How to add a tail element at end of the list

Question

How to add a tail element at end of the list

When I try to use this statement : tail->prev = newElement; then entire program just shuts down. I really wonder why.

struct LinkedList{
    string var_name;
    string scope_name;
    int scope; // 0 = global, 1 = public, 2 = private, 3 = ?
    LinkedList* next = NULL;
    LinkedList* prev = NULL;
};

struct LinkedList*  head;
struct LinkedList*  tail;  //I made this two global

void insert(LinkedList* &head, LinkedList* &newElement, LinkedList* &tail){
    newElement->next =NULL;

    if(!head){
        head = newElement;
        return;
    }
    else{
        LinkedList* last = head;
        while(last->next != NULL){
            last=last->next;
        }
        last->next = newElement;
        newElement->prev = last;
        tail = newElement; 
        tail = newElement->next;
        tail->prev = newElement;
    }
}

void LexicalAnalyzer:: var_list(){
    LinkedList* new_node = new LinkedList[sizeof(LinkedList)];
    GetToken();
    new_node->var_name = tmp.lexeme;
    new_node->scope_name = currentScope;
    if(currentScope == "global")
        new_node->scope = 0;
    else if(pubOrPri == 1)
        new_node->scope = 1;
    else if(pubOrPri == 2)
        new_node->scope = 2;
    insert(head, new_node, tail);
    //tail->prev = new_node;
    display();
    if(tmp.token_type == ID){
        GetToken();
        if(tmp.token_type == COMMA)
            var_list();
        else if(tmp.token_type == SEMICOLON){
            return;
        }
        else
            syntaxError();
    }
    else
        syntaxError();
}

The output gave me

Process returned -1073741819 (0xC0000005) execution time : 3.269 s.

But if I delete tail-> newElement; and take away the parameter tail in the function. Everything's fine.

And I thought, tail is global then why wouldn't I just directly try tail->prev = new_Node; in the var_list() function, didn't work as well.

c++

doubly-linked-list

asked on Stack Overflow Aug 2, 2019 by

zichang wang • edited Aug 2, 2019 by

zichang wang

2 Answers

You don't have any logic to deal with the initial state of tail, which is nullptr.

Change

if(!head){
    head = newElement;
    return;
}

to

if(!head){
    head = tail = newElement;
    return;
}

In addition, the line

    tail->prev = newElement;

needs to be

    tail = newElement;

in the else block.

Disclaimer I haven't tested whether the suggestion fixes the problem.

answered on Stack Overflow Aug 2, 2019 by

R Sahu • edited Aug 2, 2019 by

R Sahu

try this logic,

void insert(LinkedList* &head, LinkedList* &newElement, LinkedList* &tail){
        newElement->next =NULL;

        if(!head){
                head = tail = newElement; // your first node is both head and tail;
                return;
        }
        else{
                LinkedList* last = head;
                while(last->next != NULL){
                        last=last->next;
                }
                last->next = newElement;
                newElement->prev = last; // newElement is inserted to end of the list
                tail = newElement; // tail is moved to last element in the list
                //tail = newElement->next;
                newElement->next = NULL; // tail of next is always NULL; end of the list
                //tail->prev = newElement;
                tail->prev = last; // tail of prev must be last 
        }
}

answered on Stack Overflow Aug 2, 2019 by

Sarath Padakandla

User contributions licensed under CC BY-SA 3.0