Single Sorted Linked List - Infinite Loop - c++

Question

I'm working with a single linked list and I want to sort it from lower to higher values of integer. I though I had the idea but then the execution enters in a infinite loop and I can't see clearly why. This is the part of the code I worked with:
class Node {
int data;
Node* next;
public:
Node() { };
void SetData(int aData) { data = aData; };
void SetNext(Node* aNext) { next = aNext; };
int Data() { return data; };
Node* Next() { return next; };
};
class List {
Node *head;
public:
List() { head = NULL; };
void Print();
void Append(int data);
void Delete(int data);
};
void List::Append(int data) {
// Create a new node
Node* newNode = new Node();
newNode->SetData(data);
newNode->SetNext(NULL);
// Create a temp pointer
Node *tmp = head;
if ( tmp != NULL ) {
// Nodes already present in the list
// Parse to end of list anytime the next data has lower value
while ( tmp->Next() != NULL && tmp->Next()->Data() <= newNode->Data() ) {
tmp = tmp->Next();
}
// Point the lower value node to the new node
tmp->SetNext(newNode);
newNode->SetNext(tmp->Next());
}
else {
// First node in the list
head = newNode;
}
}
void List::Print() {
// Temp pointer
Node *tmp = head;
// No nodes
if ( tmp == NULL ) {
cout << "EMPTY" << endl;
return;
}
// One node in the list
if ( tmp->Next() == NULL ) {
cout << tmp->Data();
cout << " --> ";
cout << "NULL" << endl;
}
else {
// Parse and print the list
do {
cout << tmp->Data();
cout << " --> ";
tmp = tmp->Next();
}
while ( tmp != NULL );
cout << "NULL" << endl;
}
}
I'm confused if the list increases infinitely or the error comes from the Print function...
Sorry for the, perhaps, dummy errors.
Thanks.

Answer 1

Your problem is these two lines:
tmp->SetNext(newNode);
newNode->SetNext(tmp->Next());
You should reverse them. Right now, you set tmp.next = newNode, and then newNode.next = tmp.next (= newNode), so newNode points to itself. Then traversing past newNode leads to an infinite loop.

Answer 2

You are failing to link two existing nodes properly. Look at this fragment of your code:
tmp->SetNext(newNode);
newNode->SetNext(tmp->Next());
If for example you have this list:
head -> 5
and you want to insert a node with number 4 you will have:
head -> 5 <- tmp
newNode -> 4
With tmp->SetNext(newNode)
head (and tmp) -> 5 -> 4 <- newNode
With newNode->SetNext(tmp->Next());
head (and tmp) -> 5 -> 4 <- newNode
So any attempt to iterate through the list will result in and infinite loop:
5
4
4
4
4 ...... forever

Answer 3

// Point the lower value node to the new node
tmp->SetNext(newNode);
newNode->SetNext(tmp->Next());
Since you overwrite tmp->next in the first call, your second call is actually pointing newNode->next towards newNode itself, creating a cycle. This causes your infinite loop when calling Print ;-)
The solution is to do something like as follows...
Node* _next = tmp->Next();
tmp->SetNext(newNode);
newNode->SetNext(_next);
That being said, your code is still broken. The problems is that you don't check whether you need to place before the head of the list; try doing something as follows..
if ( tmp ) {
// Check whether to become new head
if ( tmp->Data() > newNode->Data() ) {
newNode->SetNext(tmp);
head = newNode;
}
else {
// Nodes already present in the list
// Parse to end of list anytime the next data has lower value
while ( tmp->Next() && tmp->Next()->Data() <= newNode->Data() ) {
tmp = tmp->Next();
}
// Point the lower value node to the new node
Node* _next = tmp->Next();
tmp->SetNext(newNode);
newNode->SetNext(_next);
}
}
else {
// First node in the list
head = newNode;
}
Side note: you can use an initializer list to rewrite code like...
List() : head(NULL) { };
In addition, if NULL is equivalent to 0, you can simplify conditions of the form X == NULL and X != NULL as merely X and !X, respectively.
See my ideone paste for the corrected version with an example.
int main() {
List l;
l.Append(15);
l.Append(40);
l.Append(7);
l.Print();
return 0;
}
... which produces
7 --> 15 --> 40 --> NULL