abed@saturn.wustl.edu (Abed M. Hammoud) (04/27/91)
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@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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