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

From	Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups	comp.lang.c
Subject	Re: Floating point to string, and back via `%a'...
Date	2015-07-23 11:25 +0100
Organization	A noiseless patient Spider
Message-ID	<87380f41c9.fsf@bsb.me.uk> (permalink)
References	(1 earlier) <87mvyn51bm.fsf@bsb.me.uk> <mop2i6$bkq$1@speranza.aioe.org> <87h9ov4x0f.fsf@bsb.me.uk> <mopk6n$i8k$1@speranza.aioe.org> <87bnf3427q.fsf@bsb.me.uk>

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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:

> "Chris M. Thomasson" <nospam@nospam.nospam> writes:
<snip>
>> Alice then runs it through a local round trip from double-to-
>> string-to-double, and gets back:
>>
>> 1.0123456
>>
>> Fine... Alice then uses this new value for the public key, generates
>> the fractal cipher based on the private key, encrypts the cleartext
>> and sends both 1.0123456 and the ciphertext across
>> the wire to Bob.
>>
>> Bob converts the public key string to a double. I am HOPING that Bob
>> gets exactly:
>>
>> 1.0123456
>
> You can't be sure of that but it's very, very likely.  For one thing,
> you seem to be deliberately not using the full precision which makes it
> much more likely that the conversion will produce the same result.

Reading Keith's reply has reminded me that I'm not being clear about the
distinction between internal and external representations.  Most likely,
neither Alice nor Bob have 1.0123456 exactly.  What you can hope for
(and it's only a hope) is that those decimal digits translate to the
same binary floating point number that Alice had to start with.

As has been suggested elsewhere, if your numbers are always in some
well-known range you might try using integers instead.  Also, look at
the functions frexp and scalbn.  You can use these to simulate the
effect of the %s format that you've found you can't rely on:

     int exp;
     double d = frexp(1.0123456, &exp);
     printf("%a %d %llx\n", d, exp, (long long unsigned)scalbn(d, 53));

Produces

  0x1.032914cc3fdf1p-1 1 1032914cc3fdf1

<snip>
-- 
Ben.

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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