CODEWITHSUNDEEP
c++pointers
Ajax
Ajax

Reputation: 133

How do I actually remember the address pointed by pointer?

Working with linked lists, I am doing the following:

struct Node {
    int val;
    struct Node* prev;
    struct Node* next;
};


struct Node* head;
struct Node* tail;
struct Node* temp;

where Node is a structure for linked list, head points to first element, tail to the last, temp is for intermediate computation.

move_function(address1, address2) actually swaps node at addresses 1&2 by suitably changing their prev, next pointers.

void move(struct Node* t1, struct Node* t2)  //known that t1->next ..... ->next = t2, move from t2 to t1

{
    cout << "move";

    if (t1->next == t2)
    {
        cout << " Conseq";
        temp1 = t1->prev;
        temp2 = t2->next;

        t1->next = temp2; if (temp2 != NULL) temp2->prev = t1;
        t2->prev = temp1; if (temp1 != NULL) temp1->next = t2;
        t2->next = t1;
        t1->prev = t2;

    }

    else

    {
        cout << "...";

        t2->prev->next = t2->next;

        if (t2->next != NULL) { t2->next->prev = t2->prev; }

        if (t1 != head) { t1->prev->next = t2; t2->prev = t1->prev; t2->next = t1; t1->prev = t2; }

        else { head = t2; t2->prev = NULL; t2->next = t1; t1->prev = t2; }
    }

}

if (<node to be shifted is the last one i.e. *tail*>)
    
{    
    temp = tail->prev;    
    move_function (head, tail);    
    tail= temp;
    
}

It turns out that while writing temp = tail->prev; temp retains the definition, and not what I want i.e. the address of the element behind it (which needs to be set as tail after movement). More concretely, in the end, after executing move_function (head, tail); tail=temp; contrary to what I want, for number of elements in list = =2,tail->prev == NULL is true. What I really want is to remember my last element after movement.

What is happening here, and how do I work around? While writing, temp = tail->prev, if node at tail is shifted to head position and its prev set to NULL, does temp->prev become NULL?

Thanks!

Upvotes: 0

Views: 147

Answers (1)

AVH
AVH

Reputation: 11506

This is why it's a bad idea to declare multiple variables on a single line. The problem is that only head is a pointer. I.e. the following two pieces of code are equivalent:

// The way you declared the variables:
struct Node* head, tail, temp;

// Above is equivalent to:
struct Node* head;
struct Node tail;
struct Node temp;

Simply define each variable on a separate line. It will make the code more readable and fix it at the same time.

Live demo of the issue with your declarations: https://godbolt.org/z/f154dT.

Edit: Also, since you're writing C++, not C, there's no reason to put struct there. Just use Node * head;.

Upvotes: 1

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