HELP!

[itfreakas]

Hello everyone, I have this working program with operations on circular doubly linked list. But I have one slight problem, for example when there is only one node in the list and when I try to remove it with my remove function, afterwards instead of printing that the list is now empty, it prints me garbage like 41096232 or crashes, why is that?

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

struct node
{
    int info;
    node *next;
    node *prev;
}
*start, *last;
int counter = 0;

class double_clist
{
    public:
        node *create_node(int);
        void insert_file(int value);
        void insert_begin();
        void insert_last();
        void insert_pos();
        void delete_pos();
        void display();
        void display_file();
        double_clist()
        {
            start = 0;
            last = 0;
        }
};

int main()
{
    double_clist cdl;

    ifstream fin("1.txt");
    int value;
    if (!fin)
    {
        cout << "Error! Unable to read a text file " << "1.txt" << endl << endl;
        return -1;
    }
    while(fin >> value)
    {
        cdl.insert_file(value);
    }

    cout << "Formed circular doubly linked list:" << endl;
    cdl.display();
    cdl.display_file();

    int choice;
    while (1)
    {
        cout << "OPERATIONS ON CIRCULAR DOUBLY LINKED LIST!" << endl << endl;
        cout << "1. Insert element in the begining!" << endl << endl;
        cout << "2. Insert element in the end!" << endl << endl;
        cout << "3. Insert element in the middle!" << endl << endl;
        cout << "4. Remove element from the position!" << endl << endl;
        cout << "5. Display elements on the screen!" << endl <<endl;
        cout << "6. End program!" << endl << endl;
        cout << "Enter your choice: ";
        cin >> choice;
        switch(choice)
        {
        case 1:
            cdl.insert_begin();
            cdl.display();
            break;
        case 2:
            cdl.insert_last();
            cdl.display();
            break;
        case 3:
            cdl.insert_pos();
            cdl.display();
            break;
        case 4:
            cdl.delete_pos();
            cdl.display();
            break;
        case 5:
            cdl.display();
            break;
        case 6:
            cdl.display_file();
            system("start results.txt");
            exit(1);
        default:
            cout << "Wrong choice!" << endl << endl;
        }
    }
    return 0;
}

node* double_clist::create_node(int value)
{
    counter++;
    struct node *temp;
    temp = new(struct node);
    temp->info = value;
    temp->next = 0;
    temp->prev = 0;
    return temp;
}

void double_clist::insert_file(int value)
{
    struct node *temp;
    temp = create_node(value);
    if (start == last && start == 0)
    {
        cout << "Elements are successfully read from the text file!" << "1.txt" << "!" << endl << endl;
        start = last = temp;
        start->next = last->next = 0;
        start->prev = last->prev = 0;
    }
    else
    {
        last->next = temp;
        temp->prev = last;
        last = temp;
        start->prev = last;
        last->next = start;
    }
}

void double_clist::insert_begin()
{
    int value;
    cout << endl << "Enter the element which you want to insert at the beginning: ";
    cin >> value;
    struct node *temp;
    temp = create_node(value);
    if (start == last && start == 0)
    {
        cout << "Element successfully inserted!" << endl << endl;
        start = last = temp;
        start->next = last->next = 0;
        start->prev = last->prev = 0;
    }
    else
    {
        temp->next = start;
        start->prev = temp;
        start = temp;
        start->prev = last;
        last->next = start;
        cout << "Element successfully inserted!" << endl << endl;
    }
}

void double_clist::insert_last()
{
    int value;
    cout << endl<<"Enter the element which you want to insert at the end: ";
    cin >> value;
    struct node *temp;
    temp = create_node(value);
    if (start == last && start == 0)
    {
        cout << "Element successfully inserted!" << endl << endl;
        start = last = temp;
        start->next = last->next = 0;
        start->prev = last->prev = 0;
    }
    else
    {
        last->next = temp;
        temp->prev = last;
        last = temp;
        start->prev = last;
        last->next = start;
        cout << "Element successfully inserted!" << endl << endl;
    }
}

void double_clist::insert_pos()
{
    int value, pos, i;
    cout << endl << "Enter the element which you want to insert: ";
    cin >> value;
    cout << endl << "Enter the position: ";
    cin >> pos;
    struct node *temp, *s, *ptr;
    temp = create_node(value);
    if (start == last && start == 0)
    {
        if (pos == 1)
        {
            start = last = temp;
            start->next = last->next = 0;
            start->prev = last->prev = 0;
        }
        else
        {
            cout << "Position is unavailable!" << endl << endl;
            counter--;
            return;
        }
    }
    else
    {
        if (counter < pos)
        {
             cout << "Position is unavailable!" << endl << endl;
             counter--;
             return;
        }
        s = start;
        for (i = 1;i <= counter;i++)
        {
            ptr = s;
            s = s->next;
            if (i == pos - 1)
            {
                ptr->next = temp;
                temp->prev = ptr;
                temp->next = s;
                s->prev = temp;
                cout << "Element successfully inserted!" << endl << endl;
                break;
            }
        }
    }
}

void double_clist::delete_pos()
{
    int pos, i;
    node *ptr, *s;
    if (start == last && start == 0)
    {
        cout << "Circular doubly linked list is empty!" << endl << endl;
        return;
    }
    cout << "Enter the position of the elements which you want to remove: ";
    cin >> pos;
    if (counter < pos)
    {
        cout << "Position is unavailable!" << endl << endl;
        return;
    }
    s = start;
    if(pos == 1)
    {
        counter--;
        last->next = s->next;
        s->next->prev = last;
        start = s->next;
        delete(s);
        cout << "Element successfully removed!" << endl << endl;
        return;
    }
    for (i = 0;i < pos - 1;i++ )
    {
        s = s->next;
        ptr = s->prev;
    }
    ptr->next = s->next;
    s->next->prev = ptr;
    if (pos == counter)
    {
        last = ptr;
    }
    counter--;
    delete(s);
    cout << "Element successfully removed!" << endl << endl;
}

void double_clist::display()
{
    int i;
    struct node *s;
    if (start == last && start == 0)
    {
        cout << "Circular doubly linked list is empty!" << endl << endl;
        return;
    }
    s = start;
    for (i = 0;i < counter-1;i++)
    {
        cout <<s->info << "<->";
        s = s->next;
    }
    cout <<s->info << endl << endl;
}

void double_clist::display_file()
{
    ofstream fout;
    fout.open("results.txt", ios::app);
    int i;
    struct node *s;
    if (start == last && start == 0)
    {
        fout << "Circular doubly linked list is empty!" << endl << endl;
        return;
    }
    s = start;
    for (i = 0;i < counter-1;i++)
    {
        fout<<s->info<<"<->";
        s = s->next;
    }
    fout << s->info << endl << endl;
    fout.close();
}

[Ganado]

In the future, you should make a minimal program that we can run, preferably without having to figure out what to type, that exactly reproduces your problem. This means removing anything that isn't related to your problem while still reproducing the issue.

For example, especially since your issue is not with reading from the file itself, remove any file reading from your program; it just makes it harder for people to help you, and will dramatically lower the number of replies you get.

Okay, so let's say there's only 1 node on the list, which could mean you've only inserted 1 node, and counter is 1. Both start and last now point to the only node, I assume?
Then, you call the following, I assume:

             cdl.delete_pos();
            cdl.display();

Is the first position 0 or 1? It looks like you want it to be 1.
So then you do the following:

s = start;
        counter--;
        last->next = s->next; // set to null
        s->next->prev = last; // This looks wrong. s->next is null. So you might be dereferencing a null pointer.
        start = s->next; // set start to null.
        delete(s);
        cout << "Element successfully removed!" << endl << endl;
        return;

* so counter is now 0.
* start is null (0)
* last is not updated (still points to the the deleted data, as far as I can tell).
* You've invoked undefined behavior by deferencing a null next.

On line 277, start is no longer equal to last, so that if-statement is not reached.
Instead, it continues down your display function, and tries to dereference deleted data by deferencing start (s). et viola; you have junk.


Also,

*start, *last;
int counter = 0;

AVOID GLOBAL VARIABLES. They will bite you! For example, it's impossible for your program to have more than 1 linked list without causing bugs.