linked list C++ , question selflearning

Question

        #include <iostream>
    using namespace std;



    struct Node
    {
        int item;   // storage for the node's item
        Node* next;   // pointer to the next node 
    };

  Node* addNode(Node*& head, int data , int& count) 
{
    Node * q;     // new node
    q = new Node;  // allocate memory for the new mode
    q->item = data;  // inserting data for the new node
    q->next = head;   // point to previous node ?? how would i do that? ( am i doing it correctly?)
    count++; // keep track of number of node
    head = q;
    return q;
}



    int main()
    {
        int a, count=0;
        int data;
        bool repeat;
        Node *head= NULL;   
        //^^ assuming thats creating the first node ^^
        do
        {
        cout << "please enter the data for the next node" <<endl;
        cin >> data;
        addNode(head, data, count);
        cout << "do you wish to enter another node? (enter true or false)" << endl;
        cin >>repeat;
        }
       while (repeat == true);


       // assuming this is the print function  
          while(head != NULL)
        {
            cout << "output" << temp->item << endl;
            cout << temp->next << endl;
        }

        system("pause");
        return 0; 
    }

okey i tried adding a new element in the list how would i move the head around like a LIFO memory (stack) so the last element is on the very top..

any help would be appreciated ! The pointers and the nodes are messing with my brain lately ....

Answer 1

Since I'm not sure every answer completely answers it, here's a linked list implementation (written without testig:

// your (correct) structure
struct Node
{
    float item;   // storage for the node's item
    Node* next;   // pointer to the next node 
};

Now we need two pointers somewhere to look after the list:

/* some pointers */
struct List
{
    Node* head;
    Node* tail;
};

Now we need to create some elements. As others have said, you can do that with new:

/* create some elements we want to link in */
Node* elem1 = new Node();
Node* elem2 = new Node();
Node* elem3 = new Node();
/* maybe even set their properties! */
elem1->item = 3.14;
elem2->item = 3.14;
elem3->item = 3.14;

Notice how I didn't try to add these elements to a list yet? That's because I've got a function in mind which looks like this:

void addtolist(List &list, Node* node)
{
    /* if no head, initialise the list */
    if ( list->head == NULL )
    {
        list->head = node;
        list->tail = node;
    }
    else if ( list->head != NULL && list->tail != NULL )
    {
        /* access the tail element and set its 
           next to this ptr. 
           Move tail to this node */
        list->tail->next = node;
        list->tail = node;
    }
    else
    {
        /* probably raise an exception! */
    }
}

You can call this by doing this:

List l;
addtolist(l, elem1); /* etc */

Deleting elements is somewhat more tricky, since you have to go to that element, remember its previous element, grab it's next element, join them up and delete the Node* you're on.

Now for traversing lists... your terminology HEAD|TAIL reminds me of Erlang and tail recursion, where the current element is referred to as the head and the remainder the tail. If I write:

Node* cur = l.head;
while ( cur != NULL )
{
    // do something with cur.item ? 
    cur = cur->next;
}

You can see this happening. Replacing cur with head here would be harmless thanks to the List struct.

Finally, I've used a very C-like approach here, but there's scope for templates:

template<typename T>
struct node
{
    T item;   // storage for the node's item
    Node<T>* next;   // pointer to the next node 
};

and encapsulating the List struct as a class:

template<typename T>
class List
{
protected:
    Node<T>* head;
    Node<T>* tail;
public:

    void addtolist(Node<T>* node);
    Node<T>* gethead();
    Node<T>* gettail();
}

Which brings you a little bit closer to std::list.