1. CS 101E – Exam 2 Review Spring 2007 Michele Co

2. Announcements Review Session Tonight, 7/7:30 p.m., OLS 009 Will be announced via email In-class Exam Wednesday Lab Quiz 2 Same time as Lab Quiz 1 Quiz will become available on Sunday evening, 7:00 p.m. MUST electronically submit your solution by 8:30 p.m. Contact me by Wednesday noon if you need to take the lab quiz in lab on Thursday. (Let me know which section you’d like to attend.)

3. Textbook Chapters Chapter 3 – Decision structures Chapter 4 – Looping constructs Halting problem Chapter 5 - Methods Chapter 6 – Classes

4. Chapter 3 – Decision Structures logical expressions logical and, logical or, logical not DeMorgan’s Laws truth tables operator precedence testing for object equality string comparison short circuit evaluation

5. Chapter 3 - Decisions if statements if, if-else, if-else-if nested if statements if(Expression) Action ? : notation (x > y) ? x : y

6. Chapter 3 - Decisions switch statements switch(Expression) { case CaseExpression: Action_1; case CaseExpression: Action_n; default: Action_n+1; }

7. Chapter 3 break statement

8. Chapter 4 - Loops while do-while for loop indexing Syntax for each construct Components of each Differences between the constructs for vs. while vs. do-while How often, and when is the condition expression evaluated? nested loops break vs. continue

9. Loop Design/Analysis Questions to consider in loop design and analysis What initialization is necessary for the loop’s test expression? What initialization is necessary for the loop’s processing? What causes the loop to terminate? What actions should the loop perform? What actions are necessary to prepare for the next iteration of the loop? What conditions are true and what conditions are false when the loop is terminated? When the loop completes what actions are need to prepare for subsequent program processing?

10. Halting Problem Given a Java program P, and input I Let P be a filename for a program file on a disk somewhere Let I be a filename for a file that contains all the input the program takes in Will the program P with input I ever terminate? Meaning will program P with input I loop forever or halt? Can a computer program determine this? Can a human? First shown by Alan Turing in 1936 Before digital computers existed!

11. Halting Problem Proof Idea Consider a program P with input I Suppose that a method Oracle.CheckHalt(P,I) exists Tests if P(I) will either “loop forever” or “halt” A program is a series of bits And thus can be considered data as well Thus, we can call CheckHalt(P,P) It’s using the bytes of program P as the input to program P

12. Halting Problem Proof (cont’d) Consider a new program: public class Test { public static void main (String args[]) { if ( Oracle.CheckHalt(“Test.java”, “Test.java”) ) // if Test.java loops forever System.exit();// then halt else// else if Test.java halts while (true) { }// then loop forever }

13. Why is the Halting Problem Important? It was the first algorithm that was shown to not be able to exist by a computer It’s much harder to prove that a program can never exist

14. Chapter 5 - Methods Method syntax Trace method execution Refactoring benefits disadvantages return keyword return values returning primitive types vs. returning objects

15. Chapter 5 - Methods Parameters How parameters are passed in Java When can changes occur? Variable scoping static local variables parameters

16. Chapter 6 - Classes mutator accessor constructor specific default What set of actions occurs when Circle c = new Circle(); is executed? What is the purpose of a constructor?

17. Miscellaneous Using Scanner class to read in from a file Object-oriented programming benefits ++i vs. i++ When are variables automatically initialized in Java?