Difference in adding of elements to linked list

Question

This is how i add elements to the head of a linked list

 //typedef void* VoidPtr //typedef node* NodePtr struct Node { NodePtr next VoidPtr data }; void LinkedList::Addelement(VoidPtr horoscope) { if(head == NULL) { head = new Node; head->data = horoscope; head->next = NULL; } NodePtr temp = new Node; temp -> data = horoscope; temp -> next = head; head = temp; }

This is how i add elements to the tail of a linked list

void LinkedList::addelementfromback(VoidPtr horoscope) { NodePtr temp=head; if(head == NULL) { head = new Node; head->data = horoscope; head->next = NULL; } while( temp->next != NULL) { temp=temp->next } NodePtr element=New Node; element->data=horoscope; element->next=NULL; temp->next=element; }

i dont understand why we use temp=element for adding to the head of a linked list but for adding to the tail of a linked list we use temp->next=element. I dont understand why we cant use while temp=next for adding element to tail of linked list

you don't use while (temp != NULL) for adding to the head of a linked list — Andy Prowl
– Andy Prowl, Commented Feb 3, 2013 at 17:48
You should just draw yourself a picture and see what happens if you run the algorithm by hand. Hint: if you check for temp != NULL, the loop breaks when temp is equal to NULL. How would you get back from NULL to the last list node? — Carl Norum
– Carl Norum, Commented Feb 3, 2013 at 17:51
@Computernerd: Save yourself some development headaches and use std::list. It's already been thoroughly tested. — Thomas Matthews
– Thomas Matthews, Commented Feb 3, 2013 at 18:38

Thomas Matthews · Accepted Answer · 2013-02-03 18:57:55Z

In your Addelement method, you need an else clause because if the list is empty (head == NULL), you only need to point the head to the new node. Nothing else, there are no other nodes in the list.

Also, rather than using a void pointer, consider using templates. Templates are great for data structures and algorithms where the data type changes, not the structure or algorithm, such as stacks and linked lists.

I suggest you consider separating the node pointers from the data item as two separate structures. This will help you use common code between single linked lists and doubly linked lists. Also a great help when you don't want to use templates.

struct Node_Link { Node_Link * next; }; struct Node_Integer : public Node_Link { int data; }; struct Node_Double : public Node_Link { double data; }; struct Node_String : public Node_Link { std::string data; };

Bernhard Barker · Accepted Answer · 2013-02-03 18:49:25Z

At the end of addelementfromback, temp is a pointer to the last element in the list. If you were to say temp = element, this wouldn't change the list, since you're just giving a local pointer a new value.

temp->next, however, is the next variable stored inside the object temp points to (i.e. the last element in the list), you need to change this value to point to the new element.

For Addelement, temp is a pointer to the new element, you assign the pointer head to point to the same element, and change the next variable inside this pointed-to element to point to the original head.

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