Thread: help with multidimensional array

if 4 lems are created in a 2 by 2 array, why does the for statement beneath the while statement output index 2 and 3 as being NULL?

Could it be the fact that I flattened the array by accessing it sequentially without the row/column notation? I thought that a multidimensional array was stored in memory sequentially anyways, so the way I am doing it should be fine....

Code:

	for (int r=0; r <= rows; r++)
	{
		for (int	c=0; c <= cols; c++)
		{
			lems[r][c] = new Lem(r, c, (r*c)+c);   // this statement is executed 4 times
			cin >> lems[r][c]->agenda;
			cout << lems[r][c]->agenda << endl;			   			
		}
	}

	Lem** plems = (Lem**)lems; 

	for(int i = 0; i < size; i++)
        {
		if ( plems[i] == 0 ) 
			cout << "huh?" << i << endl;   // 2 and 3 are NULL?   
        }

Can you show the declaration of lems, rows and cols?

Since a[i][j][k] is just syntactic sugar for *(*(*(a + i) + j) + k)
it would seem that multidimensional arrays are stored contiguously.
Here's a program analogous to yours but using int instead of Lem.
It seems to work just fine.

Code:

#include <iostream>
using namespace std;

int main()
{
    const int rows = 2, cols = 2;
    int* a[rows][cols];

    for (int r = 0, i = 1; r < rows; ++r)
        for (int c = 0; c < cols; ++c)
            a[r][c] = new int(i++);

    int** p = (int**) a;
    for (int i = 0; i < rows * cols; ++i)
    {
        cout << i;
        if ( p[i] ) cout << " = " << *p[i] << endl;
        else        cout << " zero pointer\n";
    }
}