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Re: Floating point to string, and back via `%a'...

From "Chris M. Thomasson" <nospam@nospam.nospam>
Newsgroups comp.lang.c
Subject Re: Floating point to string, and back via `%a'...
Date 2015-07-22 14:34 -0700
Organization Aioe.org NNTP Server
Message-ID <mop2i6$bkq$1@speranza.aioe.org> (permalink)
References <moorva$su7$1@speranza.aioe.org> <87mvyn51bm.fsf@bsb.me.uk>

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> "Ben Bacarisse"  wrote in message news:87mvyn51bm.fsf@bsb.me.uk... 

> > "Chris M. Thomasson" <nospam@nospam.nospam> writes:

> > How in the heck can I use the `%a' format specifier in conjunction
> > with sprintf/sscanf so that I can convert a floating point to a
> > string, and back? I have some code, but it does not work with
> > `%a'. Damn!

> Judging by your posts in sci.crypt, the problem is the default precision
> use by MSVC's C library. The MSVC 2015 documentation says the default
> precision is 6 digits, but the C standard mandate that it be enough to
> represent the number (or at least to distinguish all numerical values):

> 7.19.6.1 p8:

> "... if the precision is missing and FLT_RADIX is a power of 2, then
>  the precision is sufficient for an exact representation of the value;
> if the precision is missing and FLT_RADIX is not a power of 2, then
> the precision is sufficient to distinguish 245) values of type double,
> ..."

> It looks like MSVC simply has a bug.  You could calculate the number of
> digits you need but that would be fiddly to get right in every single
> case.

Right. FWIW, %a does not work on MSVC for sscanf always returns zero.

So, I am going to use %lf and stick to a precision of %.10 for 
now. I do not care if the double to string back to double 
conversion looses precision because the fractal can be generated 
on the end result of the conversion, not the original double. So, 
I get some doubles, convert then to strings and back, then used 
the new results regards of any lost precision.

Yup. This should work...

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Thread

Floating point to string, and back via `%a'... "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-22 12:46 -0700
  Re: Floating point to string, and back via `%a'... James Kuyper <jameskuyper@verizon.net> - 2015-07-22 16:19 -0400
    Re: Floating point to string, and back via `%a'... "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-22 13:49 -0700
      Re: Floating point to string, and back via `%a'... Keith Thompson <kst-u@mib.org> - 2015-07-22 14:01 -0700
      Re: Floating point to string, and back via `%a'... James Kuyper <jameskuyper@verizon.net> - 2015-07-22 17:18 -0400
        Re: Floating point to string, and back via `%a'... "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-22 14:22 -0700
  Re: Floating point to string, and back via `%a'... Keith Thompson <kst-u@mib.org> - 2015-07-22 13:40 -0700
    Re: Floating point to string, and back via `%a'... "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-22 14:01 -0700
      Re: Floating point to string, and back via `%a'... Keith Thompson <kst-u@mib.org> - 2015-07-22 14:12 -0700
        Re: Floating point to string, and back via `%a'... "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-22 14:20 -0700
          Re: Floating point to string, and back via `%a'... "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-22 14:24 -0700
          Re: Floating point to string, and back via `%a'... glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-07-22 21:30 +0000
            Re: Floating point to string, and back via `%a'... "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-22 14:39 -0700
          Re: Floating point to string, and back via `%a'... Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-07-22 19:49 -0600
  Re: Floating point to string, and back via `%a'... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-22 22:28 +0100
    Re: Floating point to string, and back via `%a'... "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-22 14:34 -0700
      Re: Floating point to string, and back via `%a'... Keith Thompson <kst-u@mib.org> - 2015-07-22 14:58 -0700
        Re: Floating point to string, and back via `%a'... "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-22 15:43 -0700
        Re: Floating point to string, and back via `%a'... "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-22 15:45 -0700
        Re: Floating point to string, and back via `%a'... "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-22 15:51 -0700
          Re: Floating point to string, and back via `%a'... Robert Wessel <robertwessel2@yahoo.com> - 2015-07-22 18:21 -0500
            Re: Floating point to string, and back via `%a'... Keith Thompson <kst-u@mib.org> - 2015-07-22 16:58 -0700
        Re: Floating point to string, and back via `%a'... Keith Thompson <kst-u@mib.org> - 2015-07-22 16:52 -0700
        Re: Floating point to string, and back via `%a'... Keith Thompson <kst-u@mib.org> - 2015-07-22 17:08 -0700
      Re: Floating point to string, and back via `%a'... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-23 00:01 +0100
        Re: Floating point to string, and back via `%a'... "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-22 19:40 -0700
          Re: Floating point to string, and back via `%a'... Keith Thompson <kst-u@mib.org> - 2015-07-22 22:32 -0700
          Re: Floating point to string, and back via `%a'... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-23 11:07 +0100
            Re: Floating point to string, and back via `%a'... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-07-23 11:25 +0100
              Re: Floating point to string, and back via `%a'... "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-24 11:44 -0700
            Re: Floating point to string, and back via `%a'... "Chris M. Thomasson" <nospam@nospam.nospam> - 2015-07-24 11:47 -0700

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