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Re: How to get mantissa of long double?

From "Alf P. Steinbach" <alf.p.steinbach@gmail.com>
Newsgroups comp.lang.c++
Subject Re: How to get mantissa of long double?
Date 2021-10-02 21:23 +0200
Organization A noiseless patient Spider
Message-ID <sjabjd$heb$1@dont-email.me> (permalink)
References <c19306a0-c1ea-419e-9604-d0e124c358e0n@googlegroups.com> <sjaa8r$7hs$1@dont-email.me> <sjaau5$bg8$1@dont-email.me>

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On 2 Oct 2021 21:12, James Kuyper wrote:
> On 10/2/21 3:00 PM, Alf P. Steinbach wrote:
>> On 1 Oct 2021 17:37, wij wrote:
>>> // numeric_limits<long double>::digits=64.
>>> //
>>> typedef long double FType;
>>> FType x=numeric_limits<FType>::max();
>>> int iexp;
>>> int64_t mint;
>>> x=::frexpl(x,&iexp);
>>> x=::ldexpl(x,numeric_limits<FType>::digits);
>>> mint= static_cast<int64_t>(x);>>
>>> Result (mint) is a negative number, something not right!!!
>>
>> Apparently you're trying to obtain the bits of the mantissa of a `long
>> double` number, represented as an `int64_t` value.
>>
>> A `long double` is in practice either IEEE 754 64-bit, or IEEE 754
>> 80-bit. In Windows that choice depends on the compiler. With Visual C++
>> (and hence probably also Intel) it's 64-bit, same as type `double`,
>> while with MinGW g++ (and hence probably also clang) it 80-bit,
>> originally the x86-family's math coprocessor's extended format. For
>> 80-bit IEEE 754 the mantissa part is 64 bits.
>>
>> With 64-bits mantissa there is a high chance of setting the sign bit of
>> an `int64_t` to 1, resulting in a negative value. I believe that that
>> will only /not/ happen for a denormal value, but, I'm tired and might be
>> wrong about that. Anyway, instead use  unsigned types for bit handling.
>> For example, in this case, use `uint64_t`.
> 
> It would be safer and more portable to use FType; there's no portable
> guarantee that any integer type is large enough to hold the mantissa,
> but FType is.

I believe you intended to write `uintptr_t`, not `FType`.

If so, agreed.

It's late in the day for me, sorry.


>> However, instead of the shenanigans with `frexpl` and `ldexpl` I'd just
>> use `memcpy`.
> 
> The advantage of the code as written is that (if you change mint to have
> FType) it will give the correct result even if your assumption about
> IEEE 754 is false; that's not the case with memcpy().

Uhm, I'd rather assert IEEE 754 representation 
(numeric_limits::is_iec559). Dealing with the bits of just about any 
representation seems to me a hopelessly daunting task. :-o


- Alf

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How to get mantissa of long double? wij <wyniijj@gmail.com> - 2021-10-01 08:37 -0700
  Re: How to get mantissa of long double? RadicalRabbit@theburrow.co.uk - 2021-10-01 16:09 +0000
    Re: How to get mantissa of long double? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-01 14:05 -0400
      Re: How to get mantissa of long double? wij <wyniijj@gmail.com> - 2021-10-01 17:23 -0700
        Re: How to get mantissa of long double? Manfred <noname@add.invalid> - 2021-10-02 18:58 +0200
          Re: How to get mantissa of long double? Manfred <noname@add.invalid> - 2021-10-02 19:21 +0200
  Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-01 19:18 +0200
    Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-01 19:20 +0200
      Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-01 19:25 +0200
        Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-01 19:53 +0200
          Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-03 08:09 +0200
            Re: How to get mantissa of long double? Bart <bc@freeuk.com> - 2021-10-03 11:15 +0100
              Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-03 12:19 +0200
                Re: How to get mantissa of long double? red floyd <no.spam@its.invalid> - 2021-10-03 18:03 -0700
                Re: How to get mantissa of long double? David Brown <david.brown@hesbynett.no> - 2021-10-04 09:52 +0200
                Re: How to get mantissa of long double? Juha Nieminen <nospam@thanks.invalid> - 2021-10-04 09:59 +0000
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-04 13:50 +0200
                Re: How to get mantissa of long double? David Brown <david.brown@hesbynett.no> - 2021-10-04 20:04 +0200
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-05 07:12 +0200
                Re: How to get mantissa of long double? David Brown <david.brown@hesbynett.no> - 2021-10-05 09:47 +0200
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-05 16:46 +0200
                Re: How to get mantissa of long double? David Brown <david.brown@hesbynett.no> - 2021-10-05 19:04 +0200
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-05 19:13 +0200
                Re: How to get mantissa of long double? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-10-05 09:04 -0700
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-05 19:14 +0200
                Re: How to get mantissa of long double? Bart <bc@freeuk.com> - 2021-10-05 19:21 +0100
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-06 07:58 +0200
                Re: How to get mantissa of long double? David Brown <david.brown@hesbynett.no> - 2021-10-06 08:11 +0200
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-06 10:25 +0200
                Re: How to get mantissa of long double? scott@slp53.sl.home (Scott Lurndal) - 2021-10-06 14:25 +0000
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-06 16:53 +0200
                Re: How to get mantissa of long double? Bart <bc@freeuk.com> - 2021-10-06 12:12 +0100
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-06 14:48 +0200
                Re: How to get mantissa of long double? red floyd <no.spam.here@its.invalid> - 2021-10-06 09:44 -0700
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-06 18:52 +0200
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-06 19:02 +0200
                Re: How to get mantissa of long double? Juha Nieminen <nospam@thanks.invalid> - 2021-10-07 05:35 +0000
                Re: How to get mantissa of long double? David Brown <david.brown@hesbynett.no> - 2021-10-07 08:20 +0200
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-07 08:38 +0200
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-07 08:42 +0200
                Re: How to get mantissa of long double? David Brown <david.brown@hesbynett.no> - 2021-10-07 10:23 +0200
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-08 20:23 +0200
                Re: How to get mantissa of long double? David Brown <david.brown@hesbynett.no> - 2021-10-09 10:23 +0200
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-09 10:50 +0200
                Re: How to get mantissa of long double? Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2021-10-09 09:02 +0000
                Re: How to get mantissa of long double? scott@slp53.sl.home (Scott Lurndal) - 2021-10-09 14:57 +0000
                Re: How to get mantissa of long double? Bart <bc@freeuk.com> - 2021-10-09 12:16 +0100
                Re: How to get mantissa of long double? David Brown <david.brown@hesbynett.no> - 2021-10-09 15:10 +0200
                Re: How to get mantissa of long double? red floyd <no.spam.here@its.invalid> - 2021-10-09 10:26 -0700
                Re: How to get mantissa of long double? Vir Campestris <vir.campestris@invalid.invalid> - 2021-10-12 21:23 +0100
                Re: How to get mantissa of long double? RadicalRabbit@theburrow.co.uk - 2021-10-13 10:30 +0000
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-07 08:38 +0200
                Re: How to get mantissa of long double? Juha Nieminen <nospam@thanks.invalid> - 2021-10-08 04:37 +0000
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-08 07:54 +0200
                Re: How to get mantissa of long double? red floyd <no.spam.here@its.invalid> - 2021-10-07 22:44 -0700
                Re: How to get mantissa of long double? David Brown <david.brown@hesbynett.no> - 2021-10-07 08:17 +0200
                Re: How to get mantissa of long double? Juha Nieminen <nospam@thanks.invalid> - 2021-10-08 04:39 +0000
                Re: How to get mantissa of long double? David Brown <david.brown@hesbynett.no> - 2021-10-08 09:25 +0200
                Re: How to get mantissa of long double? scott@slp53.sl.home (Scott Lurndal) - 2021-10-08 14:34 +0000
                Re: How to get mantissa of long double? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2021-10-06 13:57 -0700
                Re: How to get mantissa of long double? Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2021-10-05 19:34 +0000
                Re: How to get mantissa of long double? Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2021-10-05 19:33 +0000
                Re: How to get mantissa of long double? Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2021-10-05 10:19 +0000
                Re: How to get mantissa of long double? Bart <bc@freeuk.com> - 2021-10-04 14:06 +0100
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-04 16:12 +0200
                Re: How to get mantissa of long double? Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2021-10-04 16:15 +0000
                Re: How to get mantissa of long double? Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2021-10-04 16:15 +0000
                Re: How to get mantissa of long double? David Brown <david.brown@hesbynett.no> - 2021-10-05 09:48 +0200
                Re: How to get mantissa of long double? Juha Nieminen <nospam@thanks.invalid> - 2021-10-05 05:19 +0000
                Re: How to get mantissa of long double? David Brown <david.brown@hesbynett.no> - 2021-10-05 10:00 +0200
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-05 16:47 +0200
                Re: How to get mantissa of long double? scott@slp53.sl.home (Scott Lurndal) - 2021-10-04 14:37 +0000
                Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-04 16:53 +0200
                Re: How to get mantissa of long double? scott@slp53.sl.home (Scott Lurndal) - 2021-10-04 17:08 +0000
                Re: How to get mantissa of long double? Bart <bc@freeuk.com> - 2021-10-04 20:22 +0100
    Re: How to get mantissa of long double? Bart <bc@freeuk.com> - 2021-10-01 19:09 +0100
      Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-01 20:16 +0200
  Re: How to get mantissa of long double? Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2021-10-01 18:53 +0000
    Re: How to get mantissa of long double? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-01 20:19 -0400
      Re: How to get mantissa of long double? Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2021-10-02 02:16 +0000
        Re: How to get mantissa of long double? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-02 01:28 -0400
        Re: How to get mantissa of long double? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-02 01:56 -0400
          Re: How to get mantissa of long double? Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2021-10-02 10:59 +0000
            Re: How to get mantissa of long double? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-02 14:57 -0400
            Re: How to get mantissa of long double? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-02 15:37 -0400
              Re: How to get mantissa of long double? Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2021-10-03 00:47 +0000
                Re: How to get mantissa of long double? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-03 23:42 -0400
                Re: How to get mantissa of long double? Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2021-10-04 04:35 +0000
  Re: How to get mantissa of long double? Bonita Montero <Bonita.Montero@gmail.com> - 2021-10-02 12:09 +0200
  Re: How to get mantissa of long double? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2021-10-02 21:00 +0200
    Re: How to get mantissa of long double? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-02 15:12 -0400
      Re: How to get mantissa of long double? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2021-10-02 21:23 +0200
        Re: How to get mantissa of long double? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2021-10-02 21:27 +0200
          Re: How to get mantissa of long double? Manfred <noname@add.invalid> - 2021-10-02 23:19 +0200
        Re: How to get mantissa of long double? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-03 23:43 -0400
      Re: How to get mantissa of long double? Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2021-10-03 00:45 +0000
        Re: How to get mantissa of long double? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-03 23:31 -0400
          Re: How to get mantissa of long double? Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2021-10-04 04:34 +0000
            Re: How to get mantissa of long double? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-10-04 11:30 -0400
              Re: How to get mantissa of long double? Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2021-10-04 16:19 +0000
    Re: How to get mantissa of long double? wij <wyniijj@gmail.com> - 2021-10-03 00:35 -0700
  Re: How to get mantissa of long double? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-10-03 15:02 +0100
    Re: How to get mantissa of long double? wij <wyniijj@gmail.com> - 2021-10-03 18:46 -0700

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