From: Stefan Rusterholz <apeiros@...>
Date: 2007-09-04T11:02:28+09:00
Subject: Re: wierd floating point output

William James wrote:
> On Sep 3, 5:32 pm, Stefan Rusterholz <apei...@gmx.net> wrote:
>> >> using delta comparison anyway too.
>> end
>>
>> (0.01 + 0.05).in_delta?(0.06) # => true
>> 0.01 + 0.05 =~ 0.06 # => true # can't set the delta, is similarly evil
>> as ==
> 
> class Numeric
>   def in_delta?(other, delta=Float::EPSILON*16) # chose the delta
> wisely
>     (self-other).abs < delta
>   end
>   alias =~ in_delta?
> end
> 
> p 1.23456789e4.in_delta?( 1.234567892e4 )
> p 1.23456789e-1.in_delta?( 1.234567892e-1 )
> p 1.23456789e-6.in_delta?( 1.234567892e-6 )
> p 1.23456789e-11.in_delta?( 1.234567892e-11 )
> 
> ==== output ====
> false
> false
> true
> true

That's why, chose your delta wisely.  It is also why the =~ is only 
marginally less surprising than == and still evil.
Maybe one could improve it a bit using a delta created using the 
magnitude (hello, dear log). Another approach might be to use the 
quotient of the two numbers (its proximity to 1).
With both suggestions I have no idea about hazardeous situations, maybe 
somebody with either more recent CS training or daily use of floats can 
shed some light on that. Also it is too late at night to have clear 
thoughts about that.

Regards
Stefan
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