©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 6...

©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 6

Repetition Statements


Chapter 6 Objectives

After you have read and studied this chapter, you should be able to• Implement repetition control in a

program by using while statements.• Implement repetition control in a

program by using do-while statements.

• Implement a generic loop-and-a-half repetition control statement.

• Implement repetition control in a program by using for statements.


Chapter 6 Objectives, cont.

After you have read and studied this chapter, you should be able to• Nest a loop repetition statement

inside another repetition statement.• Choose the appropriate repetition

control statement for a given task.• Prompt the user for a yes/no reply by

using the showConfirmDialog method from the JOptionPane class.

• (Optional) Write simple recursive methods.


6.1 The while Statement

Repetition statements control a block of code to be executed for a fixed number of times or until a certain condition is met.

Java has three repetition statements:• while• do-while• for



In Java, while statements follow a general format:

while ( <boolean expression> )<statement>

For example:int sum = 0, number = 1;while (number <= 100){sum = sum + number;number = number + 1;

}



Repetition statements are also called loop statements, and the <statement> is known as the loop body.

As long as the <boolean expression> is true, the loop body is executed.


Fig. 6.1

Correspondence of the example while statement to the general format.


Fig. 6.2

A diagram showing the control flow of a while statement.



In a count-controlled loop, the loop body is executed a fixed number of times.

In a sentinel-controlled loop, the loop body is executed repeatedly until a designated value, called a sentinel, is encountered.


6.2 Pitfalls in Writing Repetition Statements

When writing repetition statements, it is important to ensure that the loop will eventually terminate.

There are several different types of potential programming problems we should keep in mind as we develop our programs.



Infinite loop

int product = 0;

while (product < 5000) {

product = product * 5;

}

Because product is initialized to 0, product will never be larger than 5000 (0 = 0 * 5), so the loop will never terminate.



Overflow error

int count = 1;

while (count != 10) {

count = count + 2;

}

In this example, (the while statement of which is also an infinite loop), count will equal 9 and 11, but not 10.



An overflow error occurs when you attempt to assign a value larger than the maximum value the variable can hold.

In Java, an overflow does not cause program termination. With types float and double, a value that represents infinity is assigned to the variable. With type int, the value “wraps around” and becomes a negative value.



Real numbers should not be used in testing or increment, because only an approximation of real numbers can be stored in a computer.

The off-by-one error is another frequently-encountered pitfall.


6.3 The do-while Statement

The while statement is a pretest loop, because the test is done before the execution of the loop body. Therefore, the loop body may not be executed.

The do-while statement is a posttest loop. With a posttest loop statement, the loop body is executed at least once.



The format of the do-while statement is:do

<statement>

while (<boolean expression>);

The <statement> is executed until the <boolean expression> becomes false.



int sum = 0, number = 1;

do{

sum += number;

number++;

} while (sum <=1000000);


Fig. 6.3

Correspondence of the example do-while statement to the general format.


Fig. 6.4

A diagram showing the control flow of the do-while statement.


6.4 Loop-and-a-Half Repetition Control

Loop-and-a-half repetition control can be used to test a loop’s terminating condition in the middle of the loop body.

It is implemented by using reserved words while, if, and break.



String name;

while (true){

name = JOptionPane.showInputDialog(null, “Your name”);

if (name.length() > 0) break;

JOptionPane.showMessageDialog(null, “Invalid Entry.” +

“You must enter at least one character.”);

}


Fig. 6.5

A diagram showing the control flow of a loop-and-a-half statement.



Be aware of two concerns when using the loop-and-a-half control:

• The danger of an infinite loop. The boolean expression of the while statement is true, which will always evaluate to true. If we forget to include an if statement to break out of the loop, it will result in an infinite loop.



• Multiple exit points. It is possible, although complex, to write a correct control loop with multiple exit points (breaks). It is good practice to enforce the one-entry one-exit control flow.


6.5 Confirmation Dialog

A confirmation dialog can be used to prompt the user to determine whether to continue a repetition or not.

JOptionPane.showConfirmDialog(null,

/*prompt*/ “Play Another Game?”,

/*dialog title*/ “Confirmation”,

/*button options*/ JOptionPane.YES_NO_OPTION);

Executing the code above will result in Fig. 6.6.


Fig. 6.6

A confirmation dialog created by using the showConfirmDialog method of the JOptionPane class.


6.5 Confirmation Dialog

Used in a loop statement:

boolean keepPlaying = true;int selection;while (keepPlaying){

//code to play one game comes hereselection = JOptionPane.showConfirmDialog(null,

“Play Another Game”,“Confirmation”,JOptionPane.YES_NO_OPTION);

keepPlaying = (selection == JOptionPane.YES_OPTION);

}


6.6 The for Statement

The format of the for statement is as follows:

for (<initialization>; <boolean expression>; <increment>)

<statement>

int i, sum = 0;

for (i = 1,i <=100, i++){

sum += i; //equivalent to sum = sum + 1;

}


Fig. 6.7

Correspondence of the example for statement to the general format


Fig. 6.8

A diagram showing the control flow of the example for statement.



The variable i in the example statement is called a control variable. It keeps track of the number of repetitions.

The <increment> can be by any amount, positive or negative.


6.6 The for Statement/*Chapter 6 Sample Program: Dropping a Watermelon

File: Ch6DroppingWaterMelon.java*/

import javabook.*;import java.io.*;

class Ch6DroppingWaterMelon {

public static void main( String[] args ) throws IOException {

double initialHeight, position, touchTime;


6.6 The for Statement BufferedReader bufReader = new BufferedReader( new InputStreamReader( System.in ));

System.out.print("Initial Height:");

InitialHeight=Double.parseDouble(bufReader.readLine( ));

touchTime = Math.sqrt(initialHeight /16.0); touchTime = Math.round(touchTime * 10000.0)

/ 10000.0; //convert to four decimal places

System.out.println("\n\n Time t Position at Time t \n");


6.6 The for Statementfor (int time = 0; time < touchTime; time++) {

position = -16.0 * time*time +

initialHeight; System.out.print(" " + time); System.out.println(" " +

position);}

//print the last second System.out.println(" " + touchTime +

" 0.00");

System.out.println("\n\n\n"); }}



An illustration of the Ch6DroppingWaterMelon.java program.


Fig. 6.9

The positions of a watermelon dropped from a height of 500 ft.


6.7 Nested-for Statements

Nested-for statements are for statements embedded within other for statements.

int price;

for (int width = 11; width <=20, width++){

for (int length = 5, length <=25, length+=5){

price = width * length * 19; //$19 per sq. ft.

System.out.print (“ “+ price);

}

//finished one row; move on to next row

System.out.println(“”);

}


Fig. 6.10

The price table for carpets ranging in size from 11 × 5 to 20 × 25 ft. whose unit price is $19 per sq. ft.


6.7 Nested-for Statements

The outer for statement ranges from the first row (width = 11) to the last row (width = 20).

For each repetition of the outer for, the inner for statement is executed, which ranges from the first column (length = 5) to the fifth (length = 25).


6.8 Formatting Output

To align values with varying numbers of digits, we must vary the number of spaces in front of the values.

The idea behind formatted output is to allocate the same amount of space for the output values and align the values within the allocated space.


Fig. 6.11

The price table for carpets with $15 per sq. ft. and width ranging from 5-14 ft.



We call the space occupied by an output value the field. The number of characters allocated to a field is the field width.


Fig. 6.12

How to place a varying number of spaces to align the output values. Hyphen is used here to indicate the blank space.



We will define a noninstantiable helper class called Ch6Format.

The pad method has an integer argument and returns a String object with the specified number of blank spaces.


6.8 Formatting Output/*Chapter 6 Sample Program: Simple formatting class

File: Ch6Format.java*/

/**class Ch6Format {

private static final String DEFAULT_SYMBOL = " "; //blank space

public static String pad(int size) {

return pad(size, DEFAULT_SYMBOL); }


6.8 Formatting Outputpublic static String pad(int size, String symbol) {

String result = "";

for (int i = 0; i < size; i++) {

result += symbol; }

return result; }

}


Fig. 6.13

There are two versions of the pad method. The first returns the specified number of blank spaces. The second returns the specified number of designated symbols. Hyphen is used here to indicate blank space.


6.8 Formatting Output/*Chapter 6 Sample Program: Sample formatting statements

File: Ch6CarpetPriceTableWithFormat.java*/

//import javabook.*;import java.io.*;

class Ch6CarpetPriceTableWithFormat{ public static void main (String[] args) { String numStr;

int valueWidth = 8; int labelWidth = 3; int price;


6.8 Formatting OutputPrintStream output = System.out;

//print out the column labelsoutput.print(Ch6Format.pad(labelWidth));

for (int colLabel = 5; colLabel <=25; colLabel += 5) {

numStr = Integer.toString(colLabel); output.print(Ch6Format.pad(valueWidth -

numStr.length()) + numStr);}output.print("\n\n");

for (int width = 5; width <= 14; width++) {

numStr = Integer.toString(width);

output.print(Ch6Format.pad(labelWidth - numStr.length()) + numStr);


6.8 Formatting Outputfor (int length = 5; length <= 25; length += 5) { price = width * length * 15;

numStr = Integer.toString(price); output.print(Ch6Format.pad(valueWidth - numStr.length()) + numStr);

}

//finished one row; now move on to the next row

output.print("\n"); // output.print(newLine); }

output.println("\n\n\n"); }}


Fig. 6.14

Carpet price table of Fig. 6.11 with proper alignment.


6.9 Loan Tables

The goal of this exercise is to design a program that generates a loan table. The table will compare different monthly payments for a set loan amount, with varying loan periods in each column and different interest rates in each row.


6.9 Loan Tables

The start method can be expressed as

tell the user what the program does;prompt the user “Do you want to generate a loan table?”;

while (the user says YES){input the loan amount;generate the loan table;prompt the user “Do you want another loan table?”;

}


6.9 Loan Tables

The start method was expressed in pseudocode.

Pseudocode is an informal language used to express an algorithm.

It is useful in clarifying the purpose or function of the program without being tied down to the syntactic rules of a programming language.


6.9 Loan Tablespublic void start () { int response; describeProgram();

response = prompt(“Generate a loan table?”); while (response == JOptionPane.YES_OPTION){

loanAmount = getLoanAmount(); //get inputgenerateLoanTable(loanAmount);//generate

table

response = prompt(“Generate another loan table?”);

}}


6.9 Loan Tables

private int prompt(String question){

int reply;

reply = JOptionPane.showConfirmDialog(null, question,

“Confirmation”,

JOptionPane.YES_NO_OPTION);

return reply;

}


6.9 Loan Tables

Other methods in the program:

describeProgram: tells the user what the program does if the user requests it.

getLoanAmount: gets the loan amount from the user.

generateLoanTable: generates the loan table.


6.9 Loan Tables

To compute the monthly loan payment, reuse the Loan class defined in Chapter 4.


6.10 Random Number Generation

The method random is called a pseudorandom number generator and returns a number of type double that is greater than or equal to 0.0 but less than 1.0.

The generated number is called a pseudorandom number because it is not truly random.



If we want to generate an integer, and the number returned from the random method ranges from 0.0 up to (but not including) 1.0, we must perform a conversion so the number will fall within the desired range.

Use the formula:Y = {X × (max – min + 1)} + min

where X is the number returned by random.



The formula is thus expressed in Java:

//assume correct values are assigned to

//‘max’ and ‘min’

int randomNumber

= (int) (Math.floor(Math.random *

max-min+1)) + min);



The following program generates N random numbers between 1 and 4 to simulate the suit of a drawn card.

It keeps one counter for each suit, and increments the matching counter after a random number is generated.

At the end of the generation, it prints the ratio count/N.


6.10 Random Number Generation/*Chapter 6 Sample Program: Test the random number generator

File: Ch6TestRandomGenerator.java*/

import javax.swing.*;import java.util.*;

class Ch6TestRandomGenerator {

private static final int CLUB = 1; private static final int SPADE = 2; private static final int HEART = 3; private static final int DIAMOND = 4;

public static void main( String[] args ) { Ch6TestRandomGenerator tester = new

Ch6TestRandomGenerator(); tester.start(); }


6.10 Random Number Generationpublic void start( ) { long N; while (keepTesting()) { N = getSize(); generate(N); }}

private long getSize( ) { String inputStr; long size;

while (true) { inputStr = JOptionPane.showInputDialog(null,

"Enter size:"); size = Long.parseLong(inputStr); if (size > 0) break; //input okay so exit

JOptionPane.showMessageDialog(null, "Input must be positive");

} return size;}


6.10 Random Number Generationprivate void generate(long size) {

Date startTime, endTime; int suit; long clubCnt, spadeCnt, heartCnt, diamondCnt;

clubCnt = spadeCnt = heartCnt = diamondCnt = 0;

startTime = new Date(); for (int i = 0; i < size; i++) {

suit = getRandom(CLUB, DIAMOND);

switch (suit) { case CLUB: clubCnt++; break;

case SPADE: spadeCnt++; break;

case HEART: heartCnt++; break;



case DIAMOND: diamondCnt++; break;

default: //default case should never happen JOptionPane.showMessageDialog(null,

"Internal Error"); System.exit(0); //terminate the program }

}

endTime = new Date();

System.out.println("N is " + size + "\n");System.out.println("Club: " + clubCnt + " " +

(double)clubCnt/size);System.out.println("Spade: " + spadeCnt + " " +

(double)spadeCnt/size);System.out.println("Heart: " + heartCnt + " " +

(double)heartCnt/size);System.out.println("Diamond: " + diamondCnt + " " +

(double)diamondCnt/size);


6.10 Random Number Generation double elapsedTimeInSec = (double) (endTime.getTime() - startTime.getTime())/1000.0;

System.out.println("Elapsed time (sec): " + elapsedTimeInSec );

System.out.println("\n");}

private int getRandom(int min, int max) { int randomNumber = (int) (Math.floor(Math.random() * (max-min+1)) + min);

return randomNumber;}

private boolean keepTesting( ) { boolean result; int response = JOptionPane.showConfirmDialog(null, /*prompt*/ "Perform Test?", /*dialog title*/ "Random Number Generator", /*button options*/ JOptionPane.YES_NO_OPTION);


6.10 Random Number Generationif (response == JOptionPane.YES_OPTION) {

result = true;

} else {

result = false;

}

return result;

}

}

©TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 6...

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