You are required to implement a stack using an array. If the array is full, the bottom item is dropped from the stack.

Question

how to do that

Answer 1

I'd write a container class that contains the array as private member and keeps track of the amount of elements that were inserted. Also this class should offer methods like push, pop and top to add, delete and access elements.
If the array is full inserting a new element is very expensive because you need to shift the whole array...
Btw. using an array isn't a good idea and the STL has already a stack container that's usually implemented very well.
http://www.cplusplus.com/reference/stack/stack/