From bug-octave-request at bevo dot che dot wisc dot edu Wed Feb 27 14:56:19 2002
Subject: Re: Problem with exp-function
From: Paul Kienzle <pkienzle at jazz dot ncnr dot nist dot gov>
To: Dirk Eddelbuettel <edd at debian dot org>,    Andreas Helms <helms@astro.physik.uni-potsdam.de>
Cc: bug-octave at bevo dot che dot wisc dot edu
Date: Wed, 27 Feb 2002 15:54:41 -0500

MIPS gives 0.
The normalized range for 64 bit reals is e+/-308.

I'm guessing the problem is with denormalized numbers generating an
exception which is not being trapped, whereas the exp(-750) generates an
underflow which is being trapped.

Indeed, a quick google search for "gcc alpha denormalized" gives:

http://gcc.gnu.org/ml/gcc/2001-08/msg01075.html

and the follow-up:

http://gcc.gnu.org/ml/gcc/2001-08/msg01094.html

The suggested work around is to recompile octave with -mieee, which will
slow everything down, or to use the compaq library -lcpml rather than -lm.

Presumably the c++ counter-example provided was compiled with compaq's
compiler rather than gcc.  I don't know if octave can be configured to
use compaq's compiler instead of gcc --- check the octave mailing list.

Paul Kienzle
pkienzle at users dot sf dot net


On Wed, Feb 27, 2002 at 11:30:46AM -0600, Dirk Eddelbuettel wrote:
> On Wed, Feb 27, 2002 at 06:13:22PM +0100, Andreas Helms wrote:
> > octave:10> exp(-730)
> > error: floating point exception -- trying to return to prompt
> 
> I cannot reproduce this using octave 2.1.35 on Linux - I get 9.2263e-318.
> 
> Dirk
> 
> -- 
> Good judgement comes from experience; experience comes from bad judgement. 
> 							    -- Fred Brooks
> 
> 
> 
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Octave is freely available under the terms of the GNU GPL.

Octave's home on the web:  http://www.octave.org
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Subscription information:  http://www.octave.org/archive.html
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