Question

Hi I’ have come across to a code that I have hard time to understand why it is working and I would appreciate if someone could explain me
Here is the code This is actually a very simplified version of the real code which can be found here http://acts.nersc.gov/hypre/example10/ex10.cxx.html

#include <iostream>
int main(int argc, char *argv[]){
double ***elemStiff = new double**[10];
int i, k;

for (i = 0; i < 10; i++){
    elemStiff[i] = new double*[4];
    for (k = 0; k < 4; k++){
        elemStiff[i][k] = new double[4];
    }

    double **A = elemStiff[i];
    for (k = 0; k < 4; k++)
        A[k][k] = 2/3.;

    A[0][1] = A[1][0] = -1/6.;
    A[0][2] = A[2][0] = -1/3.;
    A[0][3] = A[3][0] = -1/6.;
    A[1][2] = A[2][1] = -1/6.;
    A[1][3] = A[3][1] = -1/3.;
        A[2][3] = A[3][2] = -1/6.;
    }
    std::cout << elemStiff[1][0][0] << std::endl;
}

In short, here we initialize and fill the values of the elemStiff array which has 10 rows and each row is a 4×4 matrix.

What is very weird to me is that the code fills in the entries of elemStiff array with the help of the double **A = elemStiff[i];

When I first saw I though it would give an error but it works!. The values passed on A, they are passed at the same time on elemStiff array, but I dont understand why it works

Thank you

Answer 1

Let’s start with simpler version:

int* array = new int[10];
int* ptr = array;            // equivalent to ptr = &array[0]

for (int i = 0; i < 10; ++i)
    ptr[i] = i;              // writing into the memory that ptr points to

for (int i = 0; i < 10; ++i)
    std::cout << array[i];   // outputs: 0123456789

In this example we create a pointer ptr that points to the first element of array. Then when we are writing into the memory that ptr points to, we are actually writing into that array. So then when we check values of array elements, we can see they were properly initialized.

Your code is pretty much the same, there’s just an array of 10 two-dimensional arrays:

double ***elemStiff = new double**[10];

And then just like we used ptr in my simple example, you use a temporary pointer A to make it point to each of those 10 two-dimensional arrays:

for (i = 0; i < 10; i++)
    double **A = elemStiff[i];