Re: Strange result with exponents...

[Alexei Pytel <pytel@xxxxxxxxxx>]

There is an additional catch with float point numbers.
IEEE format is based on representation by *fractional* binary number.
It means that some decimal numbers cannot be represented precisely by 
float point number at all.
E.g. if you assign a number to float point variable (that is, convert it 
from decimal representation to IEEE representation) and then print it 
(that is, convert IEEE representation back to decimal printable form) - as 
often as not, you will see different number.

    Alexei Pytel




Scott Klement <klemscot@xxxxxxxxxxxx> 
Sent by: rpg400-l-bounces@xxxxxxxxxxxx
02/20/2004 03:41 PM
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RPG programming on the AS400 / iSeries


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Subject
Re: Strange result with exponents...







Hi Buck,

> C:
> printf("%f\n", pow(16,15));
> 1152921504606847000.000000
> printf("%f\n", (pow(16,14)*16));
> 1152921504606847000.000000

This code doesn't do quite the same thing, because it doesn't convert the
result to a packed field.

Unfortunately, I have NO CLUE how you work with packed fields in C, so
I'll do the exponentiation in C and then call an RPG program to convert it
to packed:


#include <stdio.h>
#include <math.h>

#pragma linkage (MYRPG,OS)
void MYRPG(double *);

int main(int argc, char **argv) {

    double z;

    z = pow(16, 15);
    printf("%.0f\n", z);

    MYRPG(&z);

    return 0;
}



     D MYRPG           PR                  ExtPgm('MYRPG')
     D   input                        8F

     D MYRPG           PI
     D   input                        8F

     D packed          s             30P 0

      /free
         packed = input;
         dsply '' '' packed;
         *inlr = *on;
      /end-free


The result of the printf is 1152921504606847000 as it was in your example,
but the result of the DSPLY is 1152921504606846980

Which is very interesting, don't you think?
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