Path: utzoo!utgpu!water!watmath!clyde!cbosgd!ucbvax!husc6!cca!g-rh
From: g-rh@cca.CCA.COM (Richard Harter)
Newsgroups: comp.lang.c
Subject: Re: Allocating a two-dimensional array
Message-ID: <23344@cca.CCA.COM>
Date: 17 Jan 88 23:18:24 GMT
References: <1050@ogg.cgrg.ohio-state.edu> <22941@hi.unm.edu>
Reply-To: g-rh@CCA.CCA.COM.UUCP (Richard Harter)
Organization: Computer Corp. of America, Cambridge, MA
Lines: 51

In article <22941@hi.unm.edu> kurt@hi.unm.edu (Kurt Zeilenga) writes:
>Spencer@ogg.cgrg.ohio-state.edu (PROCEED) writes:
>>Maybe this can't be done.  Maybe I'm not thinking about it the right way.
>>But I can't figure out how to dynamically allocate a two-dimensional 
>>array of floats.  (I have to have an N by N matrix of floats, but don't
>>know N beforehand.)  Can anyone help me on this one?  Thanks.
>
>You can not allocate multi-dimensional arrays dynamically.  However,
>you can do is something like:

	.... macros deleted ....

I will defer the question of whether Kurt is correct or not to people
more knowledgable in the arcana of C than I am.  I don't know whether
you can cast something as being a pointer to an N by N array where N
is a variable.  However a rather simple approach is to use an array of
pointers, e.g.

	float **array;
	char *malloc();	/* No one forgets this declaration, right? */
	int i;
	.....
	array = (float **)malloc(N*sizeof(float *));
	for (i=0;i<N;i++) array[i] = (float *)malloc(N*sizeof*float));

You can then reference array[i][j] just as though it were a two
dimensional array.  For most applications this is satisfactory and,
in fact, likely to execute faster.

This technique will not work if want to treat the array both as an
N by N two dimensional array and as a N**2 length one dimensional array.
(I.e. you want to exploit contiguity of storage.)  No problem.

	float **arr2, *arr1, *tmp;
	char *malloc();
	int i;
	.....
	arr1 = (float *)malloc(N*N*sizeof(float));
	arr2 = (float **)malloc(N*sizeof(float *));
	tmp = arr1;
	for (i=0;i<N;i++) {
	  arr2[i] = tmp;
	  tmp += N;
	  }

All very simple minded, I'm sure, but it does the trick.  
-- 

In the fields of Hell where the grass grows high
Are the graves of dreams allowed to die.
	Richard Harter, SMDS  Inc.