Pointers and Dynamic Allocation of Memory method 3 in C programming language

Pointers and Dynamic Allocation of Memory

METHOD 3:

Consider the case where we do not know the number of elements in each row at compile time, i.e. both the number of rows and number of columns must be determined at run time. One way of doing this would be to create an array of pointers to type int and then allocate space for each row and point these pointers at each row. Consider:  

-------------- Program 9.2 ------------------------------------

/* Program 9.2 from PTRTUT10.HTM   6/13/97 */

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    int nrows = 5;     /* Both nrows and ncols could be evaluated */
    int ncols = 10;    /* or read in at run time */
    int row;
    int **rowptr;
    rowptr = malloc(nrows * sizeof(int *));
    if (rowptr == NULL)
    {
        puts("\nFailure to allocate room for row pointers.\n");
        exit(0);
    }

    printf("\n\n\nIndex   Pointer(hex)   Pointer(dec)   Diff.(dec)");

    for (row = 0; row < nrows; row++)
    {
        rowptr[row] = malloc(ncols * sizeof(int));
        if (rowptr[row] == NULL)
        {
            printf("\nFailure to allocate for row[%d]\n",row);
            exit(0);
        }
        printf("\n%d         %p         %d", row, rowptr[row],
rowptr[row]);
        if (row > 0)
        printf("              %d",(int)(rowptr[row] - rowptr[row-1]));
    }

    return 0;
}

--------------- End 9.2 ------------------------------------

In the above code rowptr is a pointer to pointer to type int. In this case it points to the first element of an array of pointers to type int. Consider the number of calls to malloc(): 

    To get the array of pointers              1     call
    To get space for the rows                5     calls
                                                        -----
                                Total                   6     calls

If you choose to use this approach note that while you can use the array notation to access individual elements of the array, e.g. rowptr[row][col] = 17;, it does not mean that the data in the "two dimensional array" is contiguous in memory.
You can, however, use the array notation just as if it were a continuous block of memory. For example, you can write: 

    rowptr[row][col] = 176;

just as if rowptr were the name of a two dimensional array created at compile time. Of course row and col must be within the bounds of the array you have created, just as with an array created at compile time.  If you want to have a contiguous block of memory dedicated to the storage of the elements in the array you can do it as follows: