Pointer in linked list/vector

Question

I'm trying to implement my own linked list using vectors and pointers. The problem I'm have is that I can't get the first node to point to the second node.

Here's my code and what I've tried:

struct Node {
    Node* previous;
    Node* next;

    int data;
};

// Initialize: Create Vector size 20 and first node
void LinkedList::init() {
    vecList.resize(20, NULL); // Vector of size 20
    Node* head = new Node();  // Create head node
    head->previous = NULL;    // Previous point set to null
    head->next = vecList[1];  // Next pointer set to next position
    head->data = 0;           // Data set at value 0

    vecList[0] = head; // Put head node in first position
    count = 1; // Increase count by 1
}

// Add Node to array
void LinkedList::push_back(Node* node, int data) {
    count += 1;
    node = new Node();
    node->next = vecList[count + 1];
    node->previous = vecList[count - 1];
    node->data = data;
    vecList[count - 1] = node;
}

The data has been passed in and will displayed using:

cout << linkedlist.vecList[1]->data << endl;

But if I try this way to display I get error saying the next pointer is <Unable to read memory>

cout << linkedlist.vecList[0]->next->data << endl;

Answer 1

You forgot to set the next pointer of the previous Node in the push_back method. If count is a member variable of the list containing the number of entries you have to change the method like this:

EDIT: actually you have to increment count in the end because array indices start with zero.

void LinkedList::push_back(Node * node, int data){  
    node = new Node();
    node->next = NULL;  // NULL because next element does not exist yet
    node->previous = vecList[count - 1];
    node->data = data;
    vecList[count] = node;
    vecList[count-1]->next = vecList[count];
    count++;
}

Still it's a bit strange that you try to implement a linked list with a vector or array because that actually defeats the advantages of a list...

Answer 2

It looks like vecList is a vector/array of pointers to Node.

When you init, you let the first element point to the second element:

void LinkedList::init(){
    ...
    head->next = vecList[1]; 

But at this point, the second element does not exist yet. So you cannot point to it yet. Something similar is wrong at the push_back function.