Why we can simply write like:- ptr =a;

Question

Instead of ptr =&a[0];

Answer 1

Array identifiers, when used, return the address of the first element in the array. The notation

a[0]

actually translates to

*(a + 0)

Likewise, a[1] is *(a+1), a[2] is *(a+2) and so on. Assigning a to a pointer is hence legitimate, since a represents an address which can be stored into a pointer of the corresponding type.

Try doing

int a[5];
std::cout << a;

Answer 2

This is because array name is always pointing towards first element of array
So address of first element in array i.e &a[0] is same as a because array name itself certainly gives address of first element stored in it!