Groups > comp.lang.c > #20456

Re: Operator precedence.

Message-ID	<4FA7E91A.5C12@mindspring.com> (permalink)
Date	2012-05-07 11:24 -0400
From	pete <pfiland@mindspring.com>
Organization	PF
Newsgroups	comp.lang.c
Subject	Re: Operator precedence.
References	<76630b90-04ab-4976-b1a0-76868d85ccf5@vy9g2000pbc.googlegroups.com> <4FA79A67.5017@mindspring.com> <jo8d19$bm5$1@dont-email.me>

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James Kuyper wrote:
> 
> On 05/07/2012 05:48 AM, pete wrote:
> > somenath wrote:
> ...
> >> I tried to explain myself the outcome of 2+3*5  the way Chris Torek
> >> has explained the expression   -- - x  but I am not able to construes
> >> how the precedence of * will be enforced by the C grammar with out
> >> using operator precedence.
> >
> > I can't do it either.
> > I use the precedence table in chapter 2 of K&R instead.
> 
> What does that table say about the relative precedence of ||, ?:, =, and
> the comma operator? Using that table and the corresponding information
> 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
> 
> The second one is a constraint violation - the C standard requires both
> that it be parsed that way, and that having been so parsed, it must be
> diagnosed as a constraint violation.
> 
> Those particular expressions were chosen for a message I sent out
> several months ago explaining the differences between C and C++ in the
> parsing of such expressions. If you're interested, try the C++ examples:
> 
>          a ||  b       ?  d , e  :  f   = g
>          a  ?  b  :  c ?  d , e  :  f   , g
> 
> The peculiar spacing was chosen to allow enough room to insert the
> parentheses needed to indicate the correct parse, and to line up the
> corresponding variables when viewed in a monospaced font.

  a ||  b       ?  d , e  :  f   = g

  a ||  b       ? (d , e) :  f   = g
because ? and : form a pair of book ends, like ( and ) do.

 (a ||  b)      ? (d , e) :  f   = g
because || is stronger than ?:

((a ||  b       ? (d , e) :  f)  = g
because ?: is stronger than =


/***********************************************/

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

   a  ?  b  :  c ? (d , e) :  f   , g
because ? and : form a pair of book ends, like ( and ) do.

   (a  ?  b  :  c ? (d , e) :  f)   , g
because ?: is stronger than ,

   (a  ?  b  : (c ? (d , e) :  f))  , g
because ?: associates right to left

-- 
pete

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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