Why doesn't my implementation of linked list work?

Question

After compiling -when the cmdl pops up- it doesn't terminate and waits just as awaiting input

#include <bits/stdc++.h>

using namespace std;

struct node{
    int data;
    node *next;
};

class LinkedList{
private:
    struct node *head;
    struct node *tail;
public:
    LinkedList(int val1,int val2){
        head->next = tail;
        head->data = val1;
        //tail->next = NULL;
        //tail->data = val2;
    }
    add(int val){
        struct node *n = new node;
        n->next = head->next;
        head->next = n;
        n->data = val;
    }
    display(){
        struct node *ptr = head;
        while(ptr->next!=NULL){
            cout<<ptr->data;
            ptr = ptr->next;
        }
    }
};

int main(){
    LinkedList l(1,3);
    for(int i = 0;i<5;i++) l.add(i);
    l.display();
}

What prevents the code from executing as expected? I tried some built-in functions to test the code however none of them responded to the calls and had effect.

That shouldn't even compile. As for running: what does `head` point to in your constructor? — Mat, Mar 16 '20 at 14:20
you exepct `ptr->next` to be `NULL` at some point, but as you never set it to `NULL`, thats not going to happen — 463035818_is_not_an_ai, Mar 16 '20 at 14:21
Your code can never have an empty list since you always put something in head. — stark, Mar 16 '20 at 15:23
`struct node *head;` in `c++` you omit the `struct` so `node *head;` is preferred. — drescherjm, Mar 16 '20 at 16:15
You should read [Why `#include ` is bad](https://stackoverflow.com/q/31816095/9254539). — eesiraed, Mar 16 '20 at 16:56

iverss · Accepted Answer · 2020-03-16T14:52:25.427

2

I get an access violation when running this code.

adding

head = new node;

at the beginning of your constructor fixes that.

I would also explicitly initialize head and tail to null like this

private:
     struct node *head = NULL;
     struct node *tail = NULL;

Otherwise they're filled with garbage values and you'll potentially get an infinite loop in your display code.

edited Mar 16 '20 at 14:52

answered Mar 16 '20 at 14:29

iverss

106
7

1

Also worth asking "Why is my constructor adding a node?" Having to supply a an item to the constructor will make this liked list less useful as you now must know what the first item will be when creating the node, and this is a luxury you often do not have. – user4581301 Mar 16 '20 at 15:17

Why doesn't my implementation of linked list work?

1 Answers1