Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!rutgers!apple!ames!ncar!tank!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: Another silly question
Message-ID: <17580@mimsy.UUCP>
Date: 18 May 89 05:04:53 GMT
References: <17812@cup.portal.com> <2336@Portia.Stanford.EDU>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 22

In article <2336@Portia.Stanford.EDU> mesmo@Portia.Stanford.EDU
(Chris Johnson) writes:
>The supposed proof of a[i] == i[a] rests on the faulty
>assumption that (x+y) == (y+x) in all contexts; this is
>not correct.  When "+" denotes simple (ie int/float/etc)
>arithmetic, the operation commutes; when it denotes pointer
>arithmetic, commutation is not legal/meaningful.

The latter assertion is exactly backwards.  Pointer arithmetic (in
the forms pointer+integer and integer+pointer) is guaranteed to be
commutative, while scalar addition is not: scalar addition of certain
values is not commutative on certain peculiar architectures---things
like negative zero or peculiar floating point values, for instance.

>The statement that *(a+i) == *(i+a) is therefore invalid.

Since pointer arithmetic is commutative, the above statement is wrong,
and *(a+i) is equivalent to *(i+a), so that a[i] and i[a] denote the
same object.  See K&R (either edition) chapter 5.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris