2015-T2 Lecture 17 School of Engineering and Computer Science, Victoria University of Wellington  Marcus Frean, Lindsay Groves, Peter Andreae, John Lewis, and Thomas Kuehne, VUW COMP 103 Marcus Frean Linked Structures

2 RECAP-TODAY RECAP  We were looking at costs, Binary Search, then Sorting and recursing TODAY  A whole new way to implement Collections: Linked Structures Announcements  oops, I forgot to record the QuickSort lecture – hopefully fixed now

3 (from the end of last lecture) Stable or Unstable ? Faster if almost-sorted ?  MergeSort:  Stable: doesn’t jump any item over an unsorted region ⇒ two equal items preserve their order  Same cost on all input  “natural merge” variant doesn’t sort already sorted regions ⇒ will be very fast: O(n) on almost sorted lists  QuickSort:  Unstable: Partition “jumps” items to the other end ⇒ two equal items likely to reverse their order  Cost depends on choice of pivot  simplest choice can be very slow: O(n 2 ) even on almost sorted lists  better choice (median of three) ⇒ O(n log(n)) on almost sorted lists

4 (from the end of last lecture) Some “Big-Oh” costs revisited Implementing Collections:  ArrayList: O(n) to add/remove, except at end  Stack:O(1)  ArraySet:O(n) (cost of searching)  SortedArraySetO( log(n) ) to search (with binary search) O(n) to add/remove (cost of moving up/down)  O( n 2 ) to add n items O( n log(n) ) to initialise with n items. (with fast sorting)

5 Better Implementations for Collections  So far we have relied on arrays as a supporting data structure for collection classes  → ArrayList, SortedArraySet, ArrayQueue, ArrayPriorityQueue….  Array advantages:  quick random access  Array disadvantages:  growing them is costly (remember “ensureCapacity”?)  need contiguous chunks of memory  maintaining order is costly (Lecture 12)  How to address these disadvantages?

6 Array versus Linked Structure (analogy) Acreelman.blogspot.com

7 How can we insert faster ?  Fast lookup in array ⇒ items must be sorted  Maintaining the sorting order is costly  since inserting new items requires moving old ones  ⇒ You can’t insert fast with a sorted array!  To make insert faster, we would need each item to know its neighbours not by position but by topology  But, how do we keep track of the order ? BAEDC GIH F B A ED C H I G F

8 Linked List  Put each value in an object with a field for a link to the next  Traverse the list by following the links  Insert (F) by changing links ⇒ No need to shift everything up  Remove (D) by changing links ⇒ No need to shift things down B A E D C G I H F order of operations is important

9  Every node in a linked list can be considered to be the “head” of a sublist  That’s why linked lists – and other linked structures – lend themselves to be used with recursive methods Linked Lists are Recursive Structures List of length 3 List of length 2 List of length 1 C M X P shorthand for "null": end of the list

10 Memory management  What are references/pointers ?  A pointer/reference is an address of a chunk of memory, where the data can be stored  How do you get this memory allocated ?  You’ve been doing it since the start of Comp102, using new: creating an object allocates some chunk memory for the object new returns the address of the chunk of memory copying the address does not copy the chunk of memory  Memory should be recycled after use:  In Java, you don’t have to worry about freeing memory. The garbage collector automatically frees up any memory chunks that no longer have anything pointing/referring to them  In languages without a garbage collector (eg C, C++), you have to do this yourself. OK to ignore in small programs, but requires special care in large programs!

11 A Linked Node class: public class LinkedNode { private int value; private LinkedNode next; public LinkedNode(int item, LinkedNode nextNode) { value = item; next = nextNode; } public int getValue() { return value; } public LinkedNode next(){ return next; } public void setValue(int item) { value = item; } public void setNext(LinkedNode nextNode) { next = nextNode; } } 4 5

12 A Generic Linked Node class: public class LinkedNode { private E value; private LinkedNode next; public LinkedNode(E item, LinkedNode nextNode) { value = item; next = nextNode; } public E getValue() { return value; } public LinkedNode next(){ return next; } public void setValue(E item) { value = item; } public void setNext(LinkedNode nextNode) { next = nextNode; } an E

13 Using Linked Nodes LinkedNode colours = new LinkedNode (“red”, null); colours.setNext(new LinkedNode (“blue”, null)); colours = new LinkedNode (“green”, colours); System.out.printf(“1st: %s ”,colours.getValue() ); System.out.printf(“2nd: %s ”,colours.next().getValue() ); System.out.printf(“3rd: %s ”,colours.next().next().getValue() );  1 st : green 2 nd : red 3 rd : blue colours “red”“blue”“green”

14 Using Linked Nodes  Removing a node in the middle: colours.setNext( );  “Copy” colours, then “remove” first node LinkedNode copy = colours; colours = colours.next(); “red” colours “green”“blue” copy No references. Garbage collector will get it eventually colours.next().next()

15 Using Linked Nodes  Making a list of squared ints: LinkedNode squares = null; for (int i = 1; i ( i*i, squares );  This builds the list backwards…  Exercise: Build the list working forwards 49 1 squares /

16 Iterating through a linked list LinkedNode rest = squares; while (rest != null) { UI.println(rest.getValue()); rest = rest.next(); } or for (LinkedNode rest = squares; rest != null; rest = rest.next()) { UI.println(rest.getValue()); } or we could do it with recursion squares rest /

17 list list’ T wo ways to print a linked list /** Print the values in the list starting at a node */ public void printList(LinkedNode list) { if (list == null) return; UI.println(list.getValue()); if (list.next() == null) return; printList(list.next()); } “dog”“cat” “cow” recursive call base case public void printList(LinkedNode list’) { if (list’ == null) return; UI.println(list’.getValue()); if (list’.next() == null) return; printList(list’.next()); } handling an empty list

18 /** Print the values in the list starting at a node */ public void printList(LinkedNode list ) { if (list == null) return; UI.println(list.getValue()); printList(list.next()); } Example: printList ( the recursive version ) printList( list ) "cat" "dog" "cow" “dog” “cat” “cow” animals printList( animals ); printList( list’ ) printList( list’’ ) printList( list’’’ ) call return