Newsgroups: comp.lang.c
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uupsi!rlgvax!scc
From: scc@rlgvax.Reston.ICL.COM (Stephen Carlson)
Subject: Re: One more point regarding = and == (more flamage)
Reply-To: scc@rlgvax.OPCR.ICL.COM (Stephen Carlson)
Organization: International Computers Limited
Date: Wed, 27 Mar 91 17:39:49 GMT
Message-ID: <1991Mar27.173949.25016@rlgvax.Reston.ICL.COM>
References: <925@isgtec.UUCP> <1991Mar26.180311.29125@rlgvax.Reston.ICL.COM> <7318:Mar2622:58:0391@kramden.acf.nyu.edu>

In article <7318:Mar2622:58:0391@kramden.acf.nyu.edu> brnstnd@kramden.acf.nyu.edu (Dan Bernstein) writes:
>In article <1991Mar26.180311.29125@rlgvax.Reston.ICL.COM> scc@rlgvax.OPCR.ICL.COM (Stephen Carlson) writes:
>> 	while (*s++ == *t++)
>> 		;
>> is not well set up to compare two arrays.
>
>Actually, sentinels are among the fastest ways to do a linear search
>without wasting noticeable extra space.

If you had quoted the entire context, you would have read:

| 	while (*s++ == *t++)
| 		;
| 
| is not well set up to compare two arrays.  It does not check for a zero
| termination.  It is really only useful when the programmer has already
| set up guaranteed unequal sentinals at the end of the array.  Then the
| pointers after the loop are one greater than would be useful.

My point was that that particular encoding is not well set up even for
sentinel searches.  The post-loop pointers need to be readjusted.  This
code, for example, does not suffer that disadvantage:

	while (*s == *t)
		s++, t++;

After the loop, the pointers are pointing at the unequal elements, not one
past the unequal elements--which may even end up past the array bounds!

Your general point about sentinels is correct, of course.
-- 
Stephen Carlson           | ICL OFFICEPOWER Center    | In theory, theory and
scc@rlgvax.reston.icl.com | 11490 Commerce Park Drive | practice are the same.
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