Correctly implementing a singly linked list C++

Question

I have a list with names of employers such as:

Node 1: Jill, Matt, Joe, Bob, Matt

Node 2: Jeff, James, John, J

Answer 1

Well, let's see:

When you hit if (ptr->data == p->data) at that point:

• pp points to the end of the list
• p is you new node (nothing points to it and it points to nothing)
• ptr points to the node with duplicate data

In order to delete the node you need to actually need to have the next pointer pointing to ptr otherwise how can you remove ptr from the list? So you would actually need to check:

if (head && head->data == p->data)
{
    // do nothing as duplicate entry is already head of list
    delete p;
    return;
}

node *ptr = head;
while (ptr)
{
    if (ptr->next && ptr->next->data == p->data)
    {
        node *duplicate = ptr->next;
        ptr->next = duplicate->next; // skip the duplicate node
        duplicate->next = head;      // duplicate points to head
        head = duplicate;            // head is now the duplicate
        delete p;                    // otherwise leaking memory
        return;
    }
    ptr = ptr->next;
}

if (pp) // points to tail as per your code
{
    pp->next = p;
    ++N;
}

Answer 2

For what it's worth, I'd probably implement it like this.

class EmployerCollection
{
public:
    typedef std::list<std::string> EmployerList;

public:
    bool AddEmployer(const std::string& name)
    {
        EmployerList::const_iterator it = std::find(m_employers.begin(), m_employers.end(), name);
        if (it != m_employers.end()) // Already exists in list.
        {
            m_employers.splice(m_employers.begin(), m_employers, it, std::next(it));
            return true;
        }
        m_employers.push_front(name);
        return false;
    }

private:
    EmployerList m_employers;
};

int main()
{
    const int NUM_EMPLOYERS = 15;
    std::string employers[NUM_EMPLOYERS] = {"Jill", "Jeff", "Doe", "Pablo", "Loui", "Ron", "Bob", "Joe", "Monica", "Luis", "Edward", "Matt", "James", "Edward", "John"};

    EmployerCollection c;

    for (int i=0; i<NUM_EMPLOYERS; i++)
    {
        bool duplicate = c.AddEmployer(employers[i]);
        printf("Added %s to employer list - duplicate: %s \n", employers[i].c_str(), duplicate ? "True" : "False");
    }

    system("pause");
} 

Answer 3

I've added a find function

typedef struct node{
  string data;
  struct node *net, *prev;
 }node;      


class list {
public:
    list():head(NULL), N(0){}
    ~list(){
    //Implementation for cleanup
     }

void add(string name){  //rather than accessing the global data, use the value passed
    node* p = new node(name);
    p->next=p->prev=NULL;
    node* pp = find(name);
    if(pp==NULL){
      // No match found, append to rear
      if(head==NULL)
        head=p;  //list empty, add first element
      else{
        node* cur=head;
        while(cur->next!=NULL) //Keep looking until a slot is found
          cur=cur->next;
        cur->next=p;
        p->prev=cur;
      }
    }
    else{
        //Match found, detach it from its location
        node* pPrev = pp->prev;
        pPrev->next = pp->next;
        pp->next->prev=pPrev;
        p->next = head; //append it to the front & adjust pointers
        head->prev=p;
    }
    N++;
    }

    //MER: finds a matching element and returns the node otherwise returns NULL
    node* find(string name){
        node *cur=head;
        if(cur==NULL) // is it a blank list?
          return NULL;
        else if(cur->data==head) //is first element the same?
          return head;
        else   // Keep looking until the list ends
          while(cur->next!=NULL){
          if(cur->data==name)
            return cur;
            cur=cur->next;
          }
        return NULL;
}
friend ostream& operator << (ostream& os, const list& mylist);

private:
    int N;
    node *head;

};

Now some may tell you to use the list in STL n never to write your own code coz you can't beat STL, but to me it's good that you are implementing your own to get a clear idea on how it works in reality.

Answer 4

I changed your variable names to be more readable, but kept the forward-declaration in case you wanted it like that. The issues I found were that you were always inserting the node at the end of the list, regardless of if you found it was a duplicate. In addition, your commented lines appeared close, but only if alone. With the pp=p stuff, it would cause issues. Try the following and see if it works. You'll still be leaking memory, but it will get you started:

void list::put(int i) {  //Where i is a random number
    node *current =head;
    node *added =new node(employers[i]);
    node *tail =head;
    node *prev = NULL;
    bool foundRepeat = false;



    while (current!=NULL)
    {
        if (current->data == added->data)
        {  
            if (prev)
                prev->next = current->next;

            current->next=head;
            head=current;
            foundRepeat = true;
            break;
        }
        prev = current;
        current=current->next;
    }

    if (!foundRepeat)
    {
        while (tail->next) 
        {
            tail=tail->next;
        }
        tail->next=added;
    }

    N++;
}