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From: judah@uts.uucp (Judah Greenblatt)
Newsgroups: comp.lang.c
Subject: Dynamically allocated multi-dimensional arrays
Message-ID: <124@towernet.UUCP>
Date: 20 Nov 89 14:12:18 GMT
Sender: netnews@towernet.UUCP
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I've recently been playing around with dynamically allocated
multi-dimensional arrays in C.  So far I've found two ways to
handle them:

* As a one-dimensional array, using a macro to do the subscript calculations:
	double *a;
	a = (double *)malloc(rows * cols * sizeof(double));
	#define Sub(a, x, y) ((a)[(x) * cols + (y)])
	d = Sub(a, i, j);
  This involves no extra space, no extra memory references, but looks ugly.

* As an array of pointers to arrays:
	double **a;
	a = (double **)malloc(rows * sizeof(double *));
	for (i=0; i<rows; i++)
		a[i] = (double *)malloc(cols * sizeof(double));
	d = a[i][j];
  This requires space for a vector of pointers, but allows you to use
  the resulting structure exactly as throgh it were a statically-allocated
  multi-dimensional array.

(In both cases, you can pack the row and column dimensions along with the
array pointer if necessary.)

What I would like to know is how the existing matrix libraries (linpac,
eispac, imsl) handle varying-sized array arguments in C.  (How many of
these libraries have even been translated into C?)

(This problem reminds me of how the IMSL FORTRAN library handled this
case - the compiler did the subscript calculations, but you had to pass
explicit row and column size information to the subroutines.)

Please mail replys to one of the addresses below - I can no longer
afford the time to wade through all of comp.lang.c.


Judah Greenblatt, Bellcore, Morristown, New Jersey, USA, +1-201-829-3059
	judah@mrevox.bellcore.com
	...!bellcore!mrevox!judah