problem with printing linked list data

[globaltourist]

Hello!
I made a simple program to practice linked list basics with the current goal being to be able to insert a new node in front of head or before head if i choose to do so.

  #include<iostream>

using std::cout;
using std::cin;
using std::endl;


struct node {
	int data;
	node *next;
};

class list {
private:
	node *head;
	node *temp;
	int length;

public:
	list();
	list(int data);

	~list();
	
	void insertTop(int data);
	void insertBottom(int data);
	void printList();
};

list::list() {
	head = new node;
	head->data = 0;
	head->next = NULL;
	temp = head;
	length = 0;
}//constructor end

list::list(int data) {
	head = new node;
	head->data = data;
	head->next = NULL;
	temp = head;
	length = 0;
}//constructor end

list::~list() {
	node* prev = head;
	node* pnext = head;
	while (pnext) {
		prev = pnext;
		pnext = prev->next;
		if (pnext) delete prev;
	}//while end
}//destructor end

void list::insertTop(int data) {
	node *newNode;
	newNode = new node;
	newNode->data = data;
	newNode->next = NULL;
	temp->next = newNode;
	temp = temp->next;
	length++;
}//insertTop function end

void list::insertBottom(int data) {
	node* newNode;
	newNode = new node;
	newNode->data = data;
	newNode->next = head;
	head = newNode;
	length++;
}//insertBottom function end

void list::printList() {
	for (int i=0; i < length; i++) {
		temp = head;
		cout << "Data at location " << i << " is: " << temp->data << endl;
		temp = temp->next;
	}//for end
	cout << "Data at location " << length << " is:" << temp->data;
}//printList function end

int main() {
	list a;
	a.insertTop(51);
	a.insertTop(22);
	a.insertTop(31);
	a.printList();
	return 0;
}

the problem is that the last 2 inserts show that the data is 0 and the first insert shows as if it was the last one with the correct data.
Program output:
Data at location 0 is: 0 //<--the reason behind this one is the constructor(not a problem)
Data at location 1 is: 0
data at location 2 is: 0
data at location 3 is: 51

I suspect the printList function but cant pinpoint where the problem occurs.
Any help will be much appreciated.

[gunnerfunner]

in front of head or before head

reading from left to right, what is the difference b/w these two?

[globaltourist]

Sorry for the vague explanation i'l try to explain it more graphically:

in front:

head: new node
_______ ______
| data | |data |
|_____| |_____|
|next | |next |
|_____|---->|_____|

Regular insertion.

before:
newNode : head:
_______ ______
| data | |data |
|_____| |_____|
|next | |next |
|_____|---->|_____|

Here we insert newNode before head making it the new head.

[gunnerfunner]

when you insert into a singly linked list you'd have to check first whether or not the list is empty
node *temp; - a data-member is usually a representation of some characteristics for the datatype (e.g class Person would have name, age, etc as data-members). so a full-fledged data-member of the list is not necessary just to iterate through the list, you can declare local variables for that purpose

in front: ... head: new node

void insertAfterHead(const int& data)
{
    node* temp = new node(data);
    if(head)
    {
        temp -> next = head -> next;//head assigns ownership of it's next to temp
        head -> next = temp;//then head's next is reassigned to temp
    }
    else //list is empty so temp becomes head;
    {
        head = temp;
    }
}

before: ... newNode : head:

void insertBeforeHead(const int& data)//const qualify data
{
    node* temp = new node(data);//overloaded ctor
    if(head)//if list is not empty
    {
        temp -> next = head;
    }
    head = temp;
}

your list also needs a destructor as the objects allocated with new are not automatically released
finally, this link has a discussion on singly linked lists with a working print()/display() method:
http://www.cplusplus.com/forum/beginner/210637/

[globaltourist]

Thanks for the response!

when you insert into a singly linked list you'd have to check first whether or not the list is empty
node *temp; - a data-member is usually a representation of some characteristics for the datatype (e.g class Person would have name, age, etc as data-members). so a full-fledged data-member of the list is not necessary just to iterate through the list, you can declare local variables for that purpose

I understand your solution but i still cant understand why mine doesn't work. I know my way is far from perfect but my goal was to learn only the logic behind the list and try to build it myself before i watch a proper guide on how to make one.

your list also needs a destructor as the objects allocated with new are not automatically released

I know that they don't release automatically and i did make a destructor:

list::~list() {
	node* prev = head;
	node* pnext = head;
	while (pnext) {
		prev = pnext;
		pnext = prev->next;
		if (pnext) delete prev;
	}//while end
}//destructor end 

Though i don't know if it is supost to be done that way