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Groups > comp.lang.c > #20461

Re: Operator precedence.

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On 05/07/2012 09:36 AM, Ben Bacarisse wrote:
> James Kuyper <jameskuyper@verizon.net> writes:
> 
>> On 05/07/2012 08:18 AM, Ben Bacarisse wrote:
>>> James Kuyper <jameskuyper@verizon.net> writes:
>> ...
>>>> about the associativity of the operators, how do you parse the two
>>>> following expressions:
>>>>
>>>> 	 a ||  b       ?  d , e  :  f   ? g : h
>>>> 	 a  ?  b  :  c ?  d , e  :  f   = g
>>>>
>>>> Try to determine the correct parsings before reading any farther. The
>>>> correct parsings according to the C grammar cannot both be reproduced by
>>>> applying a single consistent precedence order to all of the relevant
>>>> operators. They are:
>>>>
>>>> 	(a || b)      ? (d , e) : (f   ? g : h)
>>>> 	(a  ? b  : (c ? (d , e) :  f)) = g
>>>
>>> How do you get this second parse?  It's a complex expression but it's
>>> only the top level of it that's bugging me.
>>
>> I should have reviewed the entire discussion before posting that part of
>> it. I'd forgotten that you'd already corrected me on that issue the last
>> time I discussed it.
> 
> I don't recall that.  In fact I seem to recall making a similar mistake
> about what the syntax says

You did, initially. But you got it right in your message with the
following headers:

	Subject: In how many ways should this fail?
	Date: Wed, 01 Feb 2012 16:40:09 +0000

...
>> I need to modify the expression to make the point I was trying to make.
>> I don't have time to think about it right now, but I'll see what I can
>> put together later.

I was trying to demonstrate the precise limits of the use of precedence
and associativity to describe the C grammar. I provided one example
where every operand of the ?: operator had the lowest precedence allowed
by the grammar, and one example where what would otherwise qualify as
the left and right operands of the ?: operator were expressions with the
highest precedence that could not so qualify. However, I forgot to make
sure that the second expression had a valid parse. Here's the corrected
version of the second expression:

	 a  ?  b  :  c ?  d , e  :  f   , g

and the parse required by the C standard:

	(a  ?  b  : (c ? (d , e) :  f)) , g

I was really trying to make several points:

1. The grammar of C expressions that contain no syntax errors or
conditional operators can be correctly described using precedence and
associativity.
2. Many possible expressions are syntax errors for reasons that cannot
be explained using precedence and associativity; they can only be
explained by reference to the grammar.
3. Some expressions (mainly involving the middle operand of the ?:
operator) are allowed by the C grammar which cannot be explained purely
in terms of precedence and associativity.

Insofar as item 1 is true, precedence and associativity are redundant
with the grammar; because of item 2, the grammar is NOT redundant with
precedence and associativity. Those two facts alone are sufficient to
justify relying upon the grammar, rather than precedence and
associativity. The standard should NOT use both methods of description;
every key point in the standard should be expressed exactly once in the
normative text. If a requirement is expressed in two different places,
the two different expressions might conflict with each other.

Item 3 makes it clear that precedence and associativity are not merely
redundant with the grammar, but are (in rare circumstances) in conflict
with it.

I'm not opposed to thinking in terms of precedence and associativity;
they're useful concepts for most C operators. It's just important to
learn the limits of applicability of those concepts.

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Thread

Operator precedence. somenath <somenathpal@gmail.com> - 2012-05-06 20:10 -0700
  Re: Operator precedence. somenath <somenathpal@gmail.com> - 2012-05-06 20:23 -0700
  Re: Operator precedence. James Kuyper <jameskuyper@verizon.net> - 2012-05-06 23:57 -0400
  Re: Operator precedence. Kaz Kylheku <kaz@kylheku.com> - 2012-05-07 03:59 +0000
  Re: Operator precedence. pete <pfiland@mindspring.com> - 2012-05-07 05:48 -0400
    Re: Operator precedence. James Kuyper <jameskuyper@verizon.net> - 2012-05-07 07:52 -0400
      Re: Operator precedence. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-05-07 13:18 +0100
        Re: Operator precedence. James Kuyper <jameskuyper@verizon.net> - 2012-05-07 09:08 -0400
          Re: Operator precedence. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-05-07 14:36 +0100
            Re: Operator precedence. James Kuyper <jameskuyper@verizon.net> - 2012-05-07 13:29 -0400
      Re: Operator precedence. pete <pfiland@mindspring.com> - 2012-05-07 11:24 -0400
  Re: Operator precedence. "BartC" <bc@freeuk.com> - 2012-05-07 16:35 +0100
    Re: Operator precedence. jgk@panix.com (Joe keane) - 2012-05-10 19:05 +0000
      Re: Operator precedence. "BartC" <bc@freeuk.com> - 2012-05-10 20:35 +0100
        Re: Operator precedence. James Kuyper <jameskuyper@verizon.net> - 2012-05-11 09:25 -0400
          Re: Operator precedence. "BartC" <bc@freeuk.com> - 2012-05-11 16:42 +0100
            Re: Operator precedence. James Kuyper <jameskuyper@verizon.net> - 2012-05-11 12:28 -0400
              Re: Operator precedence. "BartC" <bc@freeuk.com> - 2012-05-11 18:42 +0100
                Re: Operator precedence. James Kuyper <jameskuyper@verizon.net> - 2012-05-11 15:18 -0400
                Re: Operator precedence. "BartC" <bc@freeuk.com> - 2012-05-11 22:52 +0100
                Re: Operator precedence. James Kuyper <jameskuyper@verizon.net> - 2012-05-11 18:39 -0400
                Re: Operator precedence. Ike Naar <ike@sverige.freeshell.org> - 2012-05-11 19:20 +0000
                Re: Operator precedence. Ike Naar <ike@sverige.freeshell.org> - 2012-05-11 19:28 +0000
                Re: Operator precedence. "BartC" <bc@freeuk.com> - 2012-05-11 22:28 +0100
                Re: Operator precedence. Ike Naar <ike@sverige.freeshell.org> - 2012-05-11 21:45 +0000
              Re: Operator precedence. Keith Thompson <kst-u@mib.org> - 2012-05-12 12:11 -0700
            Re: Operator precedence. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-05-11 17:37 +0100
        Re: Operator precedence. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-05-11 16:24 +0100
          Re: Operator precedence. "BartC" <bc@freeuk.com> - 2012-05-11 20:14 +0100
            Re: Operator precedence. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-05-11 20:56 +0100
              Re: Operator precedence. "BartC" <bc@freeuk.com> - 2012-05-11 22:21 +0100
                Re: Operator precedence. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-05-12 00:05 +0100
          Re: Operator precedence. jgk@panix.com (Joe keane) - 2012-05-11 22:46 +0000
            Re: Operator precedence. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-05-12 00:17 +0100
              Re: Operator precedence. jgk@panix.com (Joe keane) - 2012-05-12 19:35 +0000
                Re: Operator precedence. "BartC" <bc@freeuk.com> - 2012-05-12 22:39 +0100
            Re: Operator precedence. Keith Thompson <kst-u@mib.org> - 2012-05-12 16:07 -0700
              Re: Operator precedence. jgk@panix.com (Joe keane) - 2012-05-13 19:08 +0000
                Re: Operator precedence. Keith Thompson <kst-u@mib.org> - 2012-05-13 13:17 -0700
                Re: Operator precedence. jgk@panix.com (Joe keane) - 2012-05-14 21:06 +0000
                Re: Operator precedence. Keith Thompson <kst-u@mib.org> - 2012-05-14 20:42 -0700
                Re: Operator precedence. Kaz Kylheku <kaz@kylheku.com> - 2012-05-13 23:20 +0000
      Re: Operator precedence. Keith Thompson <kst-u@mib.org> - 2012-05-10 15:24 -0700

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