Question

I’ve been banging my head against the wall for about 3 hours now trying to come up with the fix for this but I can’t figure it out. My test program is written as such…

int main()
{
  SimpList<int> intList;   // (empty) list of integers

  cout << "Let's build a sorted list of integers." << endl;
  cout << endl << "Uninitialized List: ";
  intList.print();
  cout << endl << "Length: " << intList.size() << endl;

  int intData[] = { 5, 3, -2, 7, 9, -8, 1, -4 };

  for (int i=0; i<8; i++)
    intList.insert( intData[i] );

  cout << endl << "After inserting 8 integers: ";
  intList.print();
  cout << endl << "Length: " << intList.size() << endl;

  system("PAUSE");
  return 0;
}

So the Link List is getting initialized from an array and a for loop. My class code for the node and list is here…

    template < typename T >   // Forward declaration of the SimpList class
class SimpList;

template < typename T >
class Node                 // Node class for the SimpList class
{
  private:
    // Constructors
    Node () { next = 0; }  // default constructor

    // Complete the definition inline
    Node ( const T &initItem, Node<T> *ptr ) { }

    // Data members
    T           item;   // Node data item
    Node        *next;  // Pointer to the next node

  friend class SimpList<T>;
};

template < typename T >
class SimpList
{
  public:

    // Constructor (add your code inline)
    SimpList ()
    {
      head = &PHONY;
      length = 0;
    }


    // List manipulation operations
    void insert ( const T &newitem );   // insert a data item
    bool remove ( T &item );            // remove data item
    bool find ( T &item ) const;        // find data item
    void clear ();                      // empty the list

    bool isEmpty () const { 
     if (length == 0)
         return true;
     else
         return false;
    }

    // length accessor method
    int size () const { 
        return length;
    }

    // print the list items
    void print () const;

  private: // data members
    Node<T> PHONY;      // empty node that anchors the list
    Node<T> *head;      // pointer to the beginning of the list
    int length;         // length of list
};

And then the insert and print functions are as follows…

template < typename T >
void SimpList<T>::print() const
{
  if (length == 0)
  {
    cout << "List is empty." << endl;
    return;
  }

  Node<T> *ptr = head->next;
  while (ptr != NULL)
  {
    cout << ptr->item << "  ";
    ptr = ptr->next;
  }
  cout << endl;
}

template < typename T >
void SimpList<T>::insert(const T& newitem) {

        Node<T> *currN = head->next;
        Node<T> *prevN = 0;
        Node<T> *tmp = new Node<T>();
        tmp->item = newitem;

        if(head->next == NULL ) {
          head->next = tmp;
        }
        else {
            prevN = head;
            while(currN != NULL && currN->item < newitem) {
                prevN = currN;
                currN = currN->next;
            }
            prevN->next = tmp;
        }
        length++;
        print();
}

I inserted the last “print()” into the insert function as a way of debugging what was happening and the output is quite perplexing as it gives me

5
3
-2
-2 7
-2 7 9
-8
-8 1
-8 -4

But I want the output to be sorted smallest to largest (-8 -4 -2 1 3 5 7 9)

edit: solved…forget to update tmp->next to currN. DERP.

Answer 1

Couple of things you need to fix:

1. PHONY->next is not null , so your initialization will fail :

   if(head->next==NULL) {
     head->next=tmp;
     return;
   }
   

2. Node constructor should initialize next to 0 instead of this , hence you will never hit NULL condition for end of your list.

3. in insert in else statement you should start search from head->next, currently you start search from currn = head which is PHONY which is not correct.

4. You also need to set tmp->next appropriately ,consider this case when you are inserting in middle of the list.