Stack implemantation in c++ using linked list

Question

#include<iostream>
#include<cstdlib>
using namespace std;

 struct node
 {
   int data;   //data
   node *next; //link
 };

 class stack // stack using linked list
 {
  public:
  node *top; // top element of stack

  public:
   stack()
   {
     top= NULL;
   }

   void push(int value)
   {
     node *temp  = new node; // create a new node
     temp-> data = value;
     temp-> next = NULL;
   if(top==NULL)          //  stack is empty
    {
      top=temp;
      temp=NULL;
    }
    else
    {
      temp-> next = top;
      top=temp;
      temp=NULL;
    }
  }
  //template <class X>
  void pop()
  {
    if(top==NULL)
    {
      cout<<"\nStackOverflow "<<endl;
      cout<<"Program Terminated "<<endl;
      exit (0);
    }
    else
    {
      top=top->next;
    }

  }

  void display()
  {

    node *temp=new node;
    temp=top;

    while(temp!=NULL)
    {
      cout<<temp->data<<" ";
      temp = temp-> next;
    }
    while(top==NULL)

     { 
        cout<<"\nStack is Empty "<<endl;  
        exit (0);

     }



  }
};

int main()
{
   stack a;

  a.push(5);
  a.display();
  a.push(10);
  a.display();
  a.pop();
  a.pop();
  a.push(20);
  a.display();
  a.pop();
  a.display();
  return 0;
}

The Output of this code is 5 10 5 20 Stack is Empty.

Which is wrong output and the correct output is 5 10 20 Stack is Empty..

Anyone tell me why this errors occured.

The Refrence of the code :[Implementation of stack using Templates and And Linked List in c++

Unrelated to your problem, but your display function have a memory leak. — Some programmer dude
– Some programmer dude, Commented Sep 28, 2018 at 7:39
As for your problem, you push two values, then pop both values (making the stack empty), then push one value and pop that value again making the stack empty. What else did you expect? Please edit your question to include the expected output. — Some programmer dude
– Some programmer dude, Commented Sep 28, 2018 at 7:41
dispaly() leaks memory somewhere iknow but how can i overcome with that — Akash
– Akash, Commented Sep 28, 2018 at 7:41
@AkkiêThakur Just change node *temp=new node; to node *temp;. There's no need to allocate a node there because the very next line says temp=top; Your version allocates a node and then throws it away, a.k.a a memory leak. — john
– john, Commented Sep 28, 2018 at 7:48

Blaze · Accepted Answer · 2018-09-28 07:52:38Z

6

No, the output is correct.

a.push(5);
a.display();

This displays the first 5.

a.push(10);
a.display();

The 5 is still on the stack, so now this displays the 10 and then the 5.

a.pop();
a.pop();
a.push(20);
a.display();

Now everything is removed, the 20 is added and the displayed, so this should just display 20.

And then the empty stack is printed with

a.pop();
a.display();

So put together, it should display 5 10 5 20 Stack is Empty.

edited Sep 28, 2018 at 7:52

answered Sep 28, 2018 at 7:41

Blaze

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4 Comments

Akash Over a year ago

sir i think the correct output will be 5 10 20 Stack is Empty

Blaze Over a year ago

Nah, the 5 is correctly printed twice. You put the 5 on the stack and call display() twice before popping the 5 off the stack, so it is printed both times.

john Over a year ago

a.push(5); a.display(); a.push(10); a.display(); you call display twice with 5 on the stack. So it will be printed twice. The output of this code is 5 10 5, as you see.

john Over a year ago

@AkkiêThakur I'm guessing that you think display should show the last item pushed onto the stack, but it doesn't it shows the whole stack.

Swordfish · Accepted Answer · 2018-09-28 15:50:35Z

4

a.push(5);     // Stack:  5
a.display();   // Output: new: 5
a.push(10);    // Stack:  10 5
a.display();   // Output: old: 5 new: 10 5
a.pop();       // Stack:  5
a.pop();       // Stack:  empty
a.push(20);    // Stack:  20
a.display();   // Output: old: 5 10 5 new: 20
a.pop();       // Stack:  empty
a.display();   // Output: old: 5 10 5 20 new:

stack()
{
    top= NULL;  // use initializ list instead of assignment
                // in constructors body
}

-->

stack() : top{ nullptr } {}

void pop()
{
    if (top == NULL)
    {
        cout << "\nStackOverflow " << endl;
        cout << "Program Terminated " << endl;
        exit(0);  // don't use exit() in C++ if there are other ways!
    }
    else
    {
        top = top->next; // the memory top pointed to
                         // before the assignment leaks!
    }
}

-->

void pop()
{
    if (!top) {
        cout << "Pop on empty stack!\n";
        return;
    }

    node *old_top = top;
    top = top->next;
    delete old_top;
}

void display()
{
    node *temp = new node;  // no need to allocate a node in a
                            // function that should only make output
    temp = top;             // the memory temp points to before the
                            // assignment leaks

    while (temp != NULL)
    {
        cout << temp->data << " ";
        temp = temp->next;
    }
    while (top == NULL)  // why a loop?
    {
        cout << "\nStack is Empty " << endl;
        exit (0);   // again ...
    }
}

-->

void display() const
{
    if (!top) {
        std::cout << "Stack is empty!\n";
        return;
    }

    for(node *current = top; current; current = current->next)
        cout << current->data << ' ';
}

edited Sep 28, 2018 at 15:50

answered Sep 28, 2018 at 7:50

Swordfish

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6 Comments

Akash Over a year ago

sir when i implemented your code it shows error in pop().

Akash Over a year ago

display () is well implemented sir. Overall thank you sir for your help

Swordfish Over a year ago

@AkkiêThakur "it shows error in pop()" yes, there was a } missing.

Akash Over a year ago

but sir it shows the output 5 10 5 20 Stack is empty! is this true sir

Akash Over a year ago

@sir why this print 5 before 20 in the output

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