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Re: Adjusting printf() rounding

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On 8 Apr 2022 14:40, Paavo Helde wrote:
> 
> To be honest, this is a bit off-topic for C++, but maybe you guys have 
> some insight here.
> 
> This is an excerpt from 
> "https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/printf-printf-l-wprintf-wprintf-l?view=msvc-170" 
> 
> 
> "Starting in Windows 10 version 2004 (build 19041), the printf family of 
> functions prints exactly representable floating point numbers according 
> to the IEEE 754 rules for rounding. In previous versions of Windows, 
> exactly representable floating point numbers ending in '5' would always 
> round up. IEEE 754 states that they must round to the closest even digit 
> (also known as "Banker's Rounding"). For example, both printf("%1.0f", 
> 1.5) and printf("%1.0f", 2.5) should round to 2. Previously, 1.5 would 
> round to 2 and 2.5 would round to 3. [...] This change only affects 
> programs built using Visual Studio 2019 version 16.2 and later."
> 
> glibc has similar page and similar rules, though they do not speak about 
> printf or iostream directly: 
> "https://www.gnu.org/software/libc/manual/html_node/Rounding.html"
> 
> I can understand that rounding-to-even rule is good in mathematical 
> operations as it is statistically unbiased. However, with printf the 
> rounding appears in decimal, not in binary, and the result is meant 
> primarily for human consumption, not for statistics. It is not clear to 
> me these things should follow the same ideology, what do you think?
> 
> What's worse, it appears MSVC have not kept the promises which they 
> made, and have reverted back to pre-MSVC19 behavior in VS2019 Update 11 
> (_MSC_VER==1929), rounding ties now up again.
> 
> For me this means we have nasty cross-platform differences which we 
> would rather try to avoid. How could we coerce glibc() to round up, or 
> failing that, coerce MSVC to round down? I was thinking about adding or 
> subtracting one ULP to the number beforehand, but this would surely ruin 
> some other outputs (in my program I have no control over the numbers or 
> precisions, these come from outside).
> 
> A demo program:
> 
> $ cat test6.cpp
> #include <cstdio>
> int main() {
>      std::printf("%.3g\n", 212.5);
> }
> $ g++ test6.cpp
> $ ./a.out
> 212
> 
> 
> 
> 
> TIA
> Paavo

I haven't used Dragonbox but apparently you can specify the rounding 
policy, so you could write something on top that has the default you 
want -- or just use the default round-to-even on all systems.

https://github.com/jk-jeon/dragonbox

- Alf

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Thread

Adjusting printf() rounding Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-08 15:40 +0300
  Re: Adjusting printf() rounding Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-04-08 06:02 -0700
  Re: Adjusting printf() rounding Marcel Mueller <news.5.maazl@spamgourmet.org> - 2022-04-08 18:21 +0200
    Re: Adjusting printf() rounding Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-08 19:38 +0300
      Re: Adjusting printf() rounding Sams Lara <samlara622@gmail.com> - 2022-04-08 18:48 +0100
        Re: Adjusting printf() rounding Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-08 23:44 +0300
          Re: Adjusting printf() rounding Marcel Mueller <news.5.maazl@spamgourmet.org> - 2022-04-09 12:58 +0200
            Re: Adjusting printf() rounding Geoff <geoff@invalid.invalid> - 2022-04-09 08:45 -0700
            Re: Adjusting printf() rounding Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-04-09 13:05 -0700
              Re: Adjusting printf() rounding red floyd <no.spam.here@its.invalid> - 2022-04-09 13:36 -0700
            Re: Adjusting printf() rounding Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-09 16:08 -0700
  Re: Adjusting printf() rounding Manfred <noname@add.invalid> - 2022-04-08 21:30 +0200
  Re: Adjusting printf() rounding "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-04-09 10:11 +0200
    Re: Adjusting printf() rounding Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-10 16:39 +0300
      Re: Adjusting printf() rounding Sams Lara <samlara622@gmail.com> - 2022-04-10 18:36 +0100
        Re: Adjusting printf() rounding Paavo Helde <eesnimi@osa.pri.ee> - 2022-04-10 21:50 +0300
  Re: Adjusting printf() rounding Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-09 09:05 -0700
    Re: Adjusting printf() rounding Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-04-09 09:10 -0700

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