Linked List traversal skips values - c++

Question

I'm new to pointers and I'm a bit confused.
I have written a function which merges and sorts two sorted linked lists. However, when I print the list after calling the function it does not have all the values of the new merged list. While debugging the code and examining the variables and memory locations, it looks like it skips over the locations and just jumps to the last memory location. After the function executes I need to have the values in a new list and leaving list1 and list2 empty. This is my method in the header file:
template <class Type>
void orderedLinkedList<Type>::mergeLists(orderedLinkedList<Type> &list1, orderedLinkedList<Type> &list2)
{
nodeType<Type> *lastSmall; //pointer to the last node of the merged list.
nodeType<Type> *first1 = list1.first;
nodeType<Type> *first2 = list2.first;
if (list1.first == NULL){ //first sublist is empty
this->first = list2.first;
list2.first = NULL;
}
else if (list2.first == NULL){ // second sublist is empty
this->first = list1.first;
list1.first = NULL;
}
else{
if (first1->info < first2->info){ //Compare first nodes
this->first = first1;
first1 = first1->link;
lastSmall = this->first;
}
else{
this->first = first2;
first2 = first2->link;
lastSmall = this->first;
}
while (first1 != NULL && first2 != NULL)
{
if(first1->info < first2->info){
lastSmall->link = first1;
lastSmall = lastSmall->link;
first1 = first1->link;
}
else{
lastSmall->link = first2;
lastSmall = lastSmall->link;
first2 = first2->link;
}
} //end while
if (first1 == NULL) //first sublist exhausted first
lastSmall->link = first2;
else //second sublist exhausted first
lastSmall->link = first1;
list1.first = NULL;
list1.last = NULL;
list2.first = NULL;
list2.last = NULL;
}
}
Then in my main.cpp I have:
int main()
{
orderedLinkedList<int> list1;
orderedLinkedList<int> list2;
orderedLinkedList<int> newList;
list1.insert(2);
list1.insert(6);
list1.insert(7);
list2.insert(3);
list2.insert(5);
list2.insert(8);
newList.mergeLists(list1, list2);
newList.print();
return 0;
}
My print function just in case:
template <class Type>
void linkedListType<Type>::print() const
{
nodeType<Type> *current; //pointer to traverse the list
current = first; //set current so that it points to
//the first node
while (current != NULL) //while more data to print
{
cout << current->info << " ";
current = current->link;
}
}//end print
Could someone please tell me what I am doing wrong here? The output should be 2 3 5 6 7 8 but instead it's 2 3 7 8.
Thanks
EDIT:
Here is my insertion function. Note that this function is from the book I'm working with. It is included in the same class that I need to add the mergeLists method to. It is written specifically for ordered lists:
template<class Type>
void orderedLinkedList<Type>::insert(const Type& newItem)
{
nodeType<Type> *current;
nodeType<Type> *trailCurrent;
nodeType<Type> *newNode;
bool found;
newNode = new nodeType<Type>;
newNode->info = newItem;
newNode->link = NULL;
//case1 list is empty
if(this->first == NULL)
{
this->first = newNode;
this->last = newNode;
this->count++;
}
else //if the list is not empty
{
current = this->first;
found = false;
while(current != NULL && !found)
{
if(current->info >= newItem)
found = true;
else
{
trailCurrent = current;
current = current->link;
}
//case2 insert newNode at the head
if(current == this->first)
{
newNode->link = this->first;
this->first = newNode;
this->count++;
}
else //case 3
{
trailCurrent->link = newNode;
newNode->link = current;
if(current == NULL)
this->last = newNode;
this->count++;
}
}
}
}
The 3 cases as according to the book are:
Case 1:
The list is initially empty. The node containing the new item is the only node
and, thus, the first node in the list.
Case 2:
The new item is smaller than the smallest item in the list. The new item goes at
the beginning of the list. In this case, we need to adjust the list’s head pointer—
that is, first. Also, count is incremented by 1.
Case 3:
The item is to be inserted somewhere in the list.
Case 3a:
The new item is larger than all the items in the list. In this case, the new
item is inserted at the end of the list. Thus, the value of current is NULL
and the new item is inserted after trailCurrent. Also, count is incremented
by 1.
Case 3b:
The new item is to be inserted somewhere in the middle of the list. In this
case, the new item is inserted between trailCurrent and current.
Also, count is incremented by 1.

Answer 1

The bug is in your insertion method. When you call print on the lists after inserting, you will note that the last value gets overriden:
list1.insert(2); list1.print();
list1.insert(6); list1.print();
list1.insert(7); list1.print();
list2.insert(3); list2.print();
list2.insert(5); list2.print();
list2.insert(8); list2.print();
will output:
2
2 6
2 7
3
3 5
3 8
This is because every iteration trailCurrent->link = newNode; gets called, cutting of the list the first time this happens.
So for example when 7 is inserted in list1, the loop will first set trailCurrent->link to 7 when trailCurrent is 2, and then continue and set trailCurrent->link to 7 when trailCurrent is 6. But since 2 now points to 7 and not to 6, the chain is lost, and you're stuck with only 2 and 7.
Get another book
The book you are using to learn this is outdated. C style pointers and manual memory allocation should not be used in modern C++. Try to get a modern book that teaches how to use smart pointers and modern collections, and that teaches proper debugging techniques, so you can easily detect problems as the one you stumbled over now.

Answer 2

Thanks to everyone who has tried to help. In the end it was a simple logic error.
I inadvertently added a curly bracket to the end of my while loop in my insertion method which resulted in the loop executing code that it wasn't supposed to more than once causing strange behavior. See the corrected code below:
while(current != NULL && !found)
{ //<-- Removed this
//code
this->count++;
}
} //<-- Removed this
And now the working code is:
template<class Type>
void orderedLinkedList<Type>::insert(const Type& newItem)
{
nodeType<Type> *current;
nodeType<Type> *trailCurrent;
nodeType<Type> *newNode;
bool found;
newNode = new nodeType<Type>;
newNode->info = newItem;
newNode->link = NULL;
//case1 list is empty
if(this->first == NULL)
{
this->first = newNode;
this->last = newNode;
this->count++;
}
else //if the list is not empty
{
current = this->first;
found = false;
while(current != NULL && !found)
if(current->info >= newItem)
found = true;
else
{
trailCurrent = current;
current = current->link;
}
//case2 insert newNode at the head
if(current == this->first)
{
newNode->link = this->first;
this->first = newNode;
this->count++;
}
else //case 3 insert newNode anywher in the list
{
trailCurrent->link = newNode;
newNode->link = current;
if(current == NULL)
this->last = newNode;
this->count++;
}
}
}