Path: utzoo!attcan!uunet!ogicse!mintaka!bloom-beacon!athena.mit.edu!rsfinn
From: rsfinn@athena.mit.edu (Russell S. Finn)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Think C Preprocessor Question
Message-ID: <1990Oct24.223243.12287@athena.mit.edu>
Date: 24 Oct 90 22:32:43 GMT
References: <13065@crdgw1.crd.ge.com>
Sender: daemon@athena.mit.edu (Mr Background)
Reply-To: rsfinn@athena.mit.edu (Russell S. Finn)
Organization: Massachusetts Institute of Technology
Lines: 24

In article <13065@crdgw1.crd.ge.com>, darweesh@zephyrus.crd.ge.com (Michael Darweesh) writes:
|> Does the C preprocessor ever do any calculations or does it only do search
|> and replace?  

No.  That is, it doesn't do calculations (within #define statements, anyway),
and it doesn't just search and replace -- let's look at your example:

|> #define foo 5
|> #define bar 8
|> #define baz ((foo * bar) >> 1)
|> 
|> integer = baz;

The preprocessor will replace "baz", then rescan the input and replace "foo"
and "bar" as well, so the compiler ultimately sees

   integer = ((5 * 8) >> 1);

Now, any self-respecting C compiler will perform this computation at
compile-time, which is what you probably wanted all along.  (I suspect that
this is not guaranteed by the standard, but then, I'm not Doug Gwyn...)

Russell S. Finn
rsfinn@{athena,lcs}.mit.edu