Marnen Laibow-Koser
http://www.marnen.org
mar...@marnen.org
Sent from my iPhone
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