RE: [boost] Re: Floating Point comparisons

From: Paul A. Bristow (boost_at_[hidden])
Date: 2001-12-22 12:45:54

Is it simply that the calculation

|d3 - 11| / 11

also has some roundoff opportunities?

Or is this too naive a suggestion?

Paul

Dr Paul A Bristow, hetp Chromatography
Prizet Farmhouse
Kendal, Cumbria
LA8 8AB      UK
[phone]
Mobile [phone]
mailto:pbristow_at_[hidden]

> -----Original Message-----
> From: rogeeff [mailto:rogeeff_at_[hidden]]
> Sent: Wednesday, December 19, 2001 8:53 AM
> To: boost_at_[hidden]
> Subject: [boost] Re: Floating Point comparisons
>
>
> Hi,
>
> After some clarification I still got one confusing example:
>
> #include <iostream>
> #include <limits>
>
>
> int main()
> {
>     double d = 11;
>     d /= 10;
>     d = d*d - d;
>
>     d *= 100;
>
>     std::cout << ((d - 11)/ d / std::numeric_limits<double>::epsilon
> () ) << std::endl;
>
>     return 0;
> }
>
> Intent is to calculate some value and comapre it with the real value.
>
> so I did:
>
> d1 = 11/10          - 1 rounding error
> d2 = d1*d1 - d1     - 2 rounding errors
> d3 = d2 * 100       - 1 rounding error
>
> The resulting value should be again 11. Now I calculate relative
> rounding error between resulting value and real value. It expected
> do not exceed (1+2+1)*1/2 * epsilon = 2 * epsilon. So expected that:
>
> |d3 - 11| / 11 <= 2* epsilon
>
> or
>
> |d3 - 11| / 11 / epsilon <= 2
>
> While I am getting from MSVC is ~4.
>
> What could it be?
>
> Gennadiy.
>
>
>
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