Lecture An introduction to computer science using java (2nd Edition): Chapter 6 - S.N. Kamin, D. Mickunas, E. Reingold

Chapter 6 - Iteration. In this chapter we will: discuss the use of repetition (iteration) in programming algorithms; describe the Java while, for, and do-while statements; demonstrate the use of loop invariants to verify the correctness of loops; show the use of loops in data entry and computer animation.

Chapter 6

Iteration

Lecture Slides to Accompany

An Introduction to Computer Science Using Java (2nd Edition)

by S.N Kamin, D Mickunas, E Reingold

Chapter Preview

In this chapter we will:

• discuss the use of repetition (iteration) in programming algorithms

• describe the Java while, for, and do-while statements

• demonstrate the use of loop invariants to verify the correctness of loops

• show the use of loops in data entry and computer animation

while Loop

• Repeated executes body of the loop which

can be a single or compound statement

• A while loop will execute as long as its

condition is true

• An infinite loop will occur if the condition

never becomes false

• Example:

count = 1;

while (count <= 10)

count = count + 1;

while Loop From Temperature

Conversion Program

OutputBox out = new OutputBox( );

out.println(“\tDEGREES C\tDEGREES F”);

// initialize the loop variable

cent = LOW_TEMP;

// test the loop variable

while (cent <= HIGH_TEMP) {

// convert C to F

fahr = (9.0 / 5.0) * cent + 32.0;

out.println(“\t” + cent “\t\t” fahr);

// increment loop variable

cent = cent + 1.0;

}

for Statement

• Used to implement pre-test loops that are controlled by incrementing a variable each time the loop is executed

• Loop variable computations for initialization, testing, and incrementing appear in a single statement

• Example:

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

Comparing for and while

• Done using for

for ( statement1;

condition;

statement2 )

statement 3;

• Equivalent while loop

statement1;

while (condition) { statement3;

statement2;

}

Temperature Conversion Program

Loop Implemented Using for

OutputBox out = new OutputBox( );

out.println(“\tDEGREES C\tDEGREES F”);

for (cent = LOW_TEMP;

cent <= HIGH_TEMP;

cent = cent + 1.0;) {

// convert C to F

fahr = (9.0 / 5.0) * cent + 32.0;

out.println(“\t” + cent “\t\t” fahr); }

do-while Statement

• Used to implement post-test loops that always

execute the loop body at least one time

• A do-while loop will continue execute as long as its condition is true

• An infinite loop will occur if the condition never

becomes false

• Example:

count = 1;

do { count = count + 1;

} while (count <= 10);

Comparing do-while and while

• Done using do-while

do

statement;

while (condition);

• Equivalent while loop

statement;

while (condition) statement;

Comparing do-while and while

• Done using do-while

numberOfDigits = 0;

rest = number;

do {

rest = rest / 10;

numberOfDigits++;

} while (rest != 0);

• Equivalent while loop

numberOfDigits = 0;

rest = number;

if (number = 0) numberOfDigits = 1; else

while (rest > 0) { rest = rest / 10; numberOfDigits++; };

Reading Input Using a Loop

• Done using do-while

read score

if ( not end of input)

do {

process score

read score

} while ( not end of input );

• Done using while

read score while ( not end of input ) { process score

read score };

Sentinel Controlled Loop

int i = 0;

InputBox in = new InputBox();

In.setPrompt(“Enter data or –1 to terminate”);

// read first score

score = in.readInt();

while ( score != -1 ) {

i++;

// insert code to process score

// read next score

score = in.readInt();

}

Reading to End of Input

int i = 0;

InputBox in = new InputBox();

In.setPrompt(“Enter score (empty input ends data”);

// read first score

score = in.readInt();

while ( !in.eoi() ) {

i++;

// insert code to process score

// read next score

score = in.readInt();

}

Loop-and-a-Half

• Done using while

read score

while ( !in.eoi( ) ) {

process score

read score

};

• Done using do-while

do { read score

if ( !in.eoi( ) ) process score } while ( !in.eoi( ) );

Infinite Loop with Escape

• Done using do-while

do {

read

if ( in.eoi( ) ) return

process

} while ( true );

• Done using while

while (true) { read

if ( in.eoi( ) ) return process score

};

Drawing in Java

// code to read and display GIF image from file import java.awt.*;

Import CSLib.*;

…

DrawingBox g = new DrawingBox();

…

Toolkit tools = Toolkit.getDefaultToolkit():

Image mouse =

tools.getImage(“C:/KMR/images/mouse.gif”); g.drawImage(mouse, x, y);

Nested Loops

int advance = 0;

// Draw clock and read input repeatedly

do {

for (int i = 1; i <= advance; i++) {

// Advance time

c.setMinute(c.getMinute()+1);

// Update clock display

d.clear();

c.display(d, SIZE/2, SIZE/2, SIZE/2);

}

in.setPrompt(“Advance how many minutes?”); advance = in.readInt();

} while (!in.eoi());