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From: markhall@pyrps5.pyramid.com (Mark Hall)
Newsgroups: comp.lang.c
Subject: Re: macros and semicolons
Summary: NO macro will work in general
Message-ID: <160662@pyramid.pyramid.com>
Date: 26 Jun 91 20:23:02 GMT
References: <1991Jun24.213932.595@otago.ac.nz> <1991Jun24.144701.8479@world.std.com>
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Organization: Pyramid Technology Corporation, Mountain View
Lines: 16

In article <1991Jun24.213932.595@otago.ac.nz> andrew@otago.ac.nz writes:
>I often get pissed off with the C pre-processor.  Here is one thats been
>getting up my wick for months.
>
>#define SWAP(a, b) {int c;  c = a; a = b; b = c}

Forgive me for this unsufferable horn-tooting; I just couldn't resist
(I also couldn't believe no one else sent this reply :-). . . .

NO macro will work for a swap.  You suffer from the call-by-name rule
which undid ALGOL in this case.  Consider the expansion of SWAP(i,a[i]):

	int c; c = i; i = a[i]; a[i] = c;

-Mark Hall (smart mailer): markhall@pyrps5.pyramid.com
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