contiguous 2d arrays, Please take a look....

[Abed M. Hammoud]

	Hello, 

	I hope this question will not upset anyone, ....

	I use two dimensional matrices very frequently. Negative
	indexing is inherent in some of the applications that I write.
	So I like being able to say for example x[-10][-20]...etc...

	For sometime I used the C code in Numerical recipes that suggest
	using the following:

	for example to declare an int matrix x with indices 
	i=-10, 10 and j=-10, 10; 
	you would call the function below:

	x = imatrix(-10, 10, -10, 10); 
		     ^   ^    ^   ^
		     nrl nrh  ncl  nch

	Then to access the ith, jth element you type x[i][j].

        Listed below is the function imatrix, 
/*----------------------------------------------------------------------*/
/*----------------------------From Numerical Recipes-------------------*/
int **imatrix(int nrl, int nrh, int ncl, int nch)
{
   int i,**m;

   m=(int **)malloc((unsigned int) (nrh-nrl+1)*sizeof(int*));
   if (!m) nrerror("allocation failure 1 in imatrix()");
   m -= nrl;

   for(i=nrl;i<=nrh;i++) {
     m[i]=(int *)malloc((unsigned int) (nch-ncl+1)*sizeof(int));
     if (!m[i]) nrerror("allocation failure 2 in imatrix()");
     m[i] -= ncl;
   }
   return m;
}
/*----------------------------------------------------------------------*/

	The above method is ok, but the array elements are not
	contiguous in memory.  So, I looked up the CFAQ to see what
	info does it have about this and I got the following....

/*----------------------------------------------------------------------*/
/* ------from CFAQ-----------------*/

You can keep the arrays contents contiguous, while making later
reallocation of individual rows difficult, with a bit of explicit
pointer arithmetic:

>>int **array = (int **)malloc(nrows * sizeof(int *));
>>array[0] = (int *)malloc(nrows * ncolumns * sizeof(int));
>>for(i = 1; i < nrows; i++)
>>  array[i] = array[0] + i * ncolumns;

/*----------------------------------------------------------------------*/

	The above code worked fine, I can access the ith, jth element
	by typing x[i][j] or by just incrementing a pointer. But the
	above code does not allow -ve indexing....I tried to modifiy it
	but I was not successfull....

	So my question is, can any body help me in making the above
	code work so that I can have -ve indexing.

	I would Appreciate any comments, No flames please....


	--------------------------------------------------------------
	| Abed  M. Hammoud			abed at saturn.wustl.edu|
	| Washington University.	        office:(314)726-7547 |
	| Electronic Systems & Signals Research Laboratory.          |
	| Dept. of Electrical/Biomedical Engineering.		     |
	| St.Louis Mo U.S.A                                          |
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