Wsrtjtyk

I am worked on a Linked list stack and a bunch of functions for it. What I do not understand currently is how come my "isEmpty" function is not working correctly. I believe that the way I have it written makes sense. By nature if Front is Null then the list should have to be empty which would mean that "isEmpty" would return false. The problem that I am having is that my program says that the list is always empty whether or not it actually is or not. I am not sure what the issue is. Any help would be appreciated.

struct node

{

    int data;

    node *next;

}*front = NULL, *rear = NULL, *p = NULL, *np = NULL;

void push(int x)

{

    np = new node;

    np->data = x;

    np->next = NULL;

    if(front == NULL)

    {

        front = rear = np;

        rear->next = NULL;

    }

    else

    {

        rear->next = np;

        rear = np;

        rear->next = NULL;

    }

}

int pop()

{

    int x;

    if (front == NULL)

    {

        cout<<"empty queuen";

    }

    else

    {

        p = front;

        x = p->data;

        front = front->next;

        delete(p);

        return(x);

    }

}



int peek()

{

    int x;

    if (front == NULL)

    {

        cout<<"empty queuen";

    }

    else

    {

        p = front;

        x = p->data;

        front = front->next;

        return(x);

    }

}



bool isEmpty()

{

    if (front == NULL)

    {

        return true;

    }

    else if (front != NULL)

    {

        return false;

    }

}



void Display()

{

cout << front;

}



int main()

{

    int n, c = 0, x;

    bool is_empty = isEmpty();

    cout<<"Enter the number of values to be pushed into queuen";

    cin>>n;

    while (c < n)

    {

    cout<<"Enter the value to be entered into queuen";

    cin>>x;

        push(x);

        c++;

    }

    cout<<endl<<"Pop value: ";



    if (front != NULL)

            cout<<pop()<<endl;



    cout<<endl<<"Peak value: ";



    if (front != NULL)

            cout<<peek()<<endl;



    if (is_empty == true)

    {

        cout<<endl<<"The list is empty";

    }

    else if (is_empty == false)

    {

        cout<<endl<<"The list is not empty";

    }





    cout << endl << "The current contents of the stack are: ";

while(front != NULL)

{

    Display();

    if(front == NULL)

        cout << "The stack is empty";

    break;

}

    getch();

}

There are some issues with your code.

You have defined an isEmpty() function, but you don't really it. You have declared a separate variable is_empty that you set to the return value of isEmpty() one time before you populate the list, and never update is_empty after making any changes to the list. That is why your code is always reporting the list is "empty" even if it is really not. You need to get rid of the is_empty variable and call isEmpty() every time you need to check for empty.

Also, the return value of peek() and pop() are indeterminate if front is NULL.

And peek() pops the front node from the list. Only pop() should be doing that.

And pop() does not check if rear is pointing at the node being popped. You are not resetting rear to NULL in that case.

And you don't free any nodes remaining in the list before exiting the program.

Try something more like this instead:

#include <iostream>

#include <limits>



struct node

{

    int data;

    node *next;



    node(int value): data(value), next(NULL) {}

};



node *front = NULL;

node *rear = NULL;



void push(int x)

{

    node **np = (rear) ? &(rear->next) : &front;

    *np = new node(x);

    rear = *np;

}



int peek()

{

    if (front)

        return front->data;

    return -1;

}



int pop()

{

    int x = peek();



    if (front)

    {

        node *p = front;

        front = front->next;

        if (p == rear) rear = NULL;

        delete p;

    }



    return x;

}



bool isEmpty()

{

    return !front;

}



void clear()

{

    while (front)

    {

        node *p = front;

        front = front->next;

        delete p;

    }

    rear = NULL;

}



void display()

{

    node *n = front;

    if (n)

    {

        cout << n->data;

        while (n = n->next)

            cout << ' ' << n->data;

    }

}



int askForNumber(const char *prompt)

{

    int n;

    cout << prompt << ": ";

    cin >> n;

    cin.ignore(numeric_limits<streamsize>::max(), 'n');

    return n;

}



int main()

{

    int n, x;



    n = askForNumber("Enter the number of values to be pushed into queue");



    for(int c = 0; c < n; ++c)

    {

        x = askForNumber("Enter the value to be entered into queue");

        push(x);

    }

    cout << endl;



    cout << "Pop value: ";

    if (!isEmpty())

        cout << pop();

    else

        cout << "empty queue";

    cout << endl;



    cout << "Peak value: ";

    if (!isEmpty())

        cout << peek();

    else

        cout << "empty queue";

    cout << endl;



    if (isEmpty())

        cout << "The list is empty";

    else

        cout << "The list is not empty";

    cout << endl;



    cout << "The current contents of the stack are:" << endl;

    display();

    cout << endl;



    clear();



    cin.get();

    return 0;

}