Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!snorkelwacker!husc6!spdcc!ima!haddock!karl
From: karl@haddock.ima.isc.com (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: a style question
Message-ID: <18370@haddock.ima.isc.com>
Date: 2 Oct 90 03:46:37 GMT
References: <7341@darkstar.ucsc.edu> <1990Sep30.050655.13212@zoo.toronto.edu> <1990Sep30.172917.2951@Neon.Stanford.EDU> <DAVIS.90Oct1025438@pacific.mps.ohio-state.edu> <1990Oct1.174941.22195@zoo.toronto.edu>
Reply-To: karl@kelp.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Cambridge, MA 02138-5302
Lines: 21

In article <1990Oct1.174941.22195@zoo.toronto.edu> henry@zoo.toronto.edu (Henry Spencer) writes:
>What *can* make a difference is to run the loop backwards, algorithm
>permitting:
>	for (x = 99; x >= 0; x--)

In keeping with the idea that x is ranging through the half-open interval
[0,100) rather than the closed interval [0,99], I often write this as
	for (x = 100; --x >= 0;)
instead.  (Possibly more efficient, too, since the decrement and the test are
not separated by a basic-block-boundary, and hence the test instruction can be
omitted on vaxlike architectures.)

Note that with either Henry's example or mine you will lose big if x is an
unsigned integer (and if you don't use lint to catch it).  I usually use
	for (x = 100; x != 0; --x)
on unsigned countdown loops where the index doesn't matter (i.e. just "do 100
times"); if you actually need the interval [0,100),
	for (x = 100; x-- != 0;)
does the job (though probably less efficiently).

Karl W. Z. Heuer (karl@kelp.ima.isc.com or ima!kelp!karl), The Walking Lint