Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!dptg!att!cbnewsc!mackey
From: mackey@cbnewsc.ATT.COM (ron.mackey)
Newsgroups: comp.lang.c
Subject: Remember precedence rules:  (was Re: use of if (!cptr))
Message-ID: <1984@cbnewsc.ATT.COM>
Date: 24 Jul 89 23:37:37 GMT
References: <10099@mpx2.mpx.com> <93@microsoft.UUCP> <9042@chinet.chi.il.us>
Organization: AT&T Bell Laboratories
Lines: 38

In article <9042@chinet.chi.il.us>, les@chinet.chi.il.us(Leslie Mikesell)writes:
> In article <10103@mpx2.mpx.com> erik@mpx2.mpx.com (Erik Murrey) writes:
> 
> >while (myptr= my_func(), myptr->x != myptr->y) {
> >	...
> I'd use: 
> while (myptr = my_func() && myptr->x != myptr->y) {
>
> which would detect a null pointer returned by my_func() as well as
> providing a guaranteed order of evaluation.  

It would also be incorrect since "&&" binds tighter than "="
Your expression would be evaluated as follows:

	while (myptr = (my_func() && (myptr->x != myptr->y))) {

The order of evaluation is as follows:

-----------------------------------------------
  	evaluate my_func() 		

  	if (my_func() returns 0) 
	then 
		assign 0 to myptr
	else
		evaluate (myptr->x != myptr->y)

		if (myptr->x != myptr->y) 
		then
			assign 1 to myptr
		else
			assign 0 to myptr
-----------------------------------------------

Not exactly what the original poster requested, eh?

Ron Mackey			ihlpb!mackey
AT&T Bell Laboratories		(312) 979-2140