Flow of Control Flow of Control Recitation 09/(11,12)/2008 CS 180 - - PowerPoint PPT Presentation

Flow of Control Flow of Control

Recitation – 09/(11,12)/2008

CS 180 CS 180 Department of Computer Science, Purdue University

Project 2

• Now posted on the class webpage.

j

• Due Wed, Sept. 17 at 10 pm.
• Start early All questions on the class

Start early. All questions on the class newsgroup.

• Evening consulting hours
• Evening consulting hours.

LWSN B146. MTW 7-10 pm.

22

Statements in Braces

• If and else parts in braces are preferred for even

a single statement a single statement.

if (count < 3) { total = 0; }

is better than is better than

if (count < 3) total = 0;

Statements in Braces

Benefits to do this: Decreases the chance of introducing a bug when

• Decreases the chance of introducing a bug when

adding a new statement.

• Makes the program a little more “self-

p g documenting” (more readable and easier to maintain).

== Is NOT Always Equal y q

• == should not be used to determine if two
• bjects have the same value including two
• bjects have the same value, including two

strings

• Even though (s1 == s2) might appear to work,

d t thi do not use this.

• Use (s1.equals(s2)) instead to compare two

strings. strings.

Multibranch if-else Statements

• if-else if-else is different from a series of

if statements if statements.

if (score >= 60) { if (score >= 60) { { result = ‘P’; } else if (score > 0) { result = ‘P’; } if (score > 0) { result = ‘F’; } else { result = ‘F’; } else else { result = ‘Z’; } else { result = ‘Z’; }

Short-circuit Evaluation

• Can prevent problems.

if ((number != 0) && (sum/number > 5)) if ((number != 0) && (sum/number > 5))

• Can produce problems!

if ((x evaluate()) && (y reassign())) if ((x.evaluate()) && (y.reassign()))

• How to run y.reassign() every time?
• Use & and | to replace && and || .

if ((x.evaluate()) & (y.reassign())) (( ()) (y g ()))

• But notice & and && are different!

Input & Output of Boolean Values p p

Example:

boolean booleanVar = false; boolean booleanVar = false; System.out.println(booleanVar); System.out.println("Enter a boolean value:"); Scanner keyboard = new Scanner(System in); Scanner keyboard = new Scanner(System.in); booleanVar = keyboard.nextBoolean(); System.out.println("You entered " + booleanVar);

Input & Output of Boolean Values p p

Dialog:

f l false Enter a boolean value: true Y t d t You entered true

The switch Statement

Syntax:

switch (Controlling Expression) switch (Controlling_Expression) { case Case_Label: Statement(s); break; case Case_Label: _ … default: … }

The switch Statement

View sample program Listing 3.4 class MultipleBirths class MultipleBirths

Sample Sample p screen

• utput

p screen

• utput

The switch Statement

• The action for each case typically ends with the

word break word break.

• The optional break statement prevents the

consideration of other cases.

• The controlling expression can be anything that

evaluates to an integral type.

The switch Statement

Example without break:

switch (grade) switch (grade) { case ‘P’: System.out.print(“You have”); case ‘F’: System.out.print(“…No, you have not”); default: } System out println(“passed ”); System.out.println( passed. );

Enumerations

• Consider a need to restrict contents of a variable

to certain values to certain values.

• An enumeration lists the values a variable can

have.

• Example

enum MovieRating {E, A, B} MovieRating rating; rating = MovieRating.A;

Enumerations

• Now possible to use in a switch statement

Enumerations

• An even better choice of descriptive identifiers for

the constants the constants.

enum MovieRating {EXCELLENT, AVERAGE, BAD} { , , } rating = MovieRating.AVERAGE; case EXCELLENT: ...

Specifying a Drawing Color p y g

g

• When drawing a shape inside an applet’s paint

method think of the drawing being done with a method, think of the drawing being done with a pen that can change colors.

• The method setColor changes the color of the

" " "pen." canvas.setColor(Color.YELLOW);

• Drawings done later appear on top of drawings
• Drawings done later appear on top of drawings

done earlier.

Specifying a Drawing Color p y g

g

View sample program, Listing 3.5 class YellowFace class YellowFace

Sample screen

• utput

Sample screen

• utput

Specifying a Drawing Color p y g g

Predefined Colors for the setColor Method:

Dialog Box for a Yes/No Question g

• Used to present the user with a yes/no question.

The window contains

• The window contains
• The question text
• Two buttons labeled yes and no.

Two buttons labeled yes and no.

Dialog Box for a Yes/No Question g

Example:

int answer = int answer = JOptionPane.showConfirmDialog(null, "End program?", "Click Yes or No:" "Click Yes or No:", JOptionPane.YES_NO_OPTION); if (answer == JOptionPane.YES_OPTION) System.exit(0); System.exit(0); else if (answer == JOptionPane.NO_OPTION) System.out.println("One more time");

Dialog Box for a Yes/No Question g

A Yes-or No-Dialog Box: