Hi Pshemek,


You are right. GSL indeed uses double and long double.

Thanks for pointing that.


However, consider this : (from
http://www.cuj.com/documents/s=8020/cuj0111ring/ Read the paragraph
"Why We Need Arbitrary Precision" for more details):


<fontfamily><param>Verdana</param><smaller>For example, consider the
two numbers <bold>N1 = 98237307.398797975997</bold> and <bold>N2 =
87733164872.98273499749</bold>. <bold>N1</bold> has 20 significant
digits; <bold>N2</bold> has 22. If you assign these to C
<bold>double</bold>s, each variable will maintain only 15-16 digits of
precision <color><param>0000,0909,6969</param>(assuming 64
bits)</color>. If you then multiply these numbers, the result will be
precise only to the 15-16 most significant digits.


</smaller></fontfamily>You can use GSL but then when you get a
GSL_ERANGE
(<bold><fontfamily><param>Times</param><bigger><bigger>GSL_ERANGE</bigger></bigger></fontfamily></bold><fontfamily><param>Times</param><bigger><bigger>:
Range error; used by mathematical functions when the result value is
not representable because of overflow or underflow)
</bigger></bigger></fontfamily>or other exception you will need to
handle this error.

If you are very careful and through on this error handling my opinion
is that you will probably end up writing your own version of an
arbitrary precision wrapper over GSL.


One interesting project might be to write a version of GSL with
templates that would allow the use of an arbitrary precision math
class instead of double.


I might be missing something here but this is getting off topic and I
think I should move to http://lists.gnu.org/mailman/listinfo/help-gsl
for further investigation.


Thanks,

Mauricio Gomes

Pensar Digital

phone: 55-11-4121-6287

mobile: 55-11-8319-9610

http://pensardigital.com


On Apr 18, 2005, at 5:11 AM, Sliwa, Przemyslaw (London) wrote:


<excerpt>Why not?


AFAIK GSL, one of the most important open source numerical libraries

uses double precision, so I do not see the point why you say double is

not good enough for scientific applications.


Regards,


Pshemek


-----Original Message-----

From: boost-users-bounces@lists.boost.org

[mailto:boost-users-bounces@lists.boost.org] On Behalf Of Mauricio
Gomes

Sent: 17 April 2005 04:12

To: boost-users@lists.boost.org

Subject: Re: [Boost-users] RE: Boost thread problem



Ben is right, you simply can't use double for any serious 

financial/scientific calculation.

And indeed I hope someday an arbitrary math precision library be part 

of the standard C++.


Look for a standard for the place where this is going to be used.

If there isn't one. Use an arbitrary math precision library anyway.


Take a look at mapm:

http://www.cuj.com/documents/s=8020/cuj0111ring/

http://www.tc.umn.edu/~ringx004/mapm-main.html


There are other comercial alternatives, a quick google search produced 

this:

http://www.boic.com/b1mnum.pdf

http://www.boic.com/numintro.htm


Hope this helps,

Mauricio Gomes

Pensar Digital

phone: 55-11-4121-6287

mobile: 55-11-8319-9610

http://pensardigital.com


On Apr 15, 2005, at 2:37 PM, Ben Hutchings wrote:


<excerpt>Sliwa, Przemyslaw (London) wrote:

<excerpt>Thanks, Ben, what should I use for representing the money
amounts? A 

boolean :)) ?

</excerpt>

If you're dealing with other people's money then there are regulations

</excerpt>

<excerpt>that specify the required intermediate precision and rounding
rules, 

which may vary from place to place.  I believe you'll need to use 

fixed-point decimal fractions, perhaps implemented as integer counts 

of hundredths or ten-thousandths of a currency unit.  I've not done 

this myself so I don't know the details, but I'm fairly sure that 

getting this wrong can put you on the wrong side of the law.


Ben.

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