linked list C++ , question selflearning

        #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;  // inser开发者_开发问答ting 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 ....

---

temp is an uninitialized pointer. So -

temp-> item = a;  // temp is not initialized or pointing to a memory location
                  // that has Node object to use operator ->

First, temp needs to be allocated memory location using new.

temp = new Node;
temp -> item = a;

And now assign it head. Similarly allocate memory for the child nodes too in the while loop. And return all the resources acquired from child to head using delete before program termination.

---

You seem to have some misunderstandings here:

Your "head" is the start of the list. It's always the start.

You add append elements to a linked list by assigning them to the last node's next pointer.

Third, you're not allocating anything.

Node *head= new Node();   
Node *temp = new Node();
cout<<"enter something into data"<<endl;
cin >> a ;
temp->item = a;
head->next = temp;

Now ... to add the next thing, you either need to keep track of the last node (tail), or traverse the list to find the last node.

Node *nextNode = new Node();
nextNode->item = 0.0;
Node *i;
for (i = head; i->next != null; i = i->next);
i->next = nextNode;

This is O(n) execution time. By keeping track of the tail you make it O(1):

Node *head= new Node();
Node *tail = head;   
Node *temp = new Node();
cout<<"enter something into data"<<endl;
cin >> a ;
temp->item = a;
tail->next = temp;
tail = temp;

Node *nextNode = new Node();
nextNode->item = 0.0;
tail->next = nextNode;
tail = nextNode;

EDIT: As pointed out, if you want to prepend to the list, you would:

temp->next = head;
head = temp;

---

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.

---

Additionally note that you are doing an implicit cast from int to float on

temp-> item = a;

as a is an int, while temp->item is a double.

To solve your problem: You want to allocate a new structure before accessing temp, thus

temp = new Node();