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From: jbuck@galileo.berkeley.edu (Joe Buck)
Newsgroups: comp.lang.c++
Subject: Re: parameter[][]
Message-ID: <37961@ucbvax.BERKELEY.EDU>
Date: 3 Aug 90 18:37:18 GMT
References: <1545@m1.cs.man.ac.uk> <26B9BF5A.7291@orion.oac.uci.edu>
Sender: usenet@ucbvax.BERKELEY.EDU
Reply-To: jbuck@galileo.berkeley.edu (Joe Buck)
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jk@cs.man.ac.uk (John Kewley ICL) writes:
> >One of our staff is whingeing about having to specify the size of all
> >dimensions for a multidimensional array (beyond the first that is).
> >Can anyone give me some good reasons why this is, so I can placate him.
> >He claims that it can be done in C so why can't it be done in C++.

eapu034@orion.oac.uci.edu (Carl F. Edman) writes:
> For starters: This is in no way possible in C or C++ and couldn't be
> as the following example might illustrate.
> In one source-file:

> char array[4][6][8];
(rest of example deleted -- you can't do it this way)


Oh, you of little imagination.  Of course it's possible.  You can make an
object that looks just like a 2-D array, and you can pass it around without
worrying about the size.  It can even be a triangular array and use half
the storage.  Here's the trick:

double **mat_alloc (int nrows, int ncols) {
	double **t = new double* [nrows];
	for (int i = 0; i < ncols; i++)
		t[i] = new double [ncols];
}


When John Kewley's friend says "it can be done in C" this is probably what
he means, except that malloc is used instead of new.  The code is a lot
easier to understand when written with new, though.

Voila.  Do the analogous thing for three dimensions.  Now you can use the
object created by this function as a multidimensional array and pass it
around to various functions.

In C++ you can get really fancy and create sparse matrix classes, by
overloading
the [] operator and making separate classes for rows, planes, etc.
--
Joe Buck
jbuck@galileo.berkeley.edu	 {uunet,ucbvax}!galileo.berkeley.edu!jbuck