Re: In how many ways should this fail?

From	James Kuyper <jameskuyper@verizon.net>
Newsgroups	comp.lang.c
Subject	Re: In how many ways should this fail?
Date	2012-01-30 14:05 -0500
Organization	A noiseless patient Spider
Message-ID	<4F26E9F6.3020101@verizon.net> (permalink)
References	<87hazdcikg.fsf@huddi.jernurt.dk> <chine.bleu-61D2CA.08302730012012@news.eternal-september.org> <jg6hcq$3mc$1@dont-email.me> <4F26D3FE.4010606@verizon.net> <jg6mrf$6it$1@dont-email.me>

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On 01/30/2012 01:16 PM, BartC wrote:
> 
> 
> "James Kuyper" <jameskuyper@verizon.net> wrote in message 
> news:4F26D3FE.4010606@verizon.net...
>> On 01/30/2012 11:43 AM, BartC wrote:
>>>
>>>
>>> "Devil with the China Blue Dress" <chine.bleu@yahoo.com> wrote in message
>>> news:chine.bleu-61D2CA.08302730012012@news.eternal-september.org...
>>>> In article <87hazdcikg.fsf@huddi.jernurt.dk>,
>>>> Anders Wegge Keller <wegge@wegge.dk> wrote:
>>>>
>>>>>  At work, we got into a talk about weird C constructs. One of my
>>>>> collegues volunteered this line:
>>>>>
>>>>>  (a>b)?a:b = 42;
>>>>
>>>>    *(a>b ? &a : &b) = 42;
>>>
>>> There's no reason why the original version shouldn't work. ...
>>
>> That depends upon what you mean by "work". The original expression is
>> equivalent to
>>
>> b = 42;
> 
> Only if the precedence of ?: is lower than for assignment. Used the other 
> way:

The ?: operator can't be explained in terms of a simple precedence
relative to other operators.

>  x = (a>b) ? a : b;
> 
> You would expect the conditional expression to be evaluated first. Why not 
> the same with the conditional on the left?

Because I when gwowen wrote his comment, it sounded familiar, like
something I already knew to be true, so I didn't bother to check. I just
checked, and he was wrong, and I was therefore wrong to agree with his
parse. The C grammar is:

	logical-OR-expression ? expression : conditional-expression

In the following lines, I've inserted spaces to make corresponding parts
line up with each other, but the alignment will not come out correctly
unless viewed using a monospaced font:

According to that grammar,
	 a || b       ?  d , e  :  f   ? g : h
must be parsed as
	(a || b)      ? (d , e) : (f   ? g : h)
but
	 a  ? b  :  c ?  d , e  :  f   = g
must be parsed as
	(a  ? b  : (c ? (d , e) :  f)) = g
(that last parse results in a constraint violation, but it's not a
syntax error).

In all other contexts, the C grammar can be understood as giving || a
higher precedence than =, giving both of them higher precedence than the
comma operator. However, there's no way to insert ?: into that
precedence hierarchy that explains both of the above parses.

The C++ grammar is different, as I said, but I had the difference backwards:
	logical-or-expression ? expression : assignment-expression

and that's probably what both gwowen and I were thinking about. In C++
	 a ||  b       ?  d , e  :  f  = g
parses as:
	(a ||  b)      ? (d , e) : (f  = g)
while
	 a  ?  b  :  c ?  d , e  :  f  , g
parses as:
        (a  ? (b  :  c ? (d , e) :  f)), g

which is even harder to explain by inserting the ?: anywhere into the
precedence hierarchy relative to ||, =, and the comma operator.

Thread

In how many ways should this fail? Anders Wegge Keller <wegge@wegge.dk> - 2012-01-30 16:32 +0100
  Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 11:07 -0500
    Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 11:11 -0500
    Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-30 17:42 +0000
  Re: In how many ways should this fail? gwowen <gwowen@gmail.com> - 2012-01-30 08:16 -0800
    Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-30 17:35 +0000
      Re: In how many ways should this fail? gwowen <gwowen@gmail.com> - 2012-01-30 23:47 -0800
      Re: In how many ways should this fail? Tim Rentsch <txr@alumni.caltech.edu> - 2012-02-01 00:00 -0800
        Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-02-01 12:14 +0000
          Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-02-01 08:04 -0500
          Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-01 13:31 +0000
        Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-01 12:59 +0000
          Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-02-01 08:37 -0500
            Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-01 16:40 +0000
              Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-02-01 12:13 -0500
                Re: In how many ways should this fail? Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-08 12:49 -0800
    Re: In how many ways should this fail? Anders Wegge Keller <wegge@wegge.dk> - 2012-01-30 19:20 +0100
  Re: In how many ways should this fail? Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-01-30 08:30 -0800
    Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 16:43 +0000
      Re: In how many ways should this fail? Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-01-30 09:17 -0800
      Re: In how many ways should this fail? Kaz Kylheku <kaz@kylheku.com> - 2012-01-30 17:31 +0000
      Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 12:31 -0500
        Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 18:16 +0000
          Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 14:05 -0500
            Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 19:30 +0000
              Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-30 12:50 -0800
              Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 16:17 -0500
              Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-30 21:48 +0000
                Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 12:43 +0000
                Re: In how many ways should this fail? Zoltan Kocsi <zoltan@bendor.com.au> - 2012-02-01 00:46 +1100
                Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-31 13:57 +0000
                Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 15:05 +0000
                Re: In how many ways should this fail? Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-01-31 07:16 -0800
                Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 17:37 +0000
                Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-31 11:34 -0500
                Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 17:39 +0000
                Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-31 19:42 -0800
                Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-02-01 11:57 +0000
                Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-02-01 08:14 -0500
                Re: In how many ways should this fail? Philip Lantz <prl@canterey.us> - 2012-02-01 00:04 -0800
                Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-31 09:05 -0500
                Re: In how many ways should this fail? Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-01-31 06:45 -0800
                Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 17:34 +0000
                Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-31 19:26 -0800
                Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-31 19:29 -0800
        Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-30 21:28 +0000
  Re: In how many ways should this fail? timprince <tprince@computer.org> - 2012-01-30 11:44 -0500
    Re: In how many ways should this fail? Kaz Kylheku <kaz@kylheku.com> - 2012-01-30 17:35 +0000
  Re: In how many ways should this fail? Kaz Kylheku <kaz@kylheku.com> - 2012-01-30 17:21 +0000
    Re: In how many ways should this fail? Anders Wegge Keller <wegge@wegge.dk> - 2012-01-30 19:22 +0100
      Re: In how many ways should this fail? 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-30 11:09 -0800
  Re: In how many ways should this fail? jacob navia <jacob@spamsink.net> - 2012-01-30 19:04 +0100
    Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 18:21 +0000
      Re: In how many ways should this fail? Jens Gustedt <jens.gustedt@loria.fr> - 2012-01-30 20:15 +0100
        Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 19:32 +0000
          Re: In how many ways should this fail? Jens Gustedt <jens.gustedt@loria.fr> - 2012-01-30 21:01 +0100
            Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 20:22 +0000
              Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-30 12:56 -0800
  Re: In how many ways should this fail? blp@cs.stanford.edu (Ben Pfaff) - 2012-01-30 10:07 -0800

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