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Re: .5 against 0.5f

From bartc <bc@freeuk.com>
Newsgroups comp.lang.c
Subject Re: .5 against 0.5f
Date 2026-10-27 21:03 +0000
Organization A noiseless patient Spider
Message-ID <11br3ih$1sscp$1@dont-email.me> (permalink)
References <11bpvs7$1e6kc$2@dont-email.me> <11bqnvh$1np7s$1@dont-email.me> <Ue6ES.2265$y_V1.1859@fx12.iad> <20261027125914.162@kylheku.com>

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On 27/10/2026 20:05, Kaz Kylheku wrote:
> On 2026-10-27, Scott Lurndal <scott@slp53.sl.home> wrote:
>> I ordered a couple of blu-rays from amazon last week, and the
>> confirmation email had the following for the total price:
>>
>>     Total
>>     75.71000000000001 USD
>>
>> Appears they may be using FP instead of integer fixed point arithmetic
>> for pricing.
> 
> That reminds of a joke I have in the TXR Lisp fortune module,
> intended for displaying a random quip at REPL startup.
> 
> Oh, look, I got it after just a couple of tries:
> 
> This is the TXR Lisp interactive listener of TXR 302.
> Quit with :quit or Ctrl-D on an empty line. Ctrl-X ? for cheatsheet.
> 1> (quip)
> "Coding around Unicode grapheme clusters is a character-building exercise."
> 2> (quip)
> "defmicro: error: expansion is larger than original form: switch to defmacro."
> 3> (prinl (quip))
> "This could be the year of the TXR desktop; I can feel it!"
> "This could be the year of the TXR desktop; I can feel it!"
> 4> (put-line (quip))
> TXR kind of supports IEEE 754.00000000000003 floating-point.
> t
> 
> :)
> 
> FP is fine for pricing: look at how ridiculously close to 75.71
> the above approximation is. You just have to make sure never to
> let enough error accumulate so that an intermediate value
> is suddenly closer to the wrong penny value than the right one.
> (Actually you don't even want to get anywhere near that level
> of error.)

I've used floats for pricing, I think even 32-bit ones. It should work 
fine within certain levels of magnitude - you want a few spare bits of 
precision beyond individual cents.

(To clarify, 32 bits may have been used to store prices in a stock file 
for example, so typical magnitudes would have been small, but 
calculations and totals would have used 64 bits.)

But apart from doing the rounding right as you suggest, the display 
should be only showing 2 decimals when displaying dollars, not 14. That 
example is very sloppy.

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Thread

.5 against 0.5f fir <profesor.fir@gmail.com> - 2026-10-27 11:53 +0100
  Re: .5 against 0.5f "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu> - 2026-10-27 17:45 +0000
    Re: .5 against 0.5f fir <profesor.fir@gmail.com> - 2026-10-27 18:50 +0100
      Re: .5 against 0.5f "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu> - 2026-10-27 19:01 +0000
        Re: .5 against 0.5f fir <profesor.fir@gmail.com> - 2026-10-27 20:15 +0100
          Re: .5 against 0.5f "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu> - 2026-10-27 19:54 +0000
            Re: .5 against 0.5f "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-10-27 13:10 -0700
              Re: .5 against 0.5f steve g <Sgonedes1977@gmail.com> - 2026-10-28 02:20 -0400
            Re: .5 against 0.5f David Brown <david.brown@hesbynett.no> - 2026-10-27 21:35 +0100
            Re: .5 against 0.5f James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-10-27 18:00 -0400
              Re: .5 against 0.5f "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu> - 2026-10-27 22:15 +0000
              Re: .5 against 0.5f Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-28 06:54 -0700
          Re: .5 against 0.5f fir <profesor.fir@gmail.com> - 2026-10-27 21:57 +0100
            Re: .5 against 0.5f "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-10-27 14:18 -0700
          Re: .5 against 0.5f steve g <Sgonedes1977@gmail.com> - 2026-10-28 02:19 -0400
            Re: .5 against 0.5f Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-10-28 21:54 +0000
              Re: .5 against 0.5f "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-10-28 14:58 -0700
      Re: .5 against 0.5f steve g <Sgonedes1977@gmail.com> - 2026-10-28 02:11 -0400
        Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-27 23:21 -0700
    Re: .5 against 0.5f scott@slp53.sl.home (Scott Lurndal) - 2026-10-27 18:40 +0000
      Re: .5 against 0.5f Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-27 12:29 -0700
        Re: .5 against 0.5f "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-10-27 13:11 -0700
          Re: .5 against 0.5f "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-10-27 13:25 -0700
      Re: .5 against 0.5f Michael S <already5chosen@yahoo.com> - 2026-10-27 21:43 +0200
        Re: .5 against 0.5f Michael S <already5chosen@yahoo.com> - 2026-10-27 22:05 +0200
          Re: .5 against 0.5f Michael S <already5chosen@yahoo.com> - 2026-10-27 22:51 +0200
      Re: .5 against 0.5f Kaz Kylheku <046-301-5902@kylheku.com> - 2026-10-27 20:05 +0000
        Re: .5 against 0.5f bartc <bc@freeuk.com> - 2026-10-27 21:03 +0000
        Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-27 15:41 -0700
          Re: .5 against 0.5f scott@slp53.sl.home (Scott Lurndal) - 2026-10-28 16:23 +0000
      Re: .5 against 0.5f steve g <Sgonedes1977@gmail.com> - 2026-10-28 02:25 -0400
      Re: .5 against 0.5f makendo <makendo@makendo.invalid> - 2026-10-29 18:04 +0800
        Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-29 05:01 -0700
          Re: .5 against 0.5f David Brown <david.brown@hesbynett.no> - 2026-10-29 13:55 +0100
            Re: .5 against 0.5f bartc <bc@freeuk.com> - 2026-10-29 14:32 +0000
          Re: .5 against 0.5f Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-29 23:19 +0100
    Re: .5 against 0.5f Kaz Kylheku <046-301-5902@kylheku.com> - 2026-10-27 19:58 +0000
    Re: .5 against 0.5f James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-10-27 17:43 -0400
      Re: .5 against 0.5f James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-10-27 18:06 -0400
        Re: .5 against 0.5f Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-10-28 00:49 +0000
          Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-27 18:23 -0700
            Re: .5 against 0.5f Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-28 16:38 +0100
              Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-28 13:27 -0700
                Re: .5 against 0.5f Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-29 00:08 +0100
                Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-28 16:56 -0700
                Re: .5 against 0.5f Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-29 18:55 +0100
                Re: .5 against 0.5f David Brown <david.brown@hesbynett.no> - 2026-10-29 09:30 +0100
                Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-29 04:54 -0700
                Re: .5 against 0.5f Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-29 19:57 +0100
                Re: .5 against 0.5f David Brown <david.brown@hesbynett.no> - 2026-10-29 20:56 +0100
                Re: .5 against 0.5f Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-29 23:06 +0100
                Re: .5 against 0.5f Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-29 13:39 -0700
          Re: .5 against 0.5f James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-10-27 21:41 -0400
            Re: .5 against 0.5f David Brown <david.brown@hesbynett.no> - 2026-10-28 08:45 +0100
          Re: .5 against 0.5f "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu> - 2026-10-28 01:50 +0000
            Re: .5 against 0.5f fir <profesor.fir@gmail.com> - 2026-10-28 07:32 +0100
              Re: .5 against 0.5f fir <profesor.fir@gmail.com> - 2026-10-28 07:51 +0100
                Re: .5 against 0.5f fir <profesor.fir@gmail.com> - 2026-10-28 07:55 +0100
                Re: .5 against 0.5f "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu> - 2026-10-28 16:27 +0000
                Re: .5 against 0.5f David Brown <david.brown@hesbynett.no> - 2026-10-28 17:44 +0100
                Re: .5 against 0.5f "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu> - 2026-10-28 16:22 +0000
                Re: .5 against 0.5f fir <profesor.fir@gmail.com> - 2026-10-28 19:15 +0100
                Re: .5 against 0.5f fir <profesor.fir@gmail.com> - 2026-10-28 19:47 +0100
                Re: .5 against 0.5f "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu> - 2026-10-28 19:10 +0000
                Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-28 13:31 -0700
                Re: .5 against 0.5f "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu> - 2026-10-28 22:44 +0000
                Re: .5 against 0.5f Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-29 00:27 +0100
                Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-28 16:39 -0700
                Re: .5 against 0.5f "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu> - 2026-10-29 00:22 +0000
                Re: .5 against 0.5f Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-28 17:59 -0700
                Re: .5 against 0.5f "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu> - 2026-10-29 01:50 +0000
                Re: .5 against 0.5f David Brown <david.brown@hesbynett.no> - 2026-10-29 10:45 +0100
                Re: .5 against 0.5f cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-29 14:56 +0000
                Re: .5 against 0.5f "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu> - 2026-10-29 16:34 +0000
                Re: .5 against 0.5f Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-29 15:47 -0700
                Re: .5 against 0.5f James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-10-29 11:56 -0400
                Re: .5 against 0.5f "Steven G. Kargl" <sgk@REMOVEtroutmask.apl.washington.edu> - 2026-10-29 17:57 +0000
                Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-29 16:06 -0700
                Re: .5 against 0.5f cross@spitfire.i.gajendra.net (Dan Cross) - 2026-10-29 19:49 +0000
                Re: .5 against 0.5f Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-29 22:55 +0100
                Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-29 16:29 -0700
                Re: .5 against 0.5f Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2026-10-29 17:58 +0100
                Re: .5 against 0.5f Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-29 15:42 -0700
          Re: .5 against 0.5f David Brown <david.brown@hesbynett.no> - 2026-10-28 08:39 +0100
            Re: .5 against 0.5f Michael S <already5chosen@yahoo.com> - 2026-10-28 22:25 +0200
              Re: .5 against 0.5f David Brown <david.brown@hesbynett.no> - 2026-10-29 10:48 +0100
                Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-29 04:59 -0700
            Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-28 14:26 -0700
              Re: .5 against 0.5f Michael S <already5chosen@yahoo.com> - 2026-10-29 00:12 +0200
                Re: .5 against 0.5f bartc <bc@freeuk.com> - 2026-10-28 22:33 +0000
                Re: .5 against 0.5f David Brown <david.brown@hesbynett.no> - 2026-10-29 11:01 +0100
                Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-28 16:05 -0700
                Re: .5 against 0.5f David Brown <david.brown@hesbynett.no> - 2026-10-29 10:59 +0100
            Re: .5 against 0.5f "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-10-28 14:34 -0700
              Re: .5 against 0.5f David Brown <david.brown@hesbynett.no> - 2026-10-29 11:06 +0100
          Re: .5 against 0.5f tTh <tth@none.invalid> - 2026-10-28 15:08 +0100
            Re: .5 against 0.5f Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-28 07:49 -0700
          Re: .5 against 0.5f "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-10-28 14:36 -0700
      Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-27 15:53 -0700
        Re: .5 against 0.5f James Kuyper <jameskuyper@alumni.caltech.edu> - 2026-10-27 21:55 -0400
          Re: .5 against 0.5f Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2026-10-27 19:02 -0700
        Re: .5 against 0.5f David Brown <david.brown@hesbynett.no> - 2026-10-28 08:56 +0100
      Re: .5 against 0.5f Tim Rentsch <tr.17687@z991.linuxsc.com> - 2026-10-28 08:01 -0700
  Re: .5 against 0.5f "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-10-27 13:08 -0700

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