From: Francis Cianfrocca <garbagecat10@...>
Date: 2006-05-18T06:40:50+09:00
Subject: Re: greatest float smaller than 1.0?

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Floating point libraries are really weird and platform-dependent, especiall=
y
in regard to subtraction, which doesn't behave very well between numbers
that are many orders of magnitude apart. What happens on your hardware if
you do 1.0 + Float::EPSILON?
Again, it seems to me that the OP might want to look for a more portable wa=
y
to solve his problem.

On 5/17/06, Joel VanderWerf <vjoel@path.berkeley.edu> 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"
> =3D> "?\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 =3D ("?\357" + "\377"*100).unpack "G"
> =3D> [1.0]
> irb(main):013:0> "%30f" % x
> =3D> "0.999999999999999888977697537484"
>
> Dunno if that's safe on all platforms...
>
> Incidentally, something is wrong with Float::EPSILON, which Mauricio
> suggested and should (AFAIK) work:
>
> irb(main):016:0* f =3D (1.0 - Float::EPSILON)
> =3D> 1.0
> irb(main):017:0> "%.20f" % f
> =3D> "0.99999999999999977796"
> irb(main):019:0> x[0]-f
> =3D> 1.11022302462516e-16
>
> Whaa???
>
> --
>       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407
>
>

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