CS 180 Problem Solving and Object Oriented Programming Fall 2011

Transcription

1 CS 180 Problem Solving and Object Oriented Programming Fall 2011 hlp:// This Week: Notes for Week : Nov 28- Dec 2, / ExcepUons 2. Recursion 3. Dynamic data structures Aditya Mathur Department of Computer Science Purdue University West LafayeLe, IN, USA

2 Readings and Exercises for Week Readings: Chapter: 8.4, 12.1, 12.2, 12.3; 18.1, 18.8 Exercises: 8.6, 12.2, 12.3, 18.1,

3 Announcements Project 5 due on Tuesday Dec 6, 11:59pm. Implement incrementally. Please DO alend the class on Monday Dec 5. Programming compeuuon. Chief Guest Dr. Tim Korb. Details of Final Exam on Wednesday Dec 7. Please complete course evaluauon. Thanks. Special class. Sunday Dec 11, pm. LWSN 3102AB 3

4 Exam 2 StaUsUcs Average: 75.8 Median: 83 Std Dev: A+: 100/98 A: 97/94 A- : 93/90 B+: 89/87 B: 86/82 B- : 81/78 C+: 77/73 C: 72/67 C- : 66/56 D: 55/40 F: 39/0 4

5 ExcepUons 5

6 ExcepUons An abnormal or unexpected condiuon during execuuon. When such a condiuon occurs, Java run Ume system does not always know how best to proceed and hence raises an excepuon. Some excepuons must be explicitly handled by the programmer. 6

7 ExcepUon: Example 1 public class ExcepUonExampleDiv0{ public stauc void main(string [] args){ int y=1, z=0; int x=y/z; java.lang.arithmeucexcepuon: / by zero at ExcepUonExampleDiv0.main(ExcepUonExampleDiv0.java:8) 7

8 ExcepUon: Example 2 public class ExcepUonExampleDiv0{ public stauc void main(string [] args){ int y=1, z=0; try{ int x=y/z; Try- catch not required catch(arithmeucexcepuon e){ System.out.println( Divide by zero: +e); Divide by zero java.lang.arithmeucexcepuon: / by zero 8

9 ExcepUon: Unchecked (or Error) Arise during runume due to some programming error. Not flagged by the compiler. Programmer may choose to handle these. Not subject to Catch requirement. Examples: ArithmeUcExcepUon ArrayIndexOutOfBounds NullPointerExcepUon 9

10 ExcepUon: Example 3 public class ExcepUonExampleFile{ public stauc void main(string [] args){ File f=new File("test.txt"); Scanner source=new Scanner(f); 1 error found: File: /Users/apm/www/courses/CS180Fall2010/Notes/ Programs/ExcepUons/ExcepUonExampleFile.java [line: 8] Error: /Users/apm/www/courses/CS180Fall2010/Notes/ Programs/ExcepUons/Except 10

11 ExcepUon: Example 4 public class ExcepUonExampleFile{ public stauc void main(string [] args){ File f=new File("test.txt"); try{ Scanner source=new Scanner(f); catch(filenotfoundexcepuon e){ System.out.println("File not found "+e); File not found java.io.filenotfoundexcepuon: test.txt (No such file or directory) 11

12 ExcepUon: Checked excepuons These are subject to the catch requirement. Programmer is expected to write code to recover from these excepuons. Example: FileNotFoundExcepUon 12

13 ExcepUon: try- catch try{ code where excepuon might occur; catch(excepuon object){ code to handle excepuon try- block catch- block A catch- block serves as an excepuon handler. 13

14 ExcepUon: try- catch: muluple catch blocks try{ code where excepuon might occur; catch(excepuon object){ code to handle excepuon; catch(another excepuon object){ code to handle excepuon; try- block catch- block Another catch- block 14

15 ExcepUon: try- catch- finally try{ Code where excepuon might occur; catch(excepuon object){ code to handle excepuon; try- block catch- block finally{ code to handle excepuon; finally- block finally- block always executes aper the try- catch blocks (unless the program exits prior to the control arriving at the finally block).

16 ExcepUon: Not handled public ExcepUonTest(){ Scanner s=new Scanner(new File ( DoesNotExist )); Error: /Users/apm/www/courses/CS180Fall2011/Notes/ Programs/Recursion/Tree/ExcepUonTest.java:7: unreported excepuon java.io.filenotfoundexcepuon; must be caught or declared to be thrown 16

17 ExcepUon: Throwing public ExcepUonTest() throws ExcepUon{ Scanner s=new Scanner(new File ( DoesNotExist )); A method NOT a class can throw an ExcepUon. This will compile. BUT 17

18 ExcepUon: Throwing but not handling later public ExcepUonTest() throws ExcepUon{ Scanner s=new Scanner(new File ( DoesNotExist ); public stauc void main(string [] arg){ Error: /Users/apm/www/courses/CS180Fall2011/Notes/ Programs/Recursion/Tree/ExcepUonTest.java:11: unreported excepuon java.lang.excepuon; must be caught or declared to be thrown new ExcepUonTest(); 18

19 ExcepUon: Throwing but handling later public ExcepUonTest() throws ExcepUon{ Scanner s=new Scanner(new File ( DoesNotExist ); public stauc void main(string [] arg){ try{ new ExcepUonTest(); catch(excepuon e){ 19

20 Recursion 20

21 What is recursion? Recursion is a technique to code funcuons (or methods) in Java. We say that a method is recursive, when it calls itself. This is known as direct recursion. Indirect recursion occurs when a method is called, before it completes, by another method. Recursion is complementary to iterauon. While solving a problem, you may choose to use either. 21

22 Example: Factorial DefiniUon: IteraUve [n>=0]!n= 1, if n=0 or 1 = 1*2*3* *n; DefiniUon: Recursive [n>=0]!n= 1, if n=0 or 1 = n*!(n- 1), if n>1 Basis Recursion 22

23 Example: Factorial: IteraUve method DefiniUon: IteraUve [n>=0]!n= 1, if n=0 or 1 = 1*2*3* *n; otherwise public int factiter(int n){ if(n==0 n==1) return 1; int prod=1; for (int i=2; i<=n; i++){ prod=prod*i; return (prod);!5=prod=1*2*3*4*5 23

24 Example: Factorial: Recursive method DefiniUon: Recursive [n>=0]!n= 1, if n=0 or 1 = n*!(n- 1), if n>1 public int factrecur(int n){ if(n==0 n==1){ return 1; else{ return (n*factrecur(n- 1)); factrecur(5)=5*factrecur(4)*factrecur(3)*factrecur(2)*factrecur(1) =5*factRecur(4)*factRecur(3)*factRecur(2)*1 =5*factRecur(4)*factRecur(3)*2*1 =5*factRecur(4)*3*2*1 =5*4*3*2*1 24

25 Recursion versus IteraUon IteraUon suffices for most problems that arise in pracuce. However, in some cases recursion allows compact and quick soluuon. An example follows. 25

26 Announcements Feast with faculty today at 6:30pm Ford Dining Hall. All are welcome. Project 4 grades are on Blackboard. If your submiled program was working but you received a low grade then please contact TA Julian Stephen. Programming compeuuon: Round 2: Saturday 6pm. LWSN Three teams will be selected for the final round. 26

27 Towers of Hanoi: The problem Given: Three towers A, B, and C. A contains n discs arranged from bolom to top as shown below. Problem: Find a sequence of moves such that at the end of these moves all discs are in C. No larger disc should ever be on top of a smaller disk. Only one disk can be moved at a Ume from one tower to another. Only the top disc can be moved from a tower. A B C 27

28 Towers of Hanoi: IniUal and final configurauons IniUal configurauon A B C Final configurauon A B C 28

29 Towers of Hanoi: IteraUve soluuon Try this! This will be difficult but certainly worth a try. 29

30 Towers of Hanoi: Recursive soluuon What is a move? Only one disc can be moved from one tower to another. Hence we will write a move as: moveadisc (TA, TB); TA is the tower from where the top disc is moved and placed on top in tower TB. move (TA, TB, TC, n); This will move n discs from tower TA to tower TB via tower TC while ensuring the constraints. 30

31 Towers of Hanoi: Recursive soluuon: VisualizaUon Start 1 3 A B C A B C 2 4 A B C A B C 31

32 Towers of Hanoi: Recursive program: Live 32

33 Towers of Hanoi: Challenge! How many moves are required to move n discs from tower A to tower C? 33

34 Recursion and Dynamic data structures 34

35 Dynamic data structures StaUc data structure: Array Useful when size of the data structure is not known in advance. Useful for designing efficient algorithms. Examples: Stack, Linked list, binary tree, etc. In Java: ArrayList, Vector 35

36 Tree (Binary) Tree with one node (the root) Lep link Right link Tree with a root linked to two other nodes. Lep link Right link Another tree. 36

37 Tree: with data 25 Tree with one node (the root) 25 Lep link Right link Tree with a root linked to two other nodes. Lep link 25 Right link Another tree Do you see a palern? 9 37

38 Tree: Binary search Lep link 25 Right link Does x exist in the tree? boolean find(int x, Node root){ if(x is at the root) return true; if (x< value at the root) return find (x, root.leplink); else return find(x, root.rightlink); 38

39 Binary search tree: CreaUon Lep and right links are null. Lep link Right link 39 Add 25. Start with an empty tree and add a node. Set root to be this node. Add 39. This is greater than the number at the root. Hence add a new node to the right subtree. Add 14. As this is less than the value at the root, add a new node to the lep of the root. Each data item exists exactly once in the tree. 39

40 Tree: Non- empty binary search tree Has a root Each node has 0, 1, or 2 links. Links are generally termed as lep and right links. Each node has a data item in it. Suppose that node n points to nodes n1 (lep) and n2 (right). Then the data at node n1 is less than that at n and the data at node n2 is greater than that at node n. 40

41 Binary search tree creauon: Exercise What will a binary search tree look like aper the following numbers are added in this sequence given: 5, 29, - 4, 23, 99, 3? 41

42 Live demo: Tree creauon and traversal. 42

43 Week : November 28- Dec 2, 2011 Hope you enjoyed this week! QuesUons? Contact your recitauon instructor. Make full use of our office hours. 43