From: Jason Roelofs <jameskilton@...>
Date: 2007-12-05T23:31:26+09:00
Subject: Re: what's wrong with these calculations?

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On Dec 5, 2007 9:15 AM, Phrogz <phrogz@mac.com> wrote:

> On Dec 5, 4:54 am, hemant <gethem...@gmail.com> wrote:
> > On Dec 5, 2007 5:22 PM, hemant <gethem...@gmail.com> wrote:
> >
> >
> >
> > > On Dec 5, 2007 5:00 PM, Ivo Dancet <caifara.subscr...@gmail.com>
> wrote:
> > > > Hi
> >
> > > > I thought, why search for two hours being totally frustrated if I
> can
> > > > just ask...
> >
> > > > why does this happen:
> >
> > > >  >> 73.07-63.00
> > > > => 10.07 # ok, normal!
> > > >  >> 73.07-64.00
> > > > => 9.06999999999999 # why oh why?
> >
> > > Thats folly of floats, you can't do much apart from :
> >
> > > sprintf("%.2f",(73.07-64.00)).to_f
> >
> > generally, I add a method to Float class like this:
> >
> > class Float
> >   def r2p places
> >     sprintf("%.#{places}f",self).to_f
> >   end
> > end
>
> Note that, due to the nature of floats, calling to_f on that string
> brings you back into the problem again:
>
> x = ( 73.07 - 64.00 )
> y = x.r2p( 2 )
> p x, y, "%.20f" % y
>
> #=> 9.06999999999999
> #=> 9.07
> #=> "9.07000000000000028422"
>
>
require 'bigdecimal'
require 'bigdecimal/math'
include BigMath

(BigDecimal("73.07") - BigDecimal("64.00")).to_f   #=> 9.07

http://www.ruby-doc.org/stdlib/libdoc/bigdecimal/rdoc/index.html

Jason

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