Lecture Notes CPSC 122 (Fall 2014) Today Quiz 7 Doubly Linked Lists (Unsorted) List ADT Assignments Program 8 and Reading 6 out S.

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1 Today Quiz 7 Doubly Linked Lists (Unsorted) List ADT Assignments Program 8 and Reading 6 out S. Bowers 1 of 11

2 Doubly Linked Lists Each node has both a next and a prev pointer head \ v1 v2 v3 \ tail struct Node item_type value; Node * next; Node * prev; ; Node * head; Node * tail; Maintain both a head and a tail pointer Can also have a circular doubly linked list... we won t cover this S. Bowers 2 of 11

3 Inserting into the front and end of a doubly linked list Q: What are the cases to consider? an empty list a non-empty list Exercise: Write a push front(item) and push back(item) function using a doubly linked list. void Deque::push_front(const item_type& item) Node * ptr = new Node; ptr->value = item; if (is_empty()) head = ptr; tail = head; else ptr->next = head; head->prev = ptr; // new step head = ptr; length++; void Deque::push_back(const item_type& item) Node * ptr = new Node; ptr->value = item; if (is_empty()) head = ptr; tail = head; else S. Bowers 3 of 11

4 ptr->prev = tail; tail->next = ptr; tail = ptr; length++; // new step S. Bowers 4 of 11

5 Removing from the front of a doubly linked list Q: What are the cases to consider? an empty list a one-element list a two or more element list Exercise: Write a pop front() and pop back() function. bool Deque::pop_front(item_type& item) if (is_empty()) return false; item = head->value; Node * ptr = head->next; delete head; if (size() == 1) head = tail = NULL; else ptr->prev = NULL; head = ptr; length--; return true; bool Deque::pop_back(item_type& item) if (is_empty()) return false; S. Bowers 5 of 11

6 item = tail->value; Node * ptr = tail->prev; delete tail; if (size() == 1) head = tail = NULL; else ptr->next = NULL; tail = ptr; length--; return true; S. Bowers 6 of 11

7 Another advantage of a doubly linked list concerns iteration e.g., it is easy to navigate from the back to the front good for applications needed forward and backward access to elements e.g., a text editor (moving up/down or within lines of text) S. Bowers 7 of 11

8 (Unsorted) List ADT An Unsorted List manages ordered collections of values An array is an example of an unsorted list We ll see sorted lists later There isn t a standard set of operations for a List ADT sometimes access to just the front and back of the list sometimes access to elements within the list We ll use index-based operations: get item(index) insert(index, item) remove(index)... get the item at the index... insert item at index (shift rest right)... remove the item at index (shift rest left) We ll also consider iteration: get next() get prev() has next() has prev() reset()... move to the next item in the list and return it... move to the previous item in the list and return it... true if more nodes to move forward to... true if more nodes to move backward to... reset current position to the first item in the list S. Bowers 8 of 11

9 Exercise: Implement insert(index, item) using a doubly linked list. Q: What are the cases? index must be less than or equal to length, and greater than zero inserting into an empty list inserting into the front of the list inserting at the end of the list inserting into the middle Consider the following situation: head \ v1 v2 v4 v5 \ tail Assume we want to insert v3 into position 2 v3 The result should be: head \ v1 v2 v3 v4 v5 \ tail Q: How do we fuse the new node into the linked list? let currptr be a pointer to the node with v4 (at index 2) let ptr be a pointer to the new node 1. ptr->prev = currptr->prev 2. currptr->prev->next = ptr 3. ptr->next = currptr 4. currptr->prev = ptr S. Bowers 9 of 11

10 Using a doubly-linked list as an iterator Assume a currpos pointer into the linked list starts off being NULL can be reset (to NULL) using the reset() function Exercise: Implement the has next() function bool List::has_next() return size() > 0 && (currpos == NULL currpos->next!= NULL); the issue is we use NULL to signal a reset if size greater than 0 and current position is NULL, then we can iterate forward if the current position isn t NULL, then it needs to have a next node S. Bowers 10 of 11

11 Exercise: Implement the get next() function Q: What are the cases? the current position is null (and not an empty list) the current position isn t null, but no next the current position isn t null, and there is a next bool List::get_next(item_type& item) if (is_empty()) return false; if (currpos == NULL) currpos = head; else currpos = currpos->next; if (currpos == NULL) return false; item = currpos->value; return true; S. Bowers 11 of 11

Discussion 2C Notes (Week 3, January 21) TA: Brian Choi Section Webpage:

Discussion 2C Notes (Week 3, January 21) TA: Brian Choi (schoi@cs.ucla.edu) Section Webpage: http://www.cs.ucla.edu/~schoi/cs32 Abstraction In Homework 1, you were asked to build a class called Bag. Let