error: use of undeclared identifier 'tail'

Question

I am writing this code for insertion at the tail of a linked list. When I call the function in the main block there is an error in the tail parameter. Can someone tell why? I made a function for insertion at head, but it works absolutely fine.

code -

class Node {
    public:
    int data;
    Node* next;

    //constructor type 1
    Node(int data) {
        this -> data = data;
        this -> next = NULL;
    }
};
 
void insertAtTail(Node* &tail, int data) {
    //New node create
    Node* temp = new Node(data);
    tail -> next = temp;
    tail = temp;
}

int main() {

 insertAtTail(tail, 20);
    print(tail);
}```

Answer 1

The error says it all: "undeclared identifier 'tail'"

Your main function suddenly references tail without first telling what tail is. It makes no sense to insert a node if you don't have anything to insert it into.

You would also need a head variable to have a reference to the first node in the list.

When you call your print method, you'd better pass the head of the list. If you pass tail, then print will only print the tail node of the list, and it seems likely you want print to print the whole list.

Finally, you need to take care of the case where your list is empty (nullptr) and where you call insertAtTail to insert the very first node of the list. In that case tail will be nullptr and it would be wrong to access tail->next in that situation. Moreover, you would need to also set the head of the list. So I'd suggest that insertAtTail takes a parameter for the head too. That way it can set the head to the new node in case it is the very first node of the list.

Code:

void insertAtTail(Node* &head, Node* &tail, int data) {
    Node* temp = new Node(data);
    if (head == nullptr) { // If this is the first node of the list
        head = temp;
    } else { // Only when there is already a tail node
        tail -> next = temp;
    }
    tail = temp;
}

void print(Node* head) {
    while (head != nullptr) {
        std::cout << head->data << " ";
        head = head->next;
    }
    std::cout << "\n";
}

int main() {
    Node* head = nullptr; // A list has a head
    Node* tail = head; // ...and a tail.
    insertAtTail(head, tail, 10); // Also pass the head
    insertAtTail(head, tail, 20);
    insertAtTail(head, tail, 30);
    print(head);  // Print the whole list, not just the tail
}