Flow of Control Recitation – 09/(18,19)/2008 CS 180 Department of Computer Science, Purdue University.

Flow of Control Recitation – 09/(18,19)/2008

CS 180

Department of Computer Science,

Purdue University

• Now posted on the class webpage.

• Due Wed, Sept. 24 at 10 pm.

• Start early. All questions on the class newsgroup.

• Exam in 2 weeks (10/1); Consult Mentor (Debbie) for more on pattern, advice etc.

• Evening consulting hours.

LWSN B146. MTW 7-10 pm.

Project 3 and Exam

• Loop statements

• while

• do-while

• for

• break and continue statements

• Display text in Applets

Outline

• A portion of a program that repeats a statement or a group of statements is called a loop

• The statement or group of statements to be repeated is called body of the loop

• A loop could be used to compute sum of N numbers

What are Loops?

• while loop continuously executes a block of statement until a particular condition is true

• Syntaxwhile (boolean_expression){

statement;}

The while statement

• Example:

int count = 1;while (count < 11)

{ System.out.println ("Count is: " +

count); count++;

}

while statement example

The do-while statement• Similar to a while statement, except that the loop

body is executed at least once

• Syntaxdo{

Statement;} while (Boolean_Expression);

• Remember the semicolon at the end of while. It helps the compiler to distinguish do-while and while statements

• Example:

int count = 1;do

{ System.out.println ("Count is: " +

count); count = count+1;

} while (count<=10);

do-while statement example

• Syntax for (Initialization; Condition;

Update){

statement;}

• Initialization expression initializes the loop and is executed once at the beginning

• Loop terminates when the Condition evaluates to false

• Update can be an increment or a decrement on a variable. It is executed after the first iteration

The for statement

for statement example 1

The action of the for loop in listing 4.5

• Example: Print all even numbers in the first 10 numbers

for (int count =1; count<=10;count++) {

if(count%2 == 0) System.out.print(count+” ”); }

Output: 2 4 6 8 10

for statement example 2

• Useful to operate on collection of data such as enumeration

• Example

enum Names {James, Joshua, Scott}

for (Names name: Names.values())

System.out.println(name+” ”);

• Names.values() represent all the values in the enumeration.

The for-each statement

The break statement• break statement can be used to end the loop

immediately• Example:

for (int n=1; n<=5;n++){

if (n==3){

break;}System.out.print(n+” ”);

}

• Output: 1 2

The continue statement• A continue statement ends current loop iteration

and begins next iteration• Example:

for (int i = 0; i < 10; i++){       if (i == 5) {        continue;       }       System.out.print(i+” ”);     }

• Output: 0 1 2 3 4 6 7 8 9• It is recommended to sparingly use break and

continue statements

loops in Applets

• A multiface Applet• Uses loop to draw several smiley faces• Uses if statement to alter appearance

• View sample program, listing 4.9class MultipleFaces

drawString Method• Similar to drawoval method, but displays text

• Examplecanvas.drawString("Hello",10, 20);• Writes word Hello at point (10, 20)

• Used to place "Kiss, Kiss" and "Tee Hee" on screen in listing 4.9