Hi Alan:
On my machine acos(-1) works better than on yours. That's why
x27 worked for me and not you:
perldl> p acos(-1)
3.14159265358979
perldl> p 4*atan2(1,1)
3.14159265358979
Could you send me a:
perl -V
and also run:
perldl
and send me the startup message? I'm curious
what versions of perl and PDL you are using.
This test is also simple enough that I can try it on one of our 64 bit
'production' machines. On this machine (which uses an earlier
version of PDL) it also works fine for me.
Regards,
Doug
dh...@ucar.edu
Software Engineer IV
UCAR - COSMIC, Tel. (303) 497-2611
On Thu, 11 Dec 2008, Alan W. Irwin wrote:
> On 2008-12-11 14:48-0000 Andrew Ross wrote:
>
>> This [example 02 issues] was merely a slight difference between examples.
>> In the perl
>> version the 0-1 numbers were multiplied by 255, as opposed to 255.001 in
>> the C version. Fix this and the results are identical. I've committed
>> this to svn.
>
> Thanks, Andrew, for spotting that.
>
>> This [example 27 issues] looks like small rounding errors to me.
>> Coordinates are never
>> out by more than 1.
>
> You were correct it was a problem with the Perl math library. The main
> trouble I had in finding this was figuring out how to print out individual
> elements of a pdl array. For future reference, this worked:
>
> for (my $i = 0; $i < int($windings)*int($steps)+1; $i++) {
> printf("%f,%f,%f,%f\n", $phi->slice($i)->slice('(0)'),
> $phiw->slice($i)->slice('(0)'), $xcoord->slice($i)->slice('(0)'),
> $ycoord->slice($i)->slice('(0)'));
> }
>
> The result showed the errors gradually increased with phi which lead me to
> the scale error in the PI value used in dphi.
>
> Here is the crux of that scale error.
> perldl> p 4*atan2(1,1)
> 3.14159265358979
> perldl> p acos(-1.)
> 3.14159274101257
>
> The first is the correct value of PI to the printed precision, the second
> is wrong in the 7th place. In contrast, the C math library (and every
> other language binding we have) calculates PI to full precision
> without problems for acos(-1.)
>
> Doug, will you please do the honors and make the appropriate bug report?
> This is ugly, and I am amazed such a perl math library error is still with
> us after all these years.
>
> I have just calculated PI using atan2 for the example (revision 9087) and
> the results are now
>
> perl
> Missing examples :
> Differing examples : 23
>
> So we are close to perfection.
>
> I notice Doug has already uploaded a fix for example 23 so we may be
> PDL-perfect shortly. However, it is taking a while to propagate version 0.46
> of Doug's PDL Graphics, PLplot package at CPAN so I will have to wait to
> make the final test.
>
> Alan
> __________________________
> Alan W. Irwin
>
> Astronomical research affiliation with Department of Physics and Astronomy,
> University of Victoria (astrowww.phys.uvic.ca).
>
> Programming affiliations with the FreeEOS equation-of-state implementation
> for stellar interiors (freeeos.sf.net); PLplot scientific plotting software
> package (plplot.org); the libLASi project (unifont.org/lasi); the Loads of
> Linux Links project (loll.sf.net); and the Linux Brochure Project
> (lbproject.sf.net).
> __________________________
>
> Linux-powered Science
> __________________________
>
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