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From: glb@ncsu.UUCP (Griff Bilbro)
Newsgroups: net.lang.c
Subject: How to pass subarrays?
Message-ID: <2839@ncsu.UUCP>
Date: Wed, 1-May-85 14:43:05 EDT
Article-I.D.: ncsu.2839
Posted: Wed May  1 14:43:05 1985
Date-Received: Fri, 3-May-85 04:44:37 EDT
Organization: N.C. State University, Raleigh
Lines: 35

<< Propitiation for the line eating trolls >>

I'm doing a lot of 2D programming lately.  Conceptually everything
behaves like 2D arrays, so I want to access things with 2 indices.
That's easy enough, but I want to pass a window of an array to a sub-
routine so that it looks like the original array except that it's
offset.  I want the subroutine to be able to use negative subscripts.
Here's an illustration:

#define S 20
main()
{
	int	big[S][S],
		AverageAtCenter;

	GenerateArray( big );
	AverageAtCenter = LocalAverage( &big[ S/2 ][ S/2 ] );
	printf( "Average at center is %d\n", AverageAtCenter );
}

LocalAverage( window )
	int	window[S][S],
		result;
{
	result = window[-1][0] + window[0][1] +
		window[1][0] + window[0][-1];
	result /= 4;
	return result;
}

At this point, I don't care about speed.  I need the code to look
like the math.  The code above works, but lint complains.  Why does
lint complain?  How can I make C do the indexing arithmetic the way
I want that lint likes?  It amounts to saying w[i][j] is equivalent
to w[i*S+j].  Can't I declare a argument pointer to index like this?