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From: dobrien@seas.gwu.edu (David O'Brien)
Newsgroups: comp.lang.ada
Subject: Re: "Subtract C, add Ada"
Date: 31 Jan 1995 22:22:05 GMT
Organization: George Washington University
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References: <3fo2ot$su2@miranda.gmrc.gecm.com> <3fode9$dap@soleil.uvsq.fr>
 <3g1shu$ksc@rational.rational.com> <3gdro9$aud@spam.csv.warwick.ac.uk>
 <3ghtke$9jc@felix.seas.gwu.edu>
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Date: 1995-01-31T22:22:05+00:00
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Michael Feldman (mfeldman@seas.gwu.edu) wrote:
: In article <3gdro9$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)