From: Konrad Meyer <konrad@...>
Date: 2007-08-10T06:12:14+09:00
Subject: Re: precision loss with pack/unpack ?

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On Thursday 09 August 2007 12:06:31 pm Luis Parravicini wrote:
> Hello
>=20
> I'm reading Java class files using ruby and found some problems when
> handling float values. The numbers are stored using IEEE 754 format.
> After several hours trying and thinking I might be using unpack the
> wrong way, I've wrote the following code which packs a Float and then
> immediately unpacks it. And the result is different from the original
> value.

Yes, small differences are common with floats. The best way to compare them
is with something like that demonstrated in:=20
http://www.cygnus-software.com/papers/comparingfloats/Comparing%20floating%=
20point%20numbers.htm#_Toc135149455

> ------------
> a =3D 0.1234567
> b =3D [a].pack('g').unpack('g')[0]
>=20
> puts "#{a} =3D? #{b} =3D> #{a =3D=3D b}"
> ------------
>=20
> outputs:
>=20
> 0.1234567 =3D? 0.123456701636314 =3D> false
>=20
> I came accross this doc, which states that in Perl this happens too
> (http://perldoc.perl.org/functions/pack.html) "Note that Perl uses
> doubles internally for all numeric calculation, and converting from
> double into float and thence back to double again will lose precision
> (i.e., unpack("f", pack("f", $foo)) will not in general equal $foo)."
> Maybe Ruby is doing the same Perl does?

Ruby uses doubles internally for floats as well. Loss of precision is
common, but the above link should work well for you.

> Any help will be appreciated
>=20
> Thanks

No problem, glad to (try to) help.

=2D-=20
Konrad Meyer <konrad@tylerc.org> http://konrad.sobertillnoon.com/

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