From: Alex Young <alex@...>
Date: 2007-11-27T18:38:27+09:00
Subject: Re: Bizarre Floating point errors in Ruby? Serious bug?

space.ship.traveller@gmail.com wrote:
>> Float#to_s is doing the formatting. If you want it like python, do it
>> like this:
>>
>> class Float
>>   def to_s
>>     "%.12f" % self
>>   end
>> end
>>
>> (-140.14 * 100)
>> # => -14013.999999999998
>>
>> Regards,
>> Jordan
> 
> Thanks for this useful information. After further investigation,
> Python also has the same problem with its pretty printing:
> 
>>>> print (-140.14 * 100)
> -14014.0
> 
> Well, actually this is the correct answer.... i.e. the result we are
> looking for if we were using real math.  But, it is not the correct
> way to round 14013.999999, which is the actual value that we get with
> floating point math... so I guess it is hard to pick what is the ideal
> behaviour... both have there pros and cons
> 
> Rounding 9.9 to 10.0 is like rounding 99 to 100, but we consider that
> only 1 significant figure is important in 100, ie 1xx. So, we are left
> with 10.0 with 1SF, but .0 conveys the idea of 1DP correctness, which
> is definitely not correct.
> 
> I'd need to consult a mathematician, but I still don't think the Ruby
> behaviour is correct, mathematically. I'll post here again once I have
> more information about the mathematical correctness of this issue.

#to_i doesn't round, it truncates.  If you want to round, use #round.

-- 
Alex