From: dobrien@seas.gwu.edu (David O'Brien)
Subject: Re: "Subtract C, add Ada"
Date: 31 Jan 1995 22:22:05 GMT
Date: 1995-01-31T22:22:05+00:00 [thread overview]
Message-ID: <3gmd6d$mbf@cronkite.seas.gwu.edu> (raw)
In-Reply-To: 3ghtke$9jc@felix.seas.gwu.edu
Michael Feldman (mfeldman@seas.gwu.edu) wrote:
: In article <3gdro9$aud@spam.csv.warwick.ac.uk>,
: Jules <csusb@csv.warwick.ac.uk> wrote:
: >Of course the correct version (and the version I still prefer) is:
: >
: >while (a[i++]!=b && i < ARRAYLIMIT);
: Hmmm - I think I learned somewhere that the && is short-circuit in C.
: So wouldn't you want to switch the two conditions, i.e. check the
: limit _before_ incrementing that index? (It's late at night; this
: comment may be off the wall.)
Switching is required, and it would use the short-circuit idiom we love
so much in C. But, the reason is that you skip that last element. Say
we have:
#define ARRAYLIMIT 3
foo a[ ARRAYLIMIT ];
When i == 2, the access ``a[ i++ ] != b'' is ok. However when we get to
``i < ARRAYLIMIT'', i == 3, and thus the statement is false and you stop
the loop for the wrong reason. ``i <= ARRAYLIMIT'' would be needed, but
it would cause you to access one element off the end of the array which
is undefined. ANSI-C only says the *address* that points to one element
beyond the end of an array must be valid. But accessing it is undefined
behavior. [ANSI-C 3.3.6]
Assuming we had:
foo a[ ARRARYLIMIT + 1 ]; /* also very common way */
Doesn't help either, you also stop early.
-- David (dobrien@seas.gwu.edu)
next prev parent reply other threads:[~1995-01-31 22:22 UTC|newest]
Thread overview: 85+ messages / expand[flat|nested] mbox.gz Atom feed top
1995-01-20 10:20 "Subtract C, add Ada" R.A.L Williams
1995-01-20 13:22 ` Renaud HEBERT
1995-01-24 3:35 ` David Moore
1995-01-25 5:38 ` Robert Dewar
1995-01-28 16:35 ` Jules
1995-01-29 8:06 ` Matt Kennel
1995-01-30 5:31 ` Michael Feldman
1995-01-31 22:22 ` David O'Brien [this message]
1995-01-24 20:23 ` N. Mellor
1995-01-25 8:50 ` Robb Nebbe
1995-01-25 14:19 ` John Volan
1995-01-26 5:07 ` Samuel Mize
1995-01-26 18:51 ` Mark A Biggar
1995-01-21 15:18 ` Robert Dewar
1995-01-21 21:03 ` David O'Brien
1995-01-23 3:09 ` Jay Martin
1995-01-23 12:50 ` Andrew McConnell
1995-01-24 0:54 ` Matt Kennel
1995-01-25 17:03 ` Norman H. Cohen
1995-01-26 1:13 ` Dr. Richard Botting
1995-01-26 14:32 ` Anders Juul Munch
1995-01-24 0:17 ` Bob Kitzberger
1995-01-23 20:46 ` Robert Firth
1995-01-24 14:25 ` Samuel Mize
1995-01-25 7:27 ` David O'Brien
1995-01-25 12:14 ` Robert A Duff
1995-01-25 5:57 ` David O'Brien
[not found] ` <3g9rf0$71k@Starbase.NeoSoft.COM>
1995-01-28 21:08 ` David O'Brien
1995-01-31 18:07 ` Samuel Mize
1995-02-01 10:23 ` Samuel Mize
1995-01-30 0:24 ` Mark S. Hathaway
1995-01-31 3:30 ` Jay Martin
1995-02-01 13:25 ` Jesper Kaagaard
-- strict thread matches above, loose matches on Subject: below --
1995-02-10 13:49 R.A.L Williams
[not found] <3gsr0e$oin@miranda.gmrc.gecm.com>
1995-02-07 16:58 ` Mark S. Hathaway
1995-02-08 7:39 ` Travis C. Porco
1995-02-08 16:07 ` Fred J. McCall
1995-02-08 21:30 ` Garlington KE
1995-01-31 9:34 R.A.L Williams
1995-02-01 16:45 ` Charles H. Sampson
1995-01-23 8:49 R.A.L Williams
1995-01-25 23:18 ` Charles H. Sampson
1995-01-20 9:33 R.A.L Williams
[not found] <3fgphd$sc3@rational.rational.com>
1995-01-20 5:51 ` RonaldS60
1995-02-07 13:55 ` Robert C. Soong
[not found] <3fdcoi$chn@miranda.gmrc.gecm.com>
1995-01-20 5:01 ` Samuel Mize
1995-01-20 22:07 ` Garlington KE
1995-01-24 5:02 ` R_Tim_Coslet
[not found] <3etund$hnr@miranda.gmrc.gecm.com>
1995-01-12 9:56 ` Erik Svensson
1995-01-12 14:44 ` Norman H. Cohen
1995-01-13 1:51 ` David O'Brien
1995-01-13 12:38 ` Laurent Gasser
1995-01-13 20:53 ` John DiCamillo
[not found] ` <3f8fnf$c8p@gamma.ois.com>
1995-01-16 11:02 ` Matt Kennel
[not found] ` <milodD2IFpG.329@netcom.com>
1995-01-17 21:39 ` R. William Beckwith
[not found] ` <3fa11q$sdh@gnat.cs.nyu.edu>
1995-01-16 20:20 ` David Moore
1995-01-14 0:24 ` David O'Brien
1995-01-20 4:43 ` Samuel Mize
1995-01-21 20:28 ` David O'Brien
1995-01-22 21:12 ` Robert Dewar
1995-01-23 18:35 ` Norman H. Cohen
1995-01-23 19:18 ` John Cosby - The Coz
1995-01-24 14:11 ` Samuel Mize
1995-01-14 10:37 ` Keith Thompson
[not found] ` <3fcjgt$b0v@cronkite.seas.gwu.edu>
1995-01-16 18:47 ` Robert Dewar
[not found] ` <D2It0r.4rp@inmet.camb.inmet.com>
1995-01-17 14:11 ` Norman H. Cohen
1994-12-30 16:06 Mitch Gart
1995-01-03 19:04 ` whiting_ms@corning.com (Matt Whiting)
1995-01-05 4:31 ` Michael Feldman
1995-01-04 21:40 ` Fred McCall
1995-01-05 4:30 ` Richard Pattis
1995-01-05 16:07 ` Kevin Weise
1995-01-06 13:06 ` Jahn Rentmeister
1995-01-06 16:47 ` Laurent Gasser
1995-01-06 17:29 ` David Weller
1995-01-06 17:30 ` David Weller
1995-01-10 18:28 ` Bob Kitzberger
1995-01-06 23:36 ` Kenneth Almquist
1995-01-04 22:45 ` Jay M. Martin
1995-01-05 4:37 ` Michael Feldman
1995-01-05 18:08 ` Jay Martin
1995-01-05 23:56 ` Robert Dewar
1995-01-08 8:04 ` Jay Martin
1995-01-06 0:07 ` Michael M. Bishop
1995-01-10 21:30 ` Jay Martin
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