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From: braner@batcomputer.tn.cornell.edu (braner)
Newsgroups: comp.lang.c
Subject: Re: pointers to arrays
Message-ID: <1554@batcomputer.tn.cornell.edu>
Date: Thu, 20-Nov-86 14:26:28 EST
Article-I.D.: batcompu.1554
Posted: Thu Nov 20 14:26:28 1986
Date-Received: Fri, 21-Nov-86 05:07:21 EST
References: <273@bms-at.UUCP> <1138@genrad.UUCP>
Reply-To: braner@batcomputer.UUCP (braner)
Organization: Theory Center, Cornell University, Ithaca NY
Lines: 25
Keywords: What? Why? How?
Summary: Arrays of pointers to arrays are more useful

[]

Pointers to arrays come in handy for efficiency in accessing 2-d
arrays.  See K&R about "sparse arrays".  Say you have the "page"
example:

	char page[25][80];

and you want to avoid the slow calculations needed for a direct access
such as "c = page[i][j];".  Set up another array of pointers to lines:

	char *line[25];

Initialize it once to point to the lines:

	for (i=0; i<25; i++)
		line[i] = &page[i][0];

Later you do "c = line[i][j];" - which looks the same but is MUCH faster,
since no multiplication is needed to calculate &line[i][j].  (Well, it
IS necessary to multiply i by sizeof(char *), but hopefully your compiler
is smart enough to do that by a shift, since the size is a power of two -
on most machines.)

- Moshe Braner,  Cornell Univ.