Question

I am trying to insert a value at the end of a doubly linked list , I get successful in inserting the value at head or first node but the second value is not getting inserted

The issue here is while entering the second value

class d_list
{
private:

    struct node
    {
        double data;
        node *next;
        node *previous;
    };

    node *first;
    node *last ;
public:
    d_list(void)
    {
        first = nullptr;
        last = nullptr;
    };
    void append(double);

};

void d_list::append(double num)
{
    node *ptr;
    node *toinsert;
    if(!first)
    {
        first = new node;
        first->previous= nullptr;
        first->data = num;
        last= new node;
        first->next= last->previous;
        last->previous = first->next;
        last->next= nullptr;

    }
    else
    {
        if(last->next == nullptr)
        {
            ptr = new node;
            ptr->next =last->previous;
            ptr->data=num;
            last->previous = ptr->next  ;
        }


        last->next= nullptr;
    }

}


int _tmain(int argc, _TCHAR* argv[])
{
    d_list aa;
    cout<<"going to append first"<<endl;
    aa.append(44);
    cout<<"going to append second"<<endl;
    aa.append(50.5);

    return 0;
}

Answer 1

You have a number of problems in your code:

• Your node next and previous members are never initialized anywhere and as a result are undefined when used. Either add a constructor to node or ensure they are initialized after allocation.
• The addition of a node to an empty list is not correct. first->next is left undefined and why are you creating two nodes, both first and last? In a list with one element then first == last. The setting of next/previous in first/last doesn’t make any sense either.
• In a well formed double-linked list then last->next should always be null, as should first->previous.
• The addition of a node into a non-empty list is also incorrect.
• While you don’t show it in the example, you’ll eventually need a destructor as well as a copy operator and copy constructor (the rule of three). At the moment you are leaking memory and if you try to delete nodes you’ll likely result in a double-free and crash.

I would suggest taking a step back from the code for a bit to ensure you properly understand the concepts behind a doubly-linked list. Draw out a list on paper with next/prev arrows and see how they need to be changed when adding nodes to an empty/non-empty list as well as how to delete and move nodes around. Once you figure out how next/prev should be set then translating that into code should be relatively straight forward.

Edit to answer comment:
To add a new node you can technically add it anywhere but it is usually added at the end (at least from what I’ve seen). See the other answers for a complete and correct code for adding new nodes in an empty and non-empty list.