Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!apple!bloom-beacon!bu-cs!buengc!bph
From: bph@buengc.BU.EDU (Blair P. Houghton)
Newsgroups: comp.lang.c
Subject: Re: Another silly question
Message-ID: <2904@buengc.BU.EDU>
Date: 18 May 89 19:15:44 GMT
References: <17812@cup.portal.com> <2336@Portia.Stanford.EDU>
Reply-To: bph@buengc.bu.edu (Blair P. Houghton)
Followup-To: comp.lang.c
Organization: Boston Univ. Col. of Eng.
Lines: 30

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.

Oh yeah?

>	When "+" denotes simple (ie int/float/etc) arithmetic, the
>	operation commutes; when it denotes pointer arithmetic,
>	commutation is not legal/meaningful.
>
>	The statement that *(a+i) == *(i+a) is therefore invalid.

it implies that you were doing

	sometype *a, *i;

	something = a[i];
	something_else = i[a];

So, like, tell me.  When do you use pointers as indices?  I.e., if one of
the two variables, a or i, is an int, and the other is a pointer, then
you have leave to say that pointer[int] == int[pointer] because
*(pointer + int) == *(int + pointer) and because
*((pointer or int) + (the other)) == (pointer or int)[the other].

				--Blair
				  "I still think I should be able
				   to add pointers together."