Path: utzoo!mnetor!uunet!rlgvax!jesse
From: jesse@rlgvax.UUCP (Jesse Barber)
Newsgroups: comp.lang.c
Subject: Re: interesting program
Message-ID: <825@rlgvax.UUCP>
Date: 4 Feb 88 00:52:02 GMT
References: <19@berick.UUCP>
Organization: Computer Consoles Inc, Reston VA
Lines: 44
Summary: order of evaluation

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);
> }

What the program prints depends on the compiler used. I used two compilers
msc 4.0 and CCI's 6/32 compiler.
msc printed 4
cci printed 5

However, neither of these compilers is wrong. Remember the old adage
      " It's a poor workman that blames his tools"
On page 49 of K&R copyright 1978 it states
"C, like most languages, does not specify in what order the operands of
an operator are evaluted"

The two results I got are because the two compilers evaluated the operands
of the "=" in a different order.

msc:
       tmp = k;        /* evaulate k */
       k = k + 1;            /* evaluate k++ */
       k = tmp;        /* evaluate = */

cci:
       k = k;          /* evaluate = */
       k = k + 1;      /* evaluat k++ */

As a matter of fact, K&R use the increment operator to demonstrate this
kind of problem.

Notice that in code which does not depend on the order of evaluation
both compilers produce "correct" responses.
-- 
..!seismo!rlgvax!jesse ( Jesse @ C.C.I. Reston, Va.)

Have brain, will travel