Expert: Joseph Moore - 2/21/2010

Question
How can STACK be implemented using array??
coz array is static memory allocation unit.
so when we POP any element frm stack it cannot be deleted frm memory...
Plz help me.....

Answer
A stack is merely a data structure concept.  The only rules that must be followed when implementing a stack is that data must be first-in-last-out.  Often times, stacks are implemented with dynamic memory and pointers, but can be just as easily implemented with an array.  

Essentially, you must simply maintain the index of the current top of the stack.  To push an item onto the stack, you simply assign your new item to the current top.  To pop an item off of the stack, you simply return the current top item and decrement your top index.  Something like:

   void push(int _item)
   {
       stack[topIndex++] = _item;
   }

   int pop()
   {
       return stack[--topIndex];
   }

In the above implementation, the topIndex value always points to the current "empty" index in the array.  It will initialize to 0 with an empty stack array.  Also, note that I'm not doing any error checking in those functions.  You should make sure that your index never goes out of bounds and error appropriately if it does.  This is just a brief synopsis of how this would be implemented.  In this implementation, the last item pushed onto the stack will always be at topIndex - 1.