From: Dan Zwell <dzwell@...>
Date: 2007-08-31T17:18:31+09:00
Subject: Re: 0.06 == 0.06 returns false in Ruby?

Daniel DeLorme wrote:
> Michael Ulm wrote:
>>> this floating problem is a faq and is very surprising on such a
>>> very high language such as ruby.  can we address this? maybe create
>>> flag like $EPSILON=0 or something, or may flag to revert to rational
>>> or bigdeci like $FLOAT_PROCESSOR=RATIONAL...
>>
>> Unfortunately, there is no easy solution to this problem. Here is a
>> catalog of often proposed solutions and why they do not work:
>>
>> 2 Always use  (x-y).abs < (x.abs + y.abs) * Float::EPSILON)  as a test 
>> for equality.
>>
>>  Better than the first proposal, but won't work if the rounding error
>>  gets too large after a complex computation.
>>  In addition, (1) and (2) suffer from the problem that x==y and y==z do
>>  not imply x==z.
> 
> But it would fix 99% of problems. It would be worth it just for the sake 
> of reducing those questions on the list :-P
> 
> Seriously though, since floating point calculations are approximative to 
> start with, what would be wrong with making them more intuitive 
> approximations? IMHO Float should use the above algorithm for == and 
> reserve normal floating point arithmetic for eql?
> 
> Daniel
> 
> 

Michael wrote very convincingly that there is no simple solution that 
will work in all cases. I'm convinced, at least. If there is no solution 
that works 100% of the time, we can't give the illusion that there is. 
To do so would be to teach bad programming practices to newcomers, and 
that's not fair.

The current "==" in okay because it works the way a moderately 
experienced programmer would expect. A perfect "==" that could deal with 
floats would be even better, but we aren't gonna get that. A "==" that 
seems like magic and almost always works is really pretty dangerous.

Dan