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Growable array based stack

We all know about Stacks also known as Last-In-First-Out(LIFO) structures. Stack primarily has two main operation namely push and pop, where push inserts an element at top and pop removes an element from top of the stack.

Now, whenever an implementation of stack is considered its size is pre-determined or fixed. Even though it is dynamically allocated, still once it is made its size cannot be changed. And hence a condition called “stack full” arises.

But what if a stack can grow as more elements are inserted or more elements are going to be inserted in future. Remember, we are talking about array based Stack. Growable Stack is the concept of allocating more memory such that “stack full” condition does not arises easily.

A Growable array-based Stack can be implemented by allocating new memory larger than previous stack memory and copying elements from old stack to new stack. And then at last change the name of new stack to the name which was given to old stack

There are two strategy for growable stack:
1. Tight Strategy : Add a constant amount to the old stack (N+c)
2. Growth Strategy : Double the size of old stack (2N)

There are two operation on growable stack:
1. Regular Push Operation: Add one element at top of stack
2. Special Push Operation: Create a new stack of size greater than old stack (according to one of the strategy above) and copy all elements from old stack and then push the new element to the new stack.



// CPP Program to implement growable array based stack
// using tight strategy
#include <iostream>
using namespace std;
  
// constant amount at which stack is increased
#define BOUND 4
  
// top of the stack
int top = -1;
  
// length of stack
int length = 0;
  
// function to create new stack
int* create_new(int* a)
{
    // allocate memory for new stack
    int* new_a = new int[length + BOUND];
  
    // copying the content of old stack
    for (int i = 0; i < length; i++)
        new_a[i] = a[i];
  
    // re-sizing the length
    length += BOUND;
    return new_a;
}
  
// function to push new element
int* push(int* a, int element)
{
    // if stack is full, create new one
    if (top == length - 1)
        a = create_new(a);
  
    // insert element at top of the stack
    a[++top] = element;
    return a;
}
  
// function to pop an element
void pop(int* a)
{
    top--;
}
  
// function to display
void display(int* a)
{
    // if top is -1, that means stack is empty
    if (top == -1)
        cout << "Stack is Empty" << endl;
    else {
        cout << "Stack: ";
        for (int i = 0; i <= top; i++)
            cout << a[i] << " ";
        cout << endl;
    }
}
  
// Driver Code
int main()
{
    // creating initial stack
    int *a = create_new(a);
  
    // pushing element to top of stack
    a = push(a, 1);
    a = push(a, 2);
    a = push(a, 3);
    a = push(a, 4);
    display(a);
  
    // pushing more element when stack is full
    a = push(a, 5);
    a = push(a, 6);
    display(a);
  
    a = push(a, 7);
    a = push(a, 8);
    display(a);
  
    // pushing more element so that stack can grow
    a = push(a, 9);
    display(a);
  
    return 0;
}

Output:

Stack: 1 2 3 4 
Stack: 1 2 3 4 5 6
Stack: 1 2 3 4 5 6 7 8
Stack: 1 2 3 4 5 6 7 8 9


This article is attributed to GeeksforGeeks.org

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