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Stack implementation in C++

Stack is a versatile data structure used in situations that need LIFO - last in first out. For an introduction to stack, read this post

 Data members

To implement a stack using an array, we will need an array and an index to the top of the stack. 

We can have a static array or a dynamic array. Dynamic array has the advantage that size of array can be given at run time. 

So let us have a dynamic array and also a data member called size.

class stack
{
    int max;
    int *arr;
    int top;
public:
    stack(int n);
    void push(int n);
    int pop();
    bool is_empty();
    bool is_full();
    int peek();
    ~stack();
    stack(stack&other);
};

Methods

In the constructor of this class we must initialize the data. We need to set top to -1, because we do not have any value in the stack currently. And as we are using dynamic array, we should allocate memory to array in the constructor.

We need to also write a destructor to release this memory. 

stack::stack(int size=10):max(size)
{
   top = -1;
   arr = new int[max];
}

stack::~stack()
{
    delete []arr;
}

What are the other methods needed for stack class? We know these. pop(), push(), peek(),isempty(), isfull().

push()

A push function should add a value to stack after making sure that stack is not full. After pushing the value top must be incremented.  So we need a statement of the type arr[++top]=value.   

void stack::push(int n)
{
     if(is_full())
       cout<<"Stack overflow.";
     else
 {
    arr[++top]=n;
        }
}
bool stack::is_full()
{
      return top>=max;
}

And we are using the function is_full(). The stack will be full if top == size of the array.

pop()

pop function should remove an element from the top of the stack and then decrement top. We need a statement of the type return arr[top--].

But the function should check if the stack is empty before popping.

int stack::pop()
{
    if(is_empty())
       cout<<"Stack empty";
     else
 return arr[top--];
}
bool stack::is_empty()
{
     return top==-1;
}

peek()

peek() must return the value at the top of stack without actually removing it. 

int stack::peek()
{
   return arr[top];
}

Now we are ready with all the necessary functions of the class.

Well, almost. Because now the class when used will crash your program. Why?

Because of dynamic allocation and lack of copy constructor.  If we use this class as a value parameter to any function, it will copied to a temporary object.

This copy is done using the default copy constructor provided by compiler which does not allocate memory for the array of new object. The temporary copy parameter and the original object use the same dynamic array. When the function exits, the temporary copy is destroyed using destructor. Its memory is deleted.

That leaves the original object with a dangling pointer and deleting it at the end of program gives run time error.

So we need one extra method for the class - not part of stack ADT and that is the copy constructor. This copy constructor should allocate memory for the new object and copy all the elements.

Copy constructor

stack::stack(stack&other)
{
    this->max = other.max;
    this->top = other.top;
    this->arr  = new int[max];//allocate memory
    for(int i=0;i<max;i++)//copy elements
 arr[i]=other.arr[i];
}

Let us write a program to test the class. You can download this program from here.


#include<iostream>
using  std::cin;
using std::cout;
class stack
{
    int max;
    int *arr;
    int top;
public:
    stack(int n);
    void push(int n);
    int pop();
    bool is_empty();
    bool is_full();
    int peek();
    ~stack();
    stack(stack&other);
};

stack::stack(int size=10):max(size)
{
   top = -1;
   arr = new int[max];
}

stack::~stack()
{
    delete []arr;
}
stack::stack(stack&other)
{
    this->max = other.max;
    this->top = other.top;
    this->arr  = new int[max];
    for(int i=0;i<max;i++)
 arr[i]=other.arr[i];
}

void stack::push(int n)
{
     if(is_full())
       cout<<"Stack overflow.";
     else
 {
    arr[++top]=n;
        }
}
int stack::pop()
{
    if(is_empty())
       cout<<"Stack empty";
     else
 return arr[top--];
}
bool stack::is_empty()
{
     return top==-1;
}
bool stack::is_full()
{
      return top>=max;
}
int stack::peek()
{
   return arr[top];
}
void process(stack &s,int n)
{
   int m;
   switch(n)
   {
 case 1:cout<<"Number to push"; 
        cin>>m;
        s.push(m);
  break;
 case 2: if(!s.isempty()){
   m = s.pop(); 
  cout<<"value popped is"<<m<<"\n";
  }else
     cout<<"Stack empty";
  break;
 case 3:  m = s.peek(); 
  cout<<"value at the top of stack is"<<m<<"\n";
  break;
     }
} 
    
int main()
{
     stack s1;
     while(true)
     {
 int n;
        cout<<"Enter 1 to push 2 to pop 3 to peek 4 to quit";
        cin>>n;
        if(n==4)
    break;
        process(s1,n);
      }
}

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