[Oberon] real math

[oberon at moravcik.info]

Hi Jörg,

just tested your code below and it outputs again 480....32 as in my other email I sent today.

Basically I need to test that 480/100*100 is 480 without some decimal digits at the end
because I am comparing two computed real numbers if they are less, greater or equal.
Comparing 480.0 with 480.0...32 does not make those two equal. 

I rounded the computed real numbers to 2 decimal digits which is enough precision in my case.
This way the problem was solved but I do not think this is the general solution.
Note that linux gcc compiler returns 480 
 printf("result: %3.15f", 480.0/100.0*100.0);


Zdenek



On Mon, Nov 10, 2014 at 06:50:02PM +0100, Jörg wrote:
> 
> Hi Zdenek
> 
> I just compiled your code with AOS on Windows.
> 
> - - - - 
> MODULE RealTest;
> IMPORT Commands;
> 
> PROCEDURE Do*(ctx: Commands.Context);
>   VAR
>     r: REAL;
>   BEGIN
>     r := 480 / 100 * 100;
>     ctx.out.Int( ENTIER(r*1000000), 0);
>   END Do;
> 
> BEGIN
> END RealTest.Do ~
> - - - - - 
> 
> On my PC the output is 480000000. So, I don't have the same effect as you
> have.
> 
> Generally, checking REALs on equality is a little dangerous. You should
> rather subtract two REALs and check for an epsilon>0.
> 
> I wonder what the use case of your code is? What do you want to find out by
> dividing 480 by 100 and multiplying it by 100 again. Do you want to round a
> number to two digits?
> 
> br
> Jörg
> 
> -----Original Message-----
> From: oberon at moravcik.info [mailto:oberon at moravcik.info] 
> Sent: Sonntag, 9. November 2014 20:36
> To: oberon at lists.inf.ethz.ch
> Subject: [Oberon] real math
> 
> Hi,
> 
> following code returns imprecise real number:
> 
>  r : REAL;
> 
>  r := 480 / 100 * 100;
>  out.Int( ENTIER(r * 1000000) , 0 );
> 
> outputs 48000019 instead of 48000000
> 
> How to check that 480/100*100 is 480 again ?
> 
> Thanks
> Zdenek
> 
> --
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