CODEWITHSUNDEEP
c++linked-listnodes
Jihad Osman
Jihad Osman

Reputation: 23

Printing nodes from REAR to FRONT enters infinite loop

I am trying to output the elements from each node starting from right (rear/tail) to left (head/front). However, my program enters an infinite loop that displays the same element over and over again. Despite its infinite loop, the function that I created DisplayFromLeftToRight() (found below the DisplayFromRightToLeft() function) works like a charm, but this doesnt...

void DisplayFromRightToLeft()
{
    node *newnode = rear;
    int num = 1;
    while (newnode != NULL)
    {
        cout << "Node # " << num << ": " << newnode->data << endl;
        newnode = newnode->previous;
        num++;
    }
    return;
}

This is the working code for printing element from each node from LEFT to RIGHT..

void DisplayFromLeftToRight() 
{
    node *newnode = front;
    int num = 1;
    while (newnode != NULL)
    {
        cout << "Node # " << num << ": " << newnode->data << endl;
        newnode = newnode->next;
        num++;
    }
    return;
}

If you believe that my DisplayFromRightToLeft() function is correct, I assume that the problem is from the INSERT function, take a look:

void INSERT(int _data)
{
    node *newnode = new node;
    newnode->data = _data;
    newnode->next = NULL;
    newnode->previous = newnode;
    rear = newnode;
    node *index = new node;
    index = front;

    if (isEmpty())
        front = newnode;
    else
    {
        while (index->next != NULL)
        {
            index = index->next;
        }
        index->next = newnode;
    }
}

Upvotes: 0

Views: 59

Answers (2)

Dronz
Dronz

Reputation: 2097

Looks like INSERT always sets previous to a valid address, so your while (newnode != NULL) loop will never end, because it sets newnode = newnode->previous each time.

You are correct that the problem seems to be in your INSERT function, and/or any other code that determines what is in your list.

Your Display...() functions look OK as far as not looping, by themselves, but by making them rely on a while(someNode != NULL) loop, they are dependent on the assumption that the list data is well-formed. Specifically, the first node must have a previous pointer to NULL, and the last node must have a next pointer to NULL, or else one of those Display functions will loop until it finds a NULL.

Looking at the INSERT function, if you follow the steps, you will see that it assigns a new valid address to newnode, and then sets previous to that valid address, while next gets NULL, and it can assign next to something else. How would previous ever be NULL? Previous also points to the inserted node itself, which looks like any loop following previous is just going to keep looking at the same node.

Whenever working with linked lists that I implement, I always step through the code and diagram what is going on - on paper is best, rather than just imagination. Only draw things you actually have your code do. Otherwise, you are probably assuming the resulting list data is what you intended, and not what you've actually coded.

Upvotes: 1

rtruszk
rtruszk

Reputation: 3922

You should change:

newnode->previous = newnode;

To:

newnode->previous = rear;

Line:

newnode->previous = newnode;

has no sense. It is like saying that I am my own father.

Upvotes: 0

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