# Boost :

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:

{
do {
eps /= two;
onepluseps = one + eps;
}
while (onepluseps != one);
return eps + eps;

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
| >
| > 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
|
| _______________________________________________
| Unsubscribe & other changes:
| http://lists.boost.org/mailman/listinfo.cgi/boost
|