What does out[i] = *(a_mat + i) do in C?

Question

while( i < a_rows * a_cols ) {
    out[i] = *(a_mat + i);  // this line
    i++;
}

What does the marked line do?

Answer 1

It gets the value of whatever is pointed to by a_mat + i. It could have been written a_mat[i] instead.

Answer 2

In C, x[i] is the same expression as *(x + i), because adding an integer to a pointer is done by scaling the integer by the size of the pointed-to object, and because it's defined that way.

This means that despite its asymmetrical appearance, the indexing operator [] in C is commutative.

The traditional demonstration of this is something like:

main() {
  int x[] = { 1, 2, 3, 4 };

        printf("%d\n", x[2]);
        printf("%d\n", 2[x]);
}

Both lines are equivalent and print the same thing.