From: Joel VanderWerf <vjoel@...>
Date: 2006-05-18T06:52:18+09:00
Subject: Re: greatest float smaller than 1.0?

Joel VanderWerf wrote:
> polypus wrote:
>> how do i get the greatest float smaller than 1.0?
>>
>> thanks
>>
> 
> Something like this maybe?
> 
> irb(main):006:0> [0.9999999999999999].pack "G"
> => "?\357\377\377\377\377\377\377"
> 
> (that was just to find out what floats look like in memory, big-endian).
> 
> irb(main):012:0> x = ("?\357" + "\377"*100).unpack "G"
> => [1.0]
> irb(main):013:0> "%30f" % x
> => "0.999999999999999888977697537484"
> 
> Dunno if that's safe on all platforms...

That was completely wrong, please ignore.

irb(main):059:0> y = 1e-305
=> 1.0e-305
irb(main):060:0> [y].pack "G"
=> "\000\234\026\305\305%5t"
irb(main):061:0> [y].pack("G").unpack("G")
=> [1.0e-305]

It's the exponent that needs to be chosen, not just the mantissa.

Still, it leaves the question open why Float::EPSILON is nowhere near
this exponent.

-- 
      vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407