Question

In C, when we access a[i][j] using pointers why do we need the second * in *(*(a + i) + j)? Using printf() I see a + i and *(a + i) print the same value.

Answer 1

a + i is a pointer to the i‘th subarray. If you dereference it, you get an lvalue to the i‘th subarray, which decays to a pointer to that array’s first element. The address of an array’s first element and that of its array is the same.

The dereference is needed to make + j calculate with the correct element byte-width. If you would not dereference, then instead of getting T* you would get a T(*)[J] pointer that you add j to, which together with i advances into memory pointing to (a + i + j) instead (advancing by sizeof(T[J]) instead of sizeof(T)).