From: space.ship.traveller@...
Date: 2007-11-27T16:50:02+09:00
Subject: Re: Bizarre Floating point errors in Ruby? Serious bug?


> 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.

Thanks for your great comments,
Samuel