Re: Strange result with exponents...

[rob@xxxxxxxxx]

I no longer have an obsolete version of OS to argue the past performance. 
And I've literally got dozens of pmr's currently open.  This just isn't 
that important to me.  I'm passing the torch.

Rob Berendt
-- 
Group Dekko Services, LLC
Dept 01.073
PO Box 2000
Dock 108
6928N 400E
Kendallville, IN 46755
http://www.dekko.com





Simon Coulter <shc@xxxxxxxxxxxxxxxxx> 
Sent by: rpg400-l-bounces@xxxxxxxxxxxx
02/19/2004 04:06 PM
Please respond to
RPG programming on the AS400 / iSeries <rpg400-l@xxxxxxxxxxxx>


To
RPG programming on the AS400 / iSeries <rpg400-l@xxxxxxxxxxxx>
cc

Fax to

Subject
Re: Strange result with exponents...







On Friday, February 20, 2004, at 05:40  AM, Joe Pluta wrote:

> And thus from a backwards compatibility standpoint, it's pretty clear
> that changing exponentiation to use a floating point value (if that's
> indeed what happened) actually reduced precision.  This should have at
> least been enough to warrant a warning somewhere in the release notes
> (and there may well have been, I haven't read the release notes for 
> V5R2
> yet).
>
> Think about it: if indeed exponentiation was changed, and somebody is
> currently using exponentiation in a production program, that program is
> now possibly giving erroneous results.  That is something IBM has been
> extremely good about in the past, and is something I'd hope they'd
> continue to adhere to: "Break no code (and if you do, let the users no
> in REALLY BIG letters)."
>

The ** operator has used floating-point since its inception. I did not 
look further back than 440 but in the RPG Reference for 440 section 
4.2.5.2.3 contains the same information Buck quoted from the 520 RPG 
Reference. It doesn't have a change flag so I presume it was documented 
the same way in earlier releases.

The only 'error' was choosing to use floating-point when both operands 
were decimal. Especially when the decimal values have more than 16 
digits on either side of the decimal point. This results in a 
conversion to float (possibly losing accuracy), the operation (possibly 
losing accuracy), and conversion back to decimal (possibly losing 
accuracy).

Anyone who uses floating point should know that it is an approximation 
and is only accurate to nearly 16 digits. I understand 16 digits of 
precision exceeds the experimental accuracy of all known constants of 
nature but that doesn't help in the business world. The approximation 
is why you should never check floating-point numbers for equality 
(i.e., IF float1 = float2 is a bad idea).

The fact that the behaviour has changed between releases is a means for 
opening a PMR with IBM to get a formal response. Given the results it 
looks to me like a rounding issue during conversion to decimal. Whether 
that is a defect or a correction of previously erroneous behaviour is 
for IBM to say. But even allowing for the inaccuracy of floating-point 
the fact the releases 440 to 510 give result of 1152921504606846976 and 
520 gives 1152921504606846980 suggests a change in rounding during 
conversion to decimal. Peter Colpaert's experiments showing the ..76 
answer for 20i 0 variables lends weight to that theory.

I see that Rob opened a PMR and got told that this is expected with 
floating point. I would point out the previous release behaviour and 
the behaviour with 20-digit integers and have them look at it again.

Has anyone tried the example on 520 in a language other than RPG IV to 
see if the behaviour is with RPG or the machine?

Regards,
Simon Coulter.
--------------------------------------------------------------------
    FlyByNight Software         AS/400 Technical Specialists

    http://www.flybynight.com.au/
    Phone: +61 3 9419 0175   Mobile: +61 0411 091 400        /"\
    Fax:   +61 3 9419 0175                                   \ /
                                                              X
                  ASCII Ribbon campaign against HTML E-Mail  / \
--------------------------------------------------------------------


_______________________________________________
This is the RPG programming on the AS400 / iSeries (RPG400-L) mailing list
To post a message email: RPG400-L@xxxxxxxxxxxx
To subscribe, unsubscribe, or change list options,
visit: http://lists.midrange.com/mailman/listinfo/rpg400-l
or email: RPG400-L-request@xxxxxxxxxxxx
Before posting, please take a moment to review the archives
at http://archive.midrange.com/rpg400-l.