From: "Wolfgang Nádasi-Donner" <wonado@...>
Date: 2007-01-20T05:20:49+09:00
Subject: Re: Negative Float conversion to Fixnum

Drew Raines schrieb:
> I'm sure this has been hashed out somewhere on the list before, but
> my searches have turned up fruitless.  I had a strange issue come up
> while diagnosing a unit test failure.
> 
> Notice the first dialog compared with the rest of this IRB session.
> What causes -3930.0 to be converted to a Fixnum as -3929 only after
> it's been multiplied by 100 or 100.0?  Even weirder is that it seems
> to only be *this* number.  Can I workaround this?
> 
>   >> (-39.30 * 100).to_i
>   => -3929
>   >> (-39.30 * 100.0).to_i
>   => -3929
>   >> -3930.0.to_i
>   => -3930
>   >> (-39.31 * 100).to_i
>   => -3931
>   >> (-9999.99 * 100).to_i
>   => -999999
>   $ ruby -v
>   ruby 1.8.5 (2006-12-04 patchlevel 2) [i686-linux]
> 
> Thanks.
> 
> -Drew

irb(main):001:0> 3930-39.30*100
=> 4.54747350886464e-013

Rounding errors are typical for floating point numbers. In this case the result 
of -39.30*100 ist a little bit smaller than 3930, so to_i works correct.