Re: [boost] Floating point truncation bug in qi::float_

Subject: Re: [boost] Floating point truncation bug in qi::float_
From: Jeffrey Lee Hellrung, Jr. (jhellrung_at_[hidden])
Date: 2010-09-28 17:25:59

On 9/28/2010 1:12 PM, Henry Tan wrote:
> Hi I am using boost 1.4.1 Spirit::Qi.
>
> I think we have a floating point truncation bug in the qi::float_ parser.
> I attached a simple program to repro the bug.
>
> Below are some examples of an input floating point gets truncated (=>) by
> the qi::float_:
>
> 12312321421421 =>  12312321720320.000000
> 123123214   =>  123123216.000000
> 123233.4124 =>  123233.406250
>
> I don't see the same issue with the qi::double_.
>
> Wondering if this is a known issue?
>
> Regards,
>
> HTan

Without being too familiar with the qi::float_ parser, I would guess it 
rounds rather than truncates.

Keep in mind that floats are typically 32-bit single precision [1], 
hence have only 24 significant bits (a bit more than 7 significant 
decimal digits), which is consistent with your results.

On the other hand, doubles are typically 64-bit double precision [2], 
hence have 53 significant bits (a bit under 16 significant decimal 
digits), so the first 2 inputs above would be stored exactly, and the 
3rd one would be pretty close.

- Jeff

[1] http://en.wikipedia.org/wiki/Single_precision_floating-point_format
[2] http://en.wikipedia.org/wiki/Double_precision_floating-point_format