Thread: linked list problem

Hello this is my first post.
I am making a linked list. The problem is that whenever I try to retrieve a value I get the value from the first node.

LinkedList.cpp

Code:

#include <iostream>

using namespace std;
/*constructor*/
List::List():head(NULL),length(0)//sets first heads pointer to null
{
    //remeber that head is not a node and does not contain any data
	//it simply points to the current first node
	//thus after the constructor is called, there trully still is no link list
}
    
/*destructor*/
List::~List()
{
    eraseAll();
}

void List::eraseAll()
{
    while(head != NULL)
    {
        NodePtr temp = head;
        head = head -> next;//the node pointed to by head, the value of next (which is an adress)
        delete temp;
    }//heads pointer is now null
}

bool List::isEmpty() const
{
    return head == NULL;//if head is null then the link list was just made, or deleteAll was just called or the list was lost (memorey leek)
}

unsigned int List::size() const
{
    int count = 0;//this means the first element (list) is at 0
    NodePtr toEnd = head;
    while(toEnd->next != NULL)
    {
        toEnd = toEnd->next;
        count++;
    }
    return count;
}
/*returns true upon sucess, otherwise false*/
bool List::insert(DataType item, unsigned int position)
{
	if(position > length)
		return false;//couldn't be inserted (would cause empty spaces in list)
	if(position == 0 && head == NULL)
	{
        cout<<"inisde insert"<<endl;
		head = new Node(item);
        cout<<"this is head "<<head<<endl;
		return true;
	}
    int count = 0;
    NodePtr temp = head;
    NodePtr previous;
	while(count != position)
	{
		previous = temp;
		temp = temp->next;
		count++;
	}//the new node should now be inserted after previous, and be followed by temp
	previous = new Node(item);
	previous->next->next = temp;
	length++;
    return true;
}

DataType List::get(unsigned int position) const
{
    NodePtr temp = head;
    if(head->next == NULL)//head's next is always null for some reason
        cout<<"head's screwed up"<<endl;
    unsigned int count = 0;
    cout<<"position: "<<position<<endl;
    while(count < position && temp->next != NULL)
    {
        count++;
        temp = temp->next;
    }

    cout<<"element from temp "<<temp->element<<endl;
    return temp->element;//problem occures here
}

this is the file I used to test it

tester.cpp

Code:

#include "LinkedList.h"
#include <iostream>

using namespace std;

int main()
{
    List aList = List();
    aList.insert(4.2, 0);
    aList.insert(22, 1);
    aList.insert(5,3);
    cout<<aList.get(3)<<endl;
    
    return 0;
}

for some reason head is always pointing to null. This confuses me because the only place I assign a variable the value of NULL is in the constructor.
actually I miss-spoke. Head is not pointing to null the first time I call the insert method, but it is pointing to null whenever I call the get method.

here it is again slimmed down:

Code:

#include <iostream>

using namespace std;

List::List():head(NULL),length(0)
{

}


/*returns true upon sucess, otherwise false*/
bool List::insert(DataType item, unsigned int position)
{
	if(position > length)
		return false;
	if(position == 0 && head == NULL)//it appears head is always true
	{
		head = new Node(item);
		return true;
	}
    int count = 0;
    NodePtr temp = head;
    NodePtr previous;
	while(count != position)
	{
		previous = temp;
		temp = temp->next;
		count++;
	}
	previous = new Node(item);
	previous->next->next = temp;
	length++;
    return true;
}

DataType List::get(unsigned int position) const
{
    NodePtr temp = head;
    unsigned int count = 0;
    while(count < position && temp->next != NULL)//this condition is never true
    {
        count++;
        temp = temp->next;
    }
    return temp->element;//problem occures here
}

tester.cpp

Code:

#include "LinkedList.h"
#include <iostream>

using namespace std;

int main()
{
    List aList = List();
	aList.insert(4.5, 0);
    aList.insert(22, 1);
    aList.insert(5,3);
    cout<<aList.get(3)<<endl;
    
    return 0;
}

LinkedList.h

Code:

#include <cstdlib>

typedef double DataType;

// our list is zero based, that is, the first element in the list is in position 0.

class List
{
    public:
        List();
        List(const List & rhs);
        List& operator=(const List & rhs);
        bool insert (DataType item, unsigned int position);
        DataType get(unsigned int position) const;
#ifndef NDEBUG
#endif
    private:
        struct Node
        {
            DataType element;
            Node* next;
            Node(DataType item):element(item),next(NULL){}
        };
        typedef List::Node * NodePtr;

        NodePtr head;//private field
		unsigned int length;
};
#include "LinkedList.cpp"