help!!!!!!!

I'm trying to print out all the elements in the linked list and I keep getting the same number printed out an infinite number of times. I don't know what condition I should put in the while statement to print out the numbers correctly.


#include<iostream.h>
using std :: cin;
using std :: cout;
using std :: endl;

#include<stddef.h>

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

node* make_a_list();
void print_list(node *list);
node* add_at_front(node* head, int addvalue);
void add_at_back(node *list, int addvalue);

int main(){
node* mylist;

// create the list and print it
mylist = make_a_list();
print_list(mylist);


// add_at_back( mylist, 40);
// print_list(mylist);


// mylist = add_at_front( mylist, 1);
// print_list(mylist);

return 0;
}

node* make_a_list()
// This function creates a linked list containing the even numbers
// from 2 to 20. The pointer of the list is returned to the
// calling function.
{
node *head = NULL, *tptr;
int num = 20;


while(num){
tptr = new node;
tptr->value = num;
tptr->next = head;

head = tptr;

num -= 2;
}

return head;
}


void print_list(node *list)
// This function accepts as input a pointer to the list
// and prints out all of the elements of the list.
// Each element is separated by a few spaces and
// the last element of the list is terminated by
// a newline.
{
node *ptr = list;

// loop through entire list
while(ptr->value != NULL) {


// print each element
cout << ptr->value;
list = ptr;
}
cout << endl;
return;
}


/* TAKE OUT COMMENTS MARKS BEFORE COMPILING

node* add_at_front(node* head, int addvalue)
// This function accepts as input a pointer to the list
// and an integer (addvalue). This integer is added
// to the front of the list. The pointer to the start
// of the list is then returned.
{

node *newptr;

// create a new node




// return pointer that points to front of list



}

*/



void add_at_back(node *list, int addvalue)
// This function accepts as input a pointer to the list
// and an integer (addvalue). This integer is added
// to the end of the list.
{

node *newptr, *tempptr=list;

// loop until you reach last node in list





// create a new node with appropriate values
// and add it to the list







return;
}

change this:

// loop through entire list
while(ptr->value != NULL) {


// print each element
cout << ptr->value;
list = ptr

to this:

while(ptr != NULL)
cout ptr->value;
ptr = ptr->next;