Question

Okay I know this is a ridiculously easy question, but for some reason I cannot get a linked list to work. It may just be because I am really tired, because I’ve done them a million times before. Boiled my program down to the simplest possible implementation, still not working.

Very basic implementation, just make a LL of integers, something I have done a million times before but for whatever reason it’s never progressing past head.

main.cpp

#include <iostream>
#include "ll.h"
using namespace std;

int main()
{
    int x;
    list ll;
    int i =0;


    while(i == 0)
    {
    cout << "Enter a value to add to the LL ";
    cin >> x;

    ll.add(x);
    ll.display();
    }

return 0;
}

ll.h

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

class list
{
    public:
    list();

    void add(int);
    void display();
    node * head;
};

ll.cpp

#include <iostream>
#include "ll.h"
using namespace std;

list::list()
{
    head = NULL;
}

void list::add(int x)
{
    if(!head)
    {
        cout << "First  " << endl;
        head = new node;
        head->val = x;
        head->next = NULL;
    }
    else
    {
        node * current = head;
        while (current)
            current = current->next;

        current = new node;
        current->val = x;
        current->next = NULL;

    }
}

void list::display()
{
    node * current = head;

    while(current)
    {
        cout << current->val << endl;
        current = current->next;
    }
}

Answer 1

It seems you want to append to the list. In this case, you loop condition shouldn’t be

while (current)

but

while (current->next)

making sure is initially non-NULL (which you do with your check for the `head).

Actually, the logic for setting up the new node is also not quite right. You probably want to have the second branch of add() look something like this:

while (current->next) {
    current = current->next;
}
current->next = new node(x);

… with a suitable constructor of node:

node::node(int x): val(x), next() {}