CS 101E – Exam 2 Review

Spring 2007Michele Co

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.)

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

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

Chapter 3 - Decisions

• if statements• if, if-else, if-else-if• nested if statements

if(Expression)Action

• ? : notation(x > y) ? x : y

Chapter 3 - Decisions

• switch statementsswitch(Expression) {

case CaseExpression:Action_1;

case CaseExpression:Action_n;

default:Action_n+1;

}

Chapter 3

• break statement

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

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?

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!

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

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

foreverSystem.exit(); // then halt

else // else if Test.java halts

while (true) { } // then loop forever}

}

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

Chapter 5 - Methods• Method syntax• Trace method execution• Refactoring

• benefits• disadvantages

• return keyword• return values

• returning primitive types vs. returning objects

Chapter 5 - Methods

• Parameters• How parameters are passed in Java• When can changes occur?

• Variable scoping• static• local variables• parameters

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?

Miscellaneous

• Using Scanner class to read in from a file

• Object-oriented programming benefits

• ++i vs. i++• When are variables automatically

initialized in Java?