Ordered Linked List errors - c++

Question

I know all of the code isn't working but when I first run the program and the first string is read in from the text field the program errors out.The main function is passing the string to "Insert list function" in the implementation.
The program is suppose to insert a node every time that a string is read in from the text file. The program call also call the delete function which i know isn't working yet(that is why it is commented out). I am just trying to find the error that is created when the insert function is called. The main function has a while loop that creates a Node for every text entry and passes the node one by one to be sorted in ABC order.
Header file:
#include <string>
using namespace std;
struct Node
{
string data;
Node * next;
};
class List
{
public:
List();
~List();
bool Insert(string);
bool Delete(string);
void Print();
bool Edit(string, string);
private:
Node * head;
Node * cur;
Node * trailer;
};
Implementation:
#include <iostream>
#include <string>
#include <fstream>
#include "List.h"
using namespace std;
List::List():head(NULL)
{}
List::~List()
{}
bool List::Insert(string data)
{
Node* newNode = new Node;
if (newNode == NULL)
{
cout << "Error: Memory Allocation Failed" << endl;
return false;
}
newNode->data = data;
cur = head;
trailer = NULL;
if (head == NULL)
{
//cout << "head is Null" << endl;
head = newNode;
cout << head -> data << endl;
newNode->next = NULL;
//return true;
}
while (newNode->data > cur->data && cur -> next != NULL)
{
trailer = cur;
cur = cur->next;
}
if (cur->next == NULL)
{
cur->next = newNode;
newNode->next = NULL;
return true;
}
else
{
trailer->next = newNode;
newNode->next = cur;
return true;
}
}
bool List::Delete(string data)
{
/*Node *temp = head->next;
while (head != NULL)
{
delete head;
head = temp;
temp = head->next;
}
return true;*/
}
bool List::Edit(string dataDelete, string dataInsert)
{
Delete(dataDelete);
Insert(dataInsert);
return true;
}
void List::Print()
{
for (Node * Count = head; Count != NULL; Count = Count->next)
{
cout << Count->data << endl;
}
}

Answer 1

#Deepak is right, the problem is when you're inserting first element the head variable is NULL and cur is set to the value of head.
To fix it you can simply place
cur = head;
trailer = NULL;
after condition
if (head == NULL)
{
//cout << "head is Null" << endl;
head = newNode;
cout << head -> data << endl;
newNode->next = NULL;
//return true;
}
Also there will be an error when you will try to insert element that should be placed in the beggining (value that is smaller than any other value in the list). It will happen when condition of the loop
trailer = NULL;
while (newNode->data > cur->data && cur -> next != NULL) { ... }
is false in the first call, so trailer will be NULL. To fix it you will need to check trailer variable, like this
if (trailer == NULL) {
newNode->next = head;
head = newNode;
return true;
}
As the result your code of Insert will look like
bool List::Insert(string data)
{
Node* newNode = new Node;
if (newNode == NULL)
{
cout << "Error: Memory Allocation Failed" << endl;
return false;
}
newNode->data = data;
if (head == NULL)
{
head = newNode;
newNode->next = NULL;
return true;
}
cur = head;
trailer = NULL;
while (newNode->data > cur->data && cur -> next != NULL)
{
trailer = cur;
cur = cur->next;
}
if (trailer == NULL) {
newNode->next = head;
head = newNode;
return true;
}
if (cur->next == NULL)
{
cur->next = newNode;
newNode->next = NULL;
return true;
}
else
{
trailer->next = newNode;
newNode->next = cur;
return true;
}
}

Answer 2

While inserting the first node you are getting error because of
while (newNode->data > cur->data && cur -> next != NULL)
At this moment value in the cur is NULL and you are trying to access cur->data.