Re: [boost] [math] floating point classification - testinghelpwanted

From: Paul A Bristow (pbristow_at_[hidden])
Date: 2005-12-04 15:17:23

Exercise for the students - get it to work with UDT ;-)

Darwin long double is really a doubledouble - I think, so really a UDT, for
which getting the righ type is critical.  NTL quad_float is the same - I
think.

First try gave 0 for epsilon, after changing to use ALL quad_float

I've got as far as:

quad_float epsilon_quad_float()
{
        using NTL::to_quad_float;
        quad_float one;
        one = to_quad_float("1.0");
        quad_float two;
        one = to_quad_float("2.0");
  quad_float eps = one;
  quad_float onepluseps; 
  do {
    eps /= two;
    onepluseps = one + eps;
  }
        while (onepluseps != one);
  return eps + eps;
} // quad_float epsilon_quad_float()

but it hangs!

Will look at this further....

Paul

 

| -----Original Message-----
| From: boost-bounces_at_[hidden] 
| [mailto:boost-bounces_at_[hidden]] On Behalf Of Matthias Troyer
| Sent: 04 December 2005 19:45
| To: boost_at_[hidden]
| Subject: Re: [boost] [math] floating point classification - 
| testinghelpwanted
| 
| 
| On Dec 3, 2005, at 8:09 PM, John Maddock wrote:
| 
| >> ld = hi + low as above is not among the bit patterns he lists as
| >> valid. If you apply the definition of epsilon to the "normal" long
| >> doubles only, then you get a value like the one Paul 
| Bristow computed
| >> for NTL's quad_float.
| >
| > I agree: that explanation is quite explicit when it says it 
| follows  
| > Kahan's
| > "double double" and the IEEE spec.  So the low part must be  
| > normalised so
| > that it's bit's "follow on" from those in the high part.  As it  
| > stands,
| > 1+numeric_limits<>::epsilon() should evaluate to 1, but we really  
| > need to
| > check this out.  Does anyone of a numerical inclination, want to  
| > run some
| > tests on Darwin?
| 
| Since i asked the students in my class to write a program 
| calculating  
| the machine epsilon as a homework, I just ran our solution 
| (posted at  
| http://www.itp.phys.ethz.ch/lectures/PT/Exercises/Uebung1/Solution/ 
| eps.cpp). Please overlook that the code is ugly, and does not 
| use any  
| templates - they just had one week of C++ training. Anyway, this code:
| 
| 
| long double epsilon_long_double() {
|    long double eps = 1.;
|    long double onepluseps;
| 
|    do {
|      eps /= 2;
|      onepluseps = 1. + eps;
|    } while ( onepluseps != 1.);
| 
|    return 2*eps;
| 
| }
| 
| 
| returns 0, hence there is really the problem that 
| numeric_limits<long  
| double>::epsilon() == numeric_limits<long double>::min()
| 
| Matthias
| 
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