Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!rochester!udel!udccvax1!gdtltr
From: gdtltr@vax1.acs.udel.EDU (Gary D Duzan)
Newsgroups: comp.lang.c
Subject: Re: Adding two pointers
Message-ID: <3596@udccvax1.acs.udel.EDU>
Date: 11 May 89 00:06:37 GMT
References: <2765@buengc.BU.EDU> <563@lzaz.ATT.COM> <4093@ficc.uu.net> <922@garcon.cso.uiuc.edu> <924@garcon.cso.uiuc.edu> <113@ssp1.idca.tds.philips.nl> <1321@infmx.UUCP>
Reply-To: gdtltr@vax1.acs.udel.EDU (Gary D Duzan)
Organization: Galadrim (really a disorganization)
Lines: 48

In article <1321@infmx.UUCP> kevinf@infmx.UUCP (Kevin Franden) writes:
=>
=>Assuming that there does exist an application that wnats/needs to add
=>pointers, why won't this 'model' work?
=>
=>	Assume an array a to be made up of elements of
=>	arbitrary size.  Assume further that this array has
=>	100 elements arranged (as we know C does) contiguously
=>	in memory.
=>
=>	if p1 is pointing to element 37 and p2 is pointing to
=>	element 4, I'd like p3 to point to a[37+4] or a[41].
=>	
=>	struct{...}foo;
=>
=>	foo p1,p2,p3;
=>
=>	x=37;
=>	y=4;
=>
=>	p1=a[x];
=>	p2=a[y];
=>	p3=(x+y)*sizeof(foo);
=>
=>
=>I realize that I missed part of this discussion, sorry if this has been
=>posted already, but won't p3 be pointing to the 41st element?  if this
=>is true, this is pointer addition, no?
=>
   No. I believe what you want is a[x+y] . p3 will be the number of
bytes taken up by x+y elements of foo. You also might be thinking
a+x+y . This should be legal and do what you want.
   Sorry if I am incorrect here, but I am a bit new to C myself. This
is a bit odd when you consider the fact that I am trying to put together
a minimal K&R C compiler (don't ask for what machine, you'll just laugh).

					Gary Duzan
					Time  Lord
				    Third Regeneration




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