Hi Alan: Looking at the PDL code, 'acos' is defined for both 'float' PDLs
and 'double' PDLs based on the C library 'acos' function. It seems like
the single precision 'acos' is being called instead of the double
precision one. The value you get from 'p acos(-1.)' is the single
precision value:
perldl> p acos(float(-1))
3.14159274101257
perldl> p acos(double(-1))
3.14159265358979
Even odder: On my machine, I get this:
perldl> p acos(-1)
3.14159265358979
perldl> p acos(-1.)
3.14159274101257
(!)
It seems that if you put a decimal point in, the value is interpreted as a
'float', not a 'double'.
PDL folks: Any idea why -1 is interpreted as 'double' and '-1.' is
interpreted as 'float'??
Regards,
Doug
dh...@ucar.edu
Software Engineer IV
UCAR - COSMIC, Tel. (303) 497-2611
On Thu, 11 Dec 2008, Doug Hunt wrote:
> Alan, Andrew: Thanks for finding/fixing these problems.
>
>> 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)'));
>> }
>
> You can also do this:
>
> wcols $phi;
>
> --That will print out all 20,000 elements of $phi in a single column. Or
> this:
>
> print $phi->mslice([0,99]);
>
> That will print out the first 100 elements of $phi.
>
>>
>> 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.
>
> It turns out that 'acos' is not a *perl* function, but it is a *PDL*
> function. So, I have more control over this--I'll look into it. Good
> catch!
>
> --Doug
>
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