Path: utzoo!utgpu!water!watmath!clyde!cbosgd!ihnp4!chinet!dag
From: dag@chinet.UUCP (Daniel A. Glasser)
Newsgroups: comp.lang.c
Subject: Re: C machine
Message-ID: <2155@chinet.UUCP>
Date: 29 Jan 88 06:39:27 GMT
References: <461@auvax.UUCP> <28700025@ccvaxa> <7159@brl-smoke.ARPA> <8728@ism780c.UUCP>
Reply-To: dag@chinet.UUCP (Daniel A. Glasser)
Organization: Chinet - Public Access Unix
Lines: 50

In article <8728@ism780c.UUCP> marv@ism780.UUCP (Marvin Rubenstein) writes:
>
[stuff deleted]
>
>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."
>
>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.
>
I think you misinterpret the meaning of where things point to.

The correct diagram for this would be

    +-----+-----+-----+-----	You see, this is not ambiguous at
    | el1 | el2 | el3 | ...	all -- there are exactly two elements
    +-----+-----+-----+-----	between the addresses pointed to by
    ^           ^          	p and q.
    |           |
    p           q

This is a common mistake for novice programmers and HLL programmers
who are not familiar with pointers to make.  Pointers genereally
contain the address where the "pointed to" item begins, not the
item itself.  When I was tutoring in college, one of the professors
overdid the "mailbox" analogy, and a number of students believed that
the computer must have different sized "boxes" for different kinds
of data...  So integer boxes were addressed the same as float boxes,
just in a different part of memory.  "I have a program that the Fortran
compiler says has too much integer data.  I'm not using any reals, so can
I use the real storage for integers?" (I actually got that question!)

I find that Basic programmers make this error most often.

-- 
Nobody at the place where I work	Daniel A. Glasser
knows anything about my opinions	...!ihnp4!chinet!dag
my postings, or me for that matter!	...!ihnp4!mwc!dag
					...!ihnp4!mwc!gorgon!dag
	One of those things that goes "BUMP!!! (ouch!)" in the night.