CS 180 Problem Solving and Object Oriented Programming Fall 2010

Transcription

1 CS 180 Problem Solving and Object Oriented Programming Fall 2010 hlp:// This Week: Notes for Week : Dec 6-10, /6 1. ExcepTons 2. Recursion Aditya Mathur Department of Computer Science Purdue University West LafayeLe, IN, USA

2 Readings and Exercises for Week Readings: Chapter: 7.4, 11.2, 11.3 Exercises: 7.6, 7.8, 11.1, 11.2, /12/2010 2

3 Revised Project Schedule AddiTonal office hours: Tuesday Dec 7: 2-4pm. LWSN 1177 Saturday Dec 11: 2-4pm. LWSN 1177 Special class: Sunday Dec 12, pm. LWSN /12/2010 3

4 Announcements Final exam: Monday Dec 13, :30pm. EE 129. Please alend the class on Wednesday Dec 8. Programming competton. Chief Guest Dr. Tim Korb. Cash prizes to winner and runner up teams. Thanks to Lockheed MarTn. I will announce the format of the final exam. Please complete course evaluaton. Thanks. 6/12/2010 4

5 ExcepTons An abnormal or unexpected conditon during executon. When such a conditon occurs, Java run Tme system does not always know how best to proceed and hence raises an excepton. Some exceptons must be explicitly handled by the programmer. 6/12/2010 5

6 ExcepTon: Example 1 public class ExcepTonExampleDiv0{ public statc void main(string [] args){ int y=1, z=0; int x=y/z; java.lang.arithmetcexcepton: / by zero at ExcepTonExampleDiv0.main(ExcepTonExampleDiv0.java:8) 6/12/2010 6

7 ExcepTon: Example 2 public class ExcepTonExampleDiv0{ public statc void main(string [] args){ int y=1, z=0; try{ int x=y/z; Try- catch not required catch(arithmetcexcepton e){ System.out.println( Divide by zero: +e); Divide by zero java.lang.arithmetcexcepton: / by zero 6/12/2010 7

8 ExcepTon: Unchecked (or Error) Arise during runtme due to some programming error. Not flagged by the compiler. Programmer may choose to handle these. Not subject to Catch requirement. Examples: ArithmeTcExcepTon ArrayIndexOutOfBounds NullPOinterExcepTon 6/12/2010 8

9 ExcepTon: Example 3 public class ExcepTonExampleFile{ public statc 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/ExcepTons/ExcepTonExampleFile.java [line: 8] Error: /Users/apm/www/courses/CS180Fall2010/Notes/ Programs/ExcepTons/Except 6/12/2010 9

10 ExcepTon: Example 4 public class ExcepTonExampleFile{ public statc void main(string [] args){ 6/12/2010 File f=new File("test.txt"); try{ Scanner source=new Scanner(f); catch(filenotfoundexcepton e){ System.out.println("File not found "+e); File not found java.io.filenotfoundexce pton: test.txt (No such file or directory) 10

11 ExcepTon: Checked exceptons These are subject to the catch requirement. Programmer is expected to write code to recover from these exceptons. Example: FileNotFoundExcepTon 6/12/

12 ExcepTon: try- catch try{ code where excepton might occur; catch(excepton object){ code to handle excepton try- block catch- block A catch- block serves as an excepton handler. 6/12/

13 ExcepTon: try- catch: multple catch blocks try{ code where excepton might occur; catch(excepton object){ code to handle excepton; catch(another excepton object{ code to handle excepton; try- block catch- block Another catch- block 6/12/

14 ExcepTon: try- catch- finally try{ Code where excepton might occur; catch(excepton object){ code to handle excepton; finally{ code to handle excepton; try- block catch- block finally- block Finally- block always executes aqer the try- block. 6/12/

15 Recursion 6/12/

16 What is recursion? Recursion is a technique to code functons (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 iteraton. While solving a problem, you may choose to use either. 6/12/2010

17 Example: Factorial DefiniTon: IteraTve [n>=0]!n= 1, if n=0 or 1 = 1*2*3* *n; DefiniTon: Recursive [n>=0]!n= 1, if n=0 or 1 = n*!(n- 1), if n>1 Basis Recursion 6/12/

18 Example: Factorial: IteraTve method DefiniTon: IteraTve [n>=0]!n= 1, if n=0 or 1 = 1*2*3* *n; 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 6/12/

19 Example: Factorial: Recursive method DefiniTon: 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 6/12/2010 =5*4*3*2*1 19

20 Recursion versus IteraTon IteraTon suffices for most problems that arise in practce. However, in some cases recursion allows compact and quick soluton. An example follows. 6/12/

21 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 Tme from one tower to another. Only the top disc can be moved from a tower. A B C 6/12/

22 Towers of Hanoi: IniTal and final configuratons IniTal configuraton A B C Final configuraton A B C 6/12/

23 Towers of Hanoi: IteraTve soluton Try this! This will be difficult but certainly worth a try. 6/12/

24 Towers of Hanoi: Recursive soluton What is a move? Only one disc can be moved from one tower to another. Hence we will write a move as: move x, y; x is the tower from where the top disc is moved and placed on top in tower y. For example: move A, B; will move the top disc on A to tower B 6/12/

25 Towers of Hanoi: Recursive soluton: VisualizaTon Start 1 3 A B C A B C 2 4 A B C A B C 6/12/

26 Towers of Hanoi: Recursive program: Live 6/12/

27 Towers of Hanoi: Challenge! What is the least number of moves required to move n discs from tower A to tower C? 6/12/

28 Week : December 6-10, 2010 Hope you enjoyed this week! QuesTons? Contact your recitaton instructor. Make full use of our office hours. 6/12/