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Re: Fixed-point arithmetic library

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On 27.02.2012 02:50, Arne Vajhøj wrote:
> On 2/26/2012 7:59 PM, glen herrmannsfeldt wrote:
>> Robert Klemme<shortcutter@googlemail.com> wrote:
>>
>> (snip)
>>>> 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
>>
>> In PL/I, if you multiply, you get the appropriate number if digits
>> after the radix point.
>>
>> FIXED DECIMAL(3,2) times FIXED DECIMAL(2,1) generates a
>> product of FIXED DECIMAL(5,3). If you force it to throw away
>> low order digits, then, yes you can lose precision.
>
> That is rather similar to how BigDecimal works.

And that is not exactly how FP arithmetic is described in Wikipedia:

"For simplicity, fixed-point multiply procedures use the same result 
format as the operands. This has the effect of keeping the middle bits; 
the I-number of least significant integer bits, and the Q-number of most 
significant fractional bits. Fractional bits lost below this value 
represent a precision loss which is common in fractional multiplication. 
If any integer bits are lost, however, the value will be radically 
inaccurate."

It boils down to how fixed "fixed" is.  In my understanding decimal 
places are fixed throughout the whole calculation which is not what BD 
does (obviously to avoid precision loss).

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

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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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