Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/18/84; site watdaisy.UUCP
Path: utzoo!watmath!watdaisy!ndiamond
From: ndiamond@watdaisy.UUCP (Norman Diamond)
Newsgroups: net.lang.c
Subject: Re: C question
Message-ID: <7181@watdaisy.UUCP>
Date: Mon, 8-Apr-85 13:05:17 EST
Article-I.D.: watdaisy.7181
Posted: Mon Apr  8 13:05:17 1985
Date-Received: Tue, 9-Apr-85 01:45:07 EST
References: <5272@tektronix.UUCP>
Organization: U of Waterloo, Ontario
Lines: 42

> An inexperienced C programmer wrote a program containing the following:
> 
> 	x = x++;
> 
> Assuming that x originally had the value 5, what should the value be after
> execution of the above expression ?
> 
> In favor of 5:
> 	According to the operator precedence table in K&R on page 49, the ++
> operator has higher precedence than the = operator.  Hence the sequence of
> operations should be:
> 	evaluate x (value == 5)
> 	increment x (x now == 6)
> 	assign the computed value to x (x now == 5 again)
> 
> In favor of 6:
> 	In K&R on page 42, the text says: "But the expression ++n increments n
> *before* using its value, while n++ increments n *after* its value has been
> used."  Hence, the sequence of operations should be:
> 	evaluate x (value == 5)
> 	assign the computed value to x (x still == 5)
> 	increment x (x now == 6)

The language definition doesn't say, and the construct is ambiguous
(semantically ambiguous, not syntactically).  Therefore all four compilers
are correct (for this statement anyway) and the program is incorrect.

Actually I am usually quick to catch ambiguities (roughly the same as
finding counter-examples to false theorems), but never would have thought
of that second interpretation.  Yes the increment occurs after binding
the old value, but in this particular case, even I never thought of other
operations sliding in between.  It's rather astounding.

-- 

   Norman Diamond

UUCP:  {decvax|utzoo|ihnp4|allegra}!watmath!watdaisy!ndiamond
CSNET: ndiamond%watdaisy@waterloo.csnet
ARPA:  ndiamond%watdaisy%waterloo.csnet@csnet-relay.arpa

"Opinions are those of the keyboard, and do not reflect on me or higher-ups."