CS18000: Programming I

Transcription

1 CS18000: Programming I Linked Data Structures 5 April 2010 Prof. Chris Clifton Multiple Items: Beyond Arrays interface Set<E> { boolean contains(e item); /* true iff x s.t. item.equals(x) */ void add(e item) throws SetFull; /* ensure contains(item); */ void remove(e item); /* ensure!contains(item); */ abstract class ArraySet<T> implements Set<E> { E[ ] items; public boolean contains(e item) { for (int i=0; i<items.length; ++i) { if ( item.equals(items[i]) ) return true; return false; abstract public void add(e item) throws SetFull; abstract public void remove(e item); 4/7/2010 CS Chris Clifton 1

2 Fixed size Is it big enough? Problems with Arrays Is it more than we need? Can extend, but adds complexity Deletion Removing items leaves holes Can manage (e.g., null values), but again adds complexity 4/7/2010 CS Solution: Linked Structures Array: List of pointers to contained objects aset: n: abc i: 3 n: hjk i: 2 n: dc i: 3 n: abc i: 25 n: def i: 7 lset: Alternative: Object has pointer to next contained object! 4/7/2010 CS Chris Clifton 2

3 What goes in a Linked Data Structure? Contents of one cell Instance variables for object Pointer to an object Pointer to next object Instance of your own class Data Data Null value for this variable means end Can even have multiple next objects Data Data 4/7/2010 CS Accessing Linked Data Structures Iterator interface next() hasnext() Loop while ( hasnext() ) next().processanitem(); Recursive if ( hasnext() ) { item = next(); item.processanitem(); processrestofitems(); 4/7/2010 CS Chris Clifton 3

4 Linked List Set Class LinkedSet<E> implements Set<E> { private E current; private LinkedSet<E> next; boolean contains(e item) { if ( item.equals(current) ) return true; else { if (next == null) return false; else return next.contains(item); void add(e item) { if (current == null) current = item; else { LinkedSet<E> newitem = new LinkedSet<E>(item, next); next = newitem; public LinkedSet() { // Creates an empty set. current = null; next = null; private LinkedSet(E item, LinkedSet<E> next) { this.current = item; this.next = next; void remove(e item) { if (item.equals(current)) current = null; if (next!= null) next.remove(item); 4/7/2010 CS What is wrong? Preceding code works, but could be better Remove leaves empty cells in list Why not just take them out? Need to expose abstraction Class aware of it s own structure Access next directly 4/7/2010 CS Chris Clifton 4

5 Linked List Set public void remove(e item) { if (item.equals(current)) { if (next == null) current = null; else { current = next.current; // move next to self next = next.next; // "forget" next remove(item); // Try again if (next!= null) next.remove(item); 4/7/2010 CS Iterator I mentioned Iterator as a way to go through the class What is this? Interface supporting next() operation Code can operate on collections by using next() Can operate on any Iterator<E> To be used by such code, implement Iterator<E> 4/7/2010 CS Chris Clifton 5

6 Iterator public class LinkedSet<E> implements Set<E>, java.util.iterator<e> { public E next() throws java.util.nosuchelementexception { if (current == null) throw new java.util.nosuchelementexception(); return current; 4/7/2010 CS Iterator public class LinkedSet<E> implements Set<E>, java.util.iterator<e> { public E next() throws java.util.nosuchelementexception { if (next == null) throw new java.util.nosuchelementexception(); return next.current; 4/7/2010 CS Chris Clifton 6

7 Issue: Variable always points to first element This is what made remove difficult Can t remove first element Need to overwrite First element needs to know iterator position Maintain position 4/7/2010 CS Iterator public class LinkedSet<E> implements Set<E>, java.util.iterator<e> { private LinkedSet<E> currentiterator = this; public boolean hasnext() { return! ( currentiterator == null currentiterator.current == null ); public E next() throws java.util.nosuchelementexception { if (! hasnext()) throw new java.util.nosuchelementexception(); E result = currentiterator.current; currentiterator = currentiterator.next; return result; 4/7/2010 CS Chris Clifton 7

8 Announcements Exam returned If yours was missing, you probably forgot your name Check your score with solution (at syllabus) Circled number near your name is Q2,4,9,10,11 If total (in blackboard) incorrect, verify with your recitation instructor Project 5 out Due a week from Friday 4/7/2010 CS CS18000: Programming I Recursion 7 April 2010 Prof. Chris Clifton 2010 Chris Clifton 8

9 Recursion Idea: break a problem down into small, similar sub-problems Write a method to solve first Call that method to solve next Sounds similar to a loop Same basic effect But often an easier way to conceptualize the solution 4/7/2010 CS Simple Example: Factorial What is n! n*(n-1)*(n-2)* *1 Easy enough with a loop int result = 1; for (int i=1; i<=n; i++) result = result*i; But think of the problem differently: n! = n * (n-1)! 4/7/2010 CS Chris Clifton 9

10 Recursive Factorial n! = n * (n-1)! public static int factorial( int n ) { if ( n==1 ) return 1; else return n * factorial( n-1 ); Key components: Base case when do we know the answer Recursive case how do we call to get the answer 4/7/2010 CS Visualizing Recursion: Call Stack Save what is left to do Call (recursive) method public static int factorial( int n ) { if ( n==1 ) return 1; else return n * factorial( n-1 ); factorial(3) factorial(3) return 3 * factorial(2) factorial(2) return 2 * factorial(1) factorial(1) return /7/2010 CS Chris Clifton 10

11 Tougher Problem: Towers of Hanoi Source: Wikipedia 4/7/2010 CS Tougher Problem: Towers of Hanoi We know how to move the top disk Just move it But how to move one underneath? 1. Move disk above to center 2. Move disk to right 3. Move upper disk from center to right 4/7/2010 CS Chris Clifton 11

12 Towers of Hanoi: Solution Source: Wikipedia 4/7/2010 CS Tougher Problem: Towers of Hanoi We know how to move the top disk Just move it But how to move one underneath? 1. Move disk above to center 2. Move disk to right 3. Move upper disk from center to right Recursive approach call move for move disk above 4/7/2010 CS Chris Clifton 12

13 Hanoi Example public class TowersOfHanoi { public static void main(string[] args) { move(4, A, C, B ); static void move(int n, String from, String to, String via) { if (n == 1) System.out.println( move disk from tower + from + to + to); else { move(n-1, from, via, to); move(1, from, to, via); move (n-1, via, to, from); 4/7/2010 CS Towers of Hanoi: Object-Oriented Thinking I am a disk, I m asked to move to right Ask disk above me to move to center Then I move to right Then ask disk above to move to right 4/7/2010 CS Chris Clifton 13