Path: utzoo!utgpu!watmath!clyde!att!occrsh!uokmax!rmtodd
From: rmtodd@uokmax.UUCP (Richard Michael Todd)
Newsgroups: comp.unix.wizards
Subject: Old-fashioned assignment operators (was Re: braces
Message-ID: <2155@uokmax.UUCP>
Date: 16 Dec 88 22:53:37 GMT
References: <9076@smoke.BRL.MIL> <14020049@hpisod2.HP.COM> <212@UNIX386.Convergent.COM> <2151@uokmax.UUCP>
Reply-To: rmtodd@uokmax.UUCP (Richard Michael Todd)
Organization: University of Oklahoma, Norman, OK
Lines: 53

In article <2151@uokmax.UUCP> randy@uokmax.UUCP (Longshot) writes:
>As an interesting aside, the cc here on our Multimax will generate warnings for
>lines like:
>
>	x=-y;	/* x = -y or x =- y? */
>
>In other words, and math operator that can be combined with = can produce an
>error. This will happen with +,-, *, and any others that fit the bill. Now, I
>have yet to hard-test it to see which case it generates. Has anyone had
>similiar messages, and checked further?

Script started on Fri Dec 16 16:06:15 1988
% cat foo.c
main() {
        int x,y;
        x = 3;
        y = 4;
        x=-y;
        printf("%d\n",x);
}
% cc foo.c
"foo.c", line 5: warning: old-fashioned assignment operator
% a.out
-1
% ^D
script done on Fri Dec 16 16:06:40 1988

It seems to be interpreting "=-" as "-=".  The message warns of an
"old-fashioned" assignment operator.  There's an interesting bit of history
here.  Back around 6th Edition Unix (I think) the augmented assignment 
operators in C were written as =+, =-, etc, not "+=" like we do today.
Unfortunately that caused an ambiguity problem: does x=-y mean x = -y or
x =- y?  That, I believe, is why the operator syntax was changed circa
7th Edition.  The compilers were changed to accept the old syntax "=-"
but issue a warning.  
  The question I have is, what exactly *is* the correct interpretation of
x=-y in modern C? As mentioned before, the Encore cc interprets it as
augmented assignment.  The compiler I use on my PC interprets it as assigning
-y to x.  Which interpretation is right? What does the ANSI C standard say
about this? 
   Of course, the existence of compilers that interpret this line differently
means that if you want your code to be portable, you *must* include proper
spaces; if you want to assign -y to x, you must say "x = -y" if you want
your code to work everywhere.  
   Finally, I question why this business of checking for old-fashioned 
assignment operators is still in Unix C compilers.  After all, V7 came
out around 10 years ago!  Does anyone really have around any code that hasn't
been converted to the new syntax by *now*?  
-- 
Richard Todd	Fido:1:147/1     USSnail:820 Annie Court,Norman OK 73069
Try one of these: rmtodd@chinet.uucp, rmtodd@killer.dallas.tx.us,
   rmtodd@uokmax.ecn.uoknor.edu  or ...!sun!texsun!uokmax!rmtodd.
"MSDOS is a Neanderthal operating system" - Henry Spencer