Path: utzoo!utgpu!water!watmath!clyde!rutgers!rochester!crowl
From: crowl@cs.rochester.edu (Lawrence Crowl)
Newsgroups: comp.lang.c
Subject: Re: interesting program
Message-ID: <6378@sol.ARPA>
Date: 30 Jan 88 04:21:28 GMT
References: <19@berick.UUCP>
Reply-To: crowl@cs.rochester.edu (Lawrence Crowl)
Organization: U of Rochester, CS Dept, Rochester, NY
Lines: 26

In article <19@berick.UUCP> jeffl@berick.UUCP (Jeff Lawhorn) writes:
]What does the following program print?  (I know of at least one
]program that prints the wrong answer).
]
]main() {
]	int k = 4;
]	k = k++;
]	printf("and the answer is %d\n", k);
]	exit(0);
]}

This program is ambiguous.  The statement "k = k++;" depends on the order
of evaluation, which is undefined.  Valid C implementation may print
either 4 or 5.  So, unless the C compiler in question is really bad, it
does not provide a "wrong" answer.  Consider the following legal executions
for the statement in question:

    r := k ;                      r := k ;
    k := k + 1 ;                  k := r ;
    k := r ;                      k := k + 1 ;

Both meet the standard, but yield different results.
-- 
  Lawrence Crowl		716-275-9499	University of Rochester
		      crowl@cs.rochester.edu	Computer Science Department
...!{allegra,decvax,rutgers}!rochester!crowl	Rochester, New York,  14627