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Re: Fixed-point arithmetic library

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On Wed, 22 Feb 2012 14:59:12 -0800, Gene Wirchenko wrote:

> On Wed, 22 Feb 2012 23:13:06 +0100, Robert Klemme
> <shortcutter@googlemail.com> wrote:
> 
>>On 22.02.2012 22:39, Tom Anderson wrote:
>>> Evening all,
>>>
>>> I would quite like to represent some numbers in fixed point, and do
>>> arithmetic with them.
>>
>>Fixed point math is susceptible to precision issues which can be more
>>severe than those of float and double: 0.01 * 0.2 -> 0.00
> 
>      Only if the target precision does not have enough decimal places.
> 
>      I have been working with fixed-point arithmetic in JavaScript so
> that I can add dollar amounts exactly.  Maybe OP has something similar
> in mind, though with the number of digits of precision that he wants
> before the decimal point, I hope it is not currency-related.
>
Not necessarily. BigDecimal has few surprises, but then its not fixed 
decimal. The only fixed decimal arithmetic I've done in anger was in 
Cobol, which will merrily do the following (Identification and 
Environment divisions omitted for brevity).

DATA DIVISION.
WORKING-STORAGE.
   01 FIXED-DECIMAL-FIELDS.
    05 FD-VALUE        PIC S9(6)v9(6) COMP SYNC.
    05 FD-DIVISOR      PIC S9(6)v9(6) COMP SYNC.
    05 FD-RESULT       PIC S9(6)v9(6) COMP SYNC.
    05 FD-REMAINDER    PIC S9(6)V9(6) COMP SYNC.

   01 DISPLAY-LINE.
    05 DL-VALUE           PIC -(6)V.9(6).
    05 FILLER             PIC X() VALUE " / ".
    05 DL-DIVISOR         PIC -(6)V.9(6).
    05 FILLER             PIC X() VALUE " = ".
    05 DL-RESULT          PIC -(6)V.9(6).
    05 FILLER             PIC X() VALUE " REMAINDER ".
    05 DL-REMAINDER       PIC -(6)V.9(6).

PROCEDURE DIVISION.
MAIN SECTION.
M-1.
   MOVE 0.8 TO FD-VALUE DL-VALUE.
   MOVE 2.0 TO FD-DIVISOR DL-DIVISOR.
   DIVIDE FD-VALUE BY FD-DIVISOR GIVING FD-RESULT REMAINDER FD-REMAINDER.
   MOVE FD-RESULT TO DL-RESULT.
   MOVE FD-REMAINDER TO DL-REMAINDER.
   DISPLAY DISPLAY-LINE.
   STOP RUN.


This would display

"     0.800000 /      2.000000 = 0.000000 REMAINDER 0.800000"

because, of course, it is exactly equivalent to dividing 800000 by 
2000000 and then adding decimal points in their correct fixed positions. 
Is that what you want or is BigDecimal doing the right thing for your 
problem space?    

Apologies for not showing a Cobol SSCE, but I don't have a COBOL compiler 
available to check it.


-- 
martin@   | Martin Gregorie
gregorie. | Essex, UK
org       |

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Fixed-point arithmetic library Tom Anderson <twic@urchin.earth.li> - 2012-02-22 21:39 +0000
  Re: Fixed-point arithmetic library Jan Burse <janburse@fastmail.fm> - 2012-02-22 22:59 +0100
    Re: Fixed-point arithmetic library Tom Anderson <twic@urchin.earth.li> - 2012-02-23 21:15 +0000
      Re: Fixed-point arithmetic library markspace <-@.> - 2012-02-23 13:53 -0800
      Re: Fixed-point arithmetic library Robert Klemme <shortcutter@googlemail.com> - 2012-02-23 23:21 +0100
      Re: Fixed-point arithmetic library Jan Burse <janburse@fastmail.fm> - 2012-02-24 00:02 +0100
  Re: Fixed-point arithmetic library Robert Klemme <shortcutter@googlemail.com> - 2012-02-22 23:13 +0100
    Re: Fixed-point arithmetic library Gene Wirchenko <genew@ocis.net> - 2012-02-22 14:59 -0800
      Re: Fixed-point arithmetic library Martin Gregorie <martin@address-in-sig.invalid> - 2012-02-23 01:05 +0000
        Re: Fixed-point arithmetic library Tom Anderson <twic@urchin.earth.li> - 2012-02-23 21:21 +0000
          Re: Fixed-point arithmetic library Gene Wirchenko <genew@ocis.net> - 2012-02-23 13:49 -0800
      Re: Fixed-point arithmetic library Tom Anderson <twic@urchin.earth.li> - 2012-02-23 21:16 +0000
        Re: Fixed-point arithmetic library Gene Wirchenko <genew@ocis.net> - 2012-02-23 14:03 -0800
          Re: Fixed-point arithmetic library Lew <noone@lewscanon.com> - 2012-02-23 14:42 -0800
            Re: Fixed-point arithmetic library Gene Wirchenko <genew@ocis.net> - 2012-02-23 15:08 -0800
        Re: Fixed-point arithmetic library Martin Gregorie <martin@address-in-sig.invalid> - 2012-02-23 23:05 +0000
    Re: Fixed-point arithmetic library glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-02-27 00:59 +0000
      Re: Fixed-point arithmetic library Arne Vajhøj <arne@vajhoej.dk> - 2012-02-26 20:50 -0500
        Re: Fixed-point arithmetic library Robert Klemme <shortcutter@googlemail.com> - 2012-02-27 07:32 +0100
          Re: Fixed-point arithmetic library Jan Burse <janburse@fastmail.fm> - 2012-02-27 13:46 +0100
  Re: Fixed-point arithmetic library Roedy Green <see_website@mindprod.com.invalid> - 2012-02-24 16:26 -0800

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