Groups > comp.lang.c > #112096

Re: Hexadecimal float constants

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: Hexadecimal float constants
Date	2017-06-12 16:01 -0700
Organization	None to speak of
Message-ID	<lnshj4g69r.fsf@kst-u.example.com> (permalink)
References	<ohmfqn$q4l$1@gioia.aioe.org> <3ebbcdb0-2c89-8e08-43a3-c54fe22fe426@verizon.net> <b71d7769-536a-487d-a141-b7b1214362df@googlegroups.com> <lna85dgj0b.fsf@kst-u.example.com> <ff149710-bb43-445e-a2f1-cc0974921344@googlegroups.com>

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supercat@casperkitty.com writes:
> On Monday, June 12, 2017 at 1:26:19 PM UTC-5, Keith Thompson wrote:
>> How so?  I can't think of any other tokenization than a ++ + ++ b that
>> would yield a syntactically valid expression, regardless of how `a` is
>> declared/defined.  (Of course the standard requires it to be tokenized
>> as a ++ ++ + b, which is a syntax error, again regardless of how `a` and
>> `b` are delcared/defined.)
>
> If a is an array and b is an int, the expression would be valid if
> tokenized as either "a + + + ++ b" or "a + + + + + b".  If a is an int
> and b is an array, it would be valid if tokenized as "a ++ + + + b" or
> "a + + + + + b".  In neither case would "a++ + ++b" be a valid parse.

I hadn't thought about the unary "+" operator, but you've snipped
relevant context.  You were talking about *parsing*, not about whether
the expression might be invalid for other reasons.

Yes, `a + + + + + b` can be valid if a and b are of numeric type.

[...]

>> 1.23E+5 is a perfectly valid floating constant.  The example we were
>> discussing was 0x1234e-1 .
>
> The simplest way to make floating-point numbers like 1.23E+5 "work" would
> be to treat such values as a sequence of tokens and then have the parser
> assemble them.

The language already defines how 1.23E+5 works.  We recently spent a
great deal of time discussing the minor glitch involving preprocessing
numbers like `0x1234e-1`.  It is, in my opinion, not a problem worth
solving.

[...]

> Implementations are free to add extensions provided that they document them,
> and provided that they don't interfere with the behavior of any strictly-
> conforming program.  Since the documentation requirement would be
> meaningless in cases where use of an extension would trigger a diagnostic,
> the only way in which the permission to include extensions would be
> meaningful would be if documented extensions are exempt from the diagnostic
> requirement.
>
> One could argue that implementations wanting to document as an extension
> their acceptance of 0x1e+x as equivalent to "0x1e + x" would be required
> to ensure that sequence of characters behaves as a single token in cases
> where a strictly-conforming program defines x as a macro and that sequence
> of characters is stringized after macro substitution, but I see no useful
> purpose to be served by requiring such behavior.

Wait, now you're talking about "0x1e+x"?  Where did that final "x" come
from?

An implementation could have an extension whereby it treats `0x1e+x` as
equivalent to `0x1e + x`.  Having and documenting such an extension does
not relieve it of the requirement to diagnose `0x1e+x` as a constraint
violation.  It can issue a non-fatal warning and then proceed to treat
it as `0x1e+x`, and/or it can do so without a warning in some
non-conforming mode.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Thread

Hexadecimal float constants Steve Keller <keller@no.invalid> - 2017-06-12 18:34 +0200
  Re: Hexadecimal float constants "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-12 12:44 -0400
    Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-12 10:54 -0700
      Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-12 11:26 -0700
        Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-12 19:38 +0100
          Re: Hexadecimal float constants scott@slp53.sl.home (Scott Lurndal) - 2017-06-12 18:45 +0000
          Re: Hexadecimal float constants Robert Wessel <robertwessel2@yahoo.com> - 2017-06-12 13:54 -0500
            Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-12 20:40 +0100
              Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-12 13:06 -0700
        Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-12 13:37 -0700
          Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-12 22:22 +0100
          Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-12 16:01 -0700
            Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-12 16:36 -0700
      Re: Hexadecimal float constants jameskuyper@verizon.net - 2017-06-12 12:39 -0700
  Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-12 18:42 +0100
  Re: Hexadecimal float constants Thomas David Rivers <rivers@dignus.com> - 2017-06-12 17:27 -0400
    Re: Hexadecimal float constants "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-12 18:03 -0400
      Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-12 23:31 +0100
        Re: Hexadecimal float constants Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-13 00:10 +0100
          Re: Hexadecimal float constants jacobnavia <jacob@jacob.remcomp.fr> - 2017-06-13 01:33 +0200
            Re: Hexadecimal float constants Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-13 01:10 +0100
      Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-12 16:01 -0700
        Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-12 17:00 -0700
          Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-13 07:51 -0700
            Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-13 09:17 -0700
              Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-13 17:45 +0100
                Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-13 11:58 -0700
                Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-13 12:57 -0700
                Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-13 13:01 -0700
                Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-13 22:11 +0100
                Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-13 14:22 -0700
                Re: Hexadecimal float constants "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-13 17:43 -0400
                Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-13 14:48 -0700
                Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-13 23:03 +0100
                Re: Hexadecimal float constants GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-06-14 01:30 +0200
              Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-13 12:24 -0700
                Re: Hexadecimal float constants jacobnavia <jacob@jacob.remcomp.fr> - 2017-06-13 21:52 +0200
                Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-13 13:12 -0700
                Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-13 16:18 -0700
                Re: Hexadecimal float constants Ike Naar <ike@iceland.freeshell.org> - 2017-06-14 21:28 +0000
    Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-12 15:21 -0700
    Re: Hexadecimal float constants Thomas David Rivers <rivers@dignus.com> - 2017-06-13 12:13 -0400

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