Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #9414

Re: confusion about behavior

Show all headers | View raw

Todd Carnes <toddcarnes@gmail.com> writes:
> On Sun, 14 Aug 2011 15:10:48 -0400, James Kuyper wrote:
>> On 08/14/2011 02:56 PM, LJ wrote:
>>> int     a = 2;
>>> int     b = 3;
>>> c = a+++b;
>>> What will be the value of c??
>>> Shouldn't it be undefined depending upon the interpretation??
>> 
>> The C standard mandates what's called the "maximal munch" rule; "If the
>> input stream has been parsed into preprocessing tokens up to a given
>> character, the next preprocessing token is the longest sequence of
>> characters that could constitute a preprocessing token." (6.4p4)
>> 
>> Therefore, there's only one permitted way of parsing the third
>> statement:
>> 
>> 	c = a++ + b;
>> 
>> That's equivalent to the following statement, where I've inserted some
>> unnecessary parenthesis to clarify the order of operations:
>> 
>> 	c = ((a++) + b);
>> 
>> Assuming that c has an arithmetic type, the behavior of that statement
>> is well-defined: 'a' ends up with a value of 3, and c ends up with a
>> value of 5 (converted, if necessary, to whatever the type of c happens
>> to be).
>
> So, c would actually be equal to 6. :)
>
> c = ((a++) + b)
> c = ((2++) + 3)
> c = (3 + 3)
> c = 6

No.

First of all, (2++) is not a valid expression.  The "++" operator can
only be applied to an lvalue (an expression that designates an object),
not to a constant.  That's because it has the side effect of modifying
the object.

Second, as I explained elsethread, it's a post-increment; the expression
a++ yields 2, not 3.

Finally, wouldn't it have been easier to try it yourself and see what it
does?  (Of course that's not definitive in the case of programs with
unspecified or undefined behavior, but it's worth trying it.)

> I never heard of the "maximal munch" thing. Thanks for the explanation.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Nokia
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

confusion about behavior LJ <djtheshowstopper@gmail.com> - 2011-08-14 11:56 -0700
  Re: confusion about behavior China Blue Tip Wrench <chine.bleu@yahoo.com> - 2011-08-14 12:05 -0700
  Re: confusion about behavior Keith Thompson <kst-u@mib.org> - 2011-08-14 12:06 -0700
    Re: confusion about behavior Todd Carnes <toddcarnes@gmail.com> - 2011-08-15 01:57 +0000
      Re: confusion about behavior Keith Thompson <kst-u@mib.org> - 2011-08-14 19:10 -0700
        Re: confusion about behavior Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-08-15 03:44 +0100
          Re: confusion about behavior Keith Thompson <kst-u@mib.org> - 2011-08-14 20:27 -0700
      Re: confusion about behavior Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-08-14 22:52 -0400
      Re: confusion about behavior Seebs <usenet-nospam@seebs.net> - 2011-08-15 04:28 +0000
        Re: confusion about behavior Kenneth Brody <kenbrody@spamcop.net> - 2011-08-15 15:31 -0400
          Re: confusion about behavior Keith Thompson <kst-u@mib.org> - 2011-08-15 12:36 -0700
          Re: confusion about behavior Seebs <usenet-nospam@seebs.net> - 2011-08-15 20:33 +0000
            Re: confusion about behavior Kenneth Brody <kenbrody@spamcop.net> - 2011-08-16 14:39 -0400
              Re: confusion about behavior Keith Thompson <kst-u@mib.org> - 2011-08-16 12:27 -0700
                Re: confusion about behavior Willem <willem@toad.stack.nl> - 2011-08-16 19:47 +0000
      Re: confusion about behavior Barry Schwarz <schwarz45@yahoo.com> - 2011-09-05 04:50 -0700
        [OT] Re: confusion about behavior Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-09-05 09:07 -0400
        Re: confusion about behavior Kenneth Brody <kenbrody@spamcop.net> - 2011-09-06 12:42 -0400
  Re: confusion about behavior James Kuyper <jameskuyper@verizon.net> - 2011-08-14 15:10 -0400
    Re: confusion about behavior Todd Carnes <toddcarnes@gmail.com> - 2011-08-15 02:04 +0000
      Re: confusion about behavior Keith Thompson <kst-u@mib.org> - 2011-08-14 19:15 -0700
        Re: confusion about behavior Todd Carnes <toddcarnes@gmail.com> - 2011-08-15 02:34 +0000
      Re: confusion about behavior James Kuyper <jameskuyper@verizon.net> - 2011-08-14 22:37 -0400
    Re: confusion about behavior Todd Carnes <toddcarnes@gmail.com> - 2011-08-15 02:26 +0000

csiph-web