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From: dave@spool.wisc.edu (Dave Cohrs)
Newsgroups: comp.unix.questions
Subject: Re: Brain-dead Unix tutor needs quick help (-1)
Message-ID: <4692@spool.wisc.edu>
Date: Wed, 18-Nov-87 13:11:07 EST
Article-I.D.: spool.4692
Posted: Wed Nov 18 13:11:07 1987
Date-Received: Sat, 21-Nov-87 08:09:52 EST
References: <387@cogen.UUCP>
Sender: news@spool.wisc.edu
Reply-To: dave@spool.wisc.edu (Dave Cohrs)
Organization: U of Wisconsin CS Dept
Lines: 32
Summary: lack of parens cause parse errors

In article <387@cogen.UUCP> alen@cogen.UUCP (Alen Shapiro) writes:
>I've hit a mental
>block about the reason ()s are used in preprocessor expressions like
>#define XXX (-1).

Consider this canned example:

#define NEGATE -1

main()
{
	int a;
	int b = 5;

	a=b NEGATE;
	printf("a should equal -5.  a = %d\n", a);
}

If NEGATE were a variable, this would generate a loud "syntax error".
But, because it's a #define, no error is generated, and the program
behaves strangely (the programmer meant to put a "*" between b and
NEGATE).

If you say

#define NEGATE (-1)

You get the "syntax error" you wanted expected.

Dave Cohrs
+1 608 262-6617                        UW-Madison Computer Sciences Department
dave@cs.wisc.edu                 ...!{harvard,ihnp4,rutgers,ucbvax}!uwvax!dave