RE: Exponential

[Buck Calabro <buck.calabro@xxxxxxxxxxxxxxxxx>]

Patrick Conner wrote:

>I'm getting the wrong answer on an eval 
>statement. Why?
>
>D#ExPo         S              1P 0   Inz(0)
>D#Base         S              4P 2   Inz(18.10)
>D#Result       S              5P 2
>
>Eval      #Result = #Base + (1 / (10 ** #Expo))
>
>     Expected answer:    19.10
>     Actual answer:      19.09
>
>We are on V4R3.
>
>If I break it down, it works.
>Eval #CalcExpo = 10 ** (#Expo)
>Eval #Result = #Base + (1/#CalcExpo)

Peter Connell gave a very good answer (intermediate results can result in
loss of least significant digits).  I just wanted to add that the ILE RPG
Reference has a section entitled "Precision Rules for Numeric Operations"
that every RPG programmer should read before using EVAL.  

Lest someone start a calumny against IBM documentation, the section on EVAL
specifically refers to the precision rules:
V3R2: http://publib.boulder.ibm.com:80/cgi-bin/bookmgr/BOOKS/QBKAQE01/4.4.34
V4R1: http://publib.boulder.ibm.com:80/cgi-bin/bookmgr/BOOKS/QBJAQE01/4.4.36
V4R2: http://publib.boulder.ibm.com:80/cgi-bin/bookmgr/BOOKS/QB3AGZ00/4.4.36
V4R3: http://publib.boulder.ibm.com:80/cgi-bin/bookmgr/BOOKS/QB3AGZ01/4.4.36
V4R4: http://publib.boulder.ibm.com:80/cgi-bin/bookmgr/BOOKS/QB3AGZ03/4.4.36
V4R5: http://publib.boulder.ibm.com:80/cgi-bin/bookmgr/BOOKS/QB3AGZ03/4.4.36

Buck Calabro
Aptis; Albany, NY
"We are what we repeatedly do.
 Excellence, then, is not an act, but a habit." --Aristotle


Billing Concepts Corp., a NASDAQ Listed Company, Symbol: BILL
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