Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!hao!oddjob!gargoyle!ihnp4!ihlpf!nevin1
From: nevin1@ihlpf.ATT.COM (00704a-Liber)
Newsgroups: comp.lang.c
Subject: Pointers and Arrays (was: C machine)
Message-ID: <3546@ihlpf.ATT.COM>
Date: 29 Jan 88 20:57:51 GMT
References: <461@auvax.UUCP> <28700025@ccvaxa> <7159@brl-smoke.ARPA> <8728@ism780c.UUCP>
Reply-To: nevin1@ihlpf.UUCP (00704a-Liber,N.J.)
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 55

In article <8728@ism780c.UUCP> marv@ism780.UUCP (Marvin Rubenstein) writes:
>BTW, the definition of pointer difference in K&R is "if p and q point to
>members of the same array , p-q is the number of elements between p and q."

I opened up my K&R, skimmed all of chapter 5 (Pointers and Arrays), and could
not find this anywhere!  (Maybe I didn't look hard enough :-))  I don't think
this is how K&R defined it (see more comments below).
>
>One way to interpret this is the following:
>
>     el1  el2  el3
>      ^         ^
>      |         |
>      p         q
>
>As can be seen from the diagram the number of elements "between" p and q is
>1 (not 2).  Furthermore, the number of elements between p and q is clearly
>the same as the number of elements between q and p.  i.e, p-q == q-p and no
>overflow is possible.  I hope the defininition in the proposed standard is
>less ambigious than the wording in K&R.

Suppose q pointed to el2 instead of el3.  Would q-p === 0?  And if this were
true (which I maintain isn't), then what would q-p be if p and q pointed to the
same element?

Also, p-q is not equal to q-p (except in the trivial case where q === p).
There is no implied absolute value taken when subtracting pointers.
(p-q) === -(q-p)
For justification of what I am saying, I submit the following program:
==============================================================================
	main()

	{
	int	array[9];
	int	*p1;
	int	*p2;

	p1 = &array[3];
	p2 = &array[7];
	printf("%d\t", (int)(p1 - p2));
	printf("%d\n", (int)(p2 - p1));
	}
==============================================================================
When compiled and executed, this program produces:
	-4	4
QED


I don't know where your ideas came from, but they certainly weren't from K&R.
Maybe you were reading the new draft standard for ANSI C :-) :-)!!
-- 
 _ __			NEVIN J. LIBER	..!ihnp4!ihlpf!nevin1	(312) 510-6194
' )  )				"The secret compartment of my ring I fill
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/  (_</_\/ <__/ / <_	These are solely MY opinions, not AT&T's, blah blah blah