Java Programming: From Problem Analysis to Program Design, D.S. Malik Chapter 4: Control Structures...

Java Programming: From Problem Analysis to Program Design, D.S. Malik

Chapter 4: Control Structures II

Java Programming: From Problem Analysis to Program Design, D.S. Malik 2

Chapter Objectives

• Learn about repetition (looping) control structures.

• Explore how to construct and use counter-controlled, sentinel-controlled, and flag-controlled structures.

• Examine break and continue statements.• Discover how to form and use nested control

structures.


Why is Repetition Needed?

• There are many situations in which the same statements need to be executed several times.

• Example:– Formulas used to find average grades for

students in a class.


Repetition

• Java has three repetition, or looping, structures that let you repeat statements over and over again until certain conditions are met:

while for do…while


The while Looping (Repetition) Structure• Syntax:while (expression) statement

• Statements must change value of expression to false. • A loop that continues to execute endlessly is called an

infinite loop (expression is always true).


The while Looping (Repetition) Structure

Example 5-1

i = 0; while (i <= 20){

System.out.print(i + " "); i = i + 5;

}System.out.println();

Output

0 5 10 15 20


The while Looping (Repetition) Structure

Typically, while loops are written in the following form:

//initialize the loop control variable(s)

while (expression) //expression tests the LCV

{

.

.

.

//update the loop control variable(s)

.

.

.

}


Counter-Controlled while Loop• Used when exact number of data or entry pieces is

known.• General form:

int N = //value input by user or specified //in programint counter = 0;while (counter < N){ . . . counter++; . .}


Counter-Controlled while Loop-Example 5-3

//Counter-controlled while loopimport java.util.*;public class CounterControlledWhileLoop{ static Scanner console = new Scanner(System.in);

public static void main(String[] args) { int limit; //store the number of items //in the list int number; //variable to store the number int sum; //variable to store the sum int counter; //loop control variable

System.out.print("Enter the number of " + "integers in the list: "); limit = console.nextInt();

System.out.println();

sum = 0;

counter = 0; System.out.println("Enter " + limit+ " integers.");


Counter-Controlled while Loop-Example 5-3 (continued)

while (counter < limit) { number = console.nextInt();

sum = sum + number; counter++; }

System.out.printf("The sum of the %d " +"numbers = %d%n", limit, sum);

if (counter != 0) System.out.printf("The average = %d%n",(sum / counter));

else System.out.println("No input."); }

}

Sample Run:Enter the number of integers I the list: 4

Enter 4 Integers2 1 5 8The sum of the 4 numbers = 16

The average = 4


Sentinel-Controlled while Loop• Used when exact number of entry pieces is

unknown, but last entry (special/sentinel value) is known.

• General form:Input the first data item into variable;

while (variable != sentinel){

. . . input a data item into variable; . . .}


Sentinel-Controlled while LoopExample 5-4

//Sentinel-controlled while loop

import java.util.*;

public class SentinelControlledWhileLoop{ static Scanner console = new Scanner(System.in);

static final int SENTINEL = -999;

public static void main (String[] args) { int number; //variable to store the number int sum = 0; //variable to store the sum int count = 0; //variable to store the total //numbers read

System.out.println("Enter positive integers " + "ending with " + SENTINEL);


Sentinel-Controlled while LoopExample 5-4 (continued)

number = console.nextInt(); while (number != SENTINEL)

{ sum = sum + number; count++;

number = console.nextInt(); }

System.out.printf("The sum of the %d " + "numbers = %d%n", count, sum);

if (count != 0) System.out.printf("The average = %d%n",(sum /

count)); else System.out.println("No input");

}}


Sentinel-Controlled while LoopExample 5-5

//This program converts uppercase letters to their// corresponding telephone digits.//********************************************************

import java.util.*;

public class TelephoneDigit{ static Scanner input = new Scanner (System.in); public static void main (String[] args) { char letter; String inputMessage; String inputString; String outputMessage;

inputMessage = "Program to convert uppercase " + "letters to their corresponding " + "telephone digits.\n" + "To stop the program enter #.\n" + "Enter a letter:";

System.out.println(inputMessage);



letter = input.next().charAt(0); while (letter != '#' )

{ outputMessage = "The letter you entered is: " + letter + "\n" + "The corresponding telephone " + "digit is: ";

if (letter >= 'A' && letter <= 'Z') { switch (letter) { case 'A': case 'B': case 'C': outputMessage = outputMessage+ "2"; break; case 'D': case 'E': case 'F': outputMessage = outputMessage+ "3"; break;



case 'G': case 'H': case 'I': outputMessage = outputMessage+ "4"; break; case 'J': case 'K': case 'L': outputMessage = outputMessage+ "5"; break;

case 'M':

case 'N': case 'O': outputMessage = outputMessage+ "6"; break; case 'P': case 'Q': case 'R': case 'S': outputMessage = outputMessage + "7"; break;



case 'T': case 'U': case 'V': outputMessage = outputMessage+ "8"; break; case 'W': case 'X': case 'Y': case 'Z': outputMessage = outputMessage+ "9"; } } else outputMessage = outputMessage + "Invalid input";

System.out.println(outputMessage);

inputMessage = "Enter another uppercase letter " + "to find its corresponding " + "telephone digit.\n" + "To stop the program enter #.\n" + "Enter a letter:"; System.out.println (inputMessage); letter = input.next().charAt(0); }//end while }}


Flag-Controlled while Loop

• Boolean value used to control loop.• General form:

boolean found = false; while (!found){ .

. . if (expression) found = true; . . .}


Flag-Controlled while Loop-Example 5-6

/Flag-controlled while loop.//Guessing the number game.

import java.util.*;

public class FlagControlledLoop{ static Scanner console = new Scanner(System.in);

public static void main (String[] args) { //declare the variables int num; //variable to store the random number int guess; //variable to store the number //guessed by the user

boolean done; //boolean variable to control the loop

num = (int) (Math.random() * 100);

done = false;


Flag-Controlled while Loop-Example 5-6 (continued)

while (!done) {

System.out.print ("Enter an integer greater" + " than or equal to 0 and " + "less than 100: ");

guess = console.nextInt(); System.out.println();

if (guess == num) { System.out.println("You guessed the " + "correct number.");

done = true; }

else if (guess < num) System.out.println("Your guess is "

+ "lower than " + "the number.\n" + "Guess again!");

else System.out.println("Your guess is “

+ "higher than " + "the number.\n" + "Guess again!"); } //end while }

}


While Loop Programming Example: Fibonacci Number

• Fibonacci formula for any Fibonacci sequence:

an = an-1 + an-2

• Input: First two Fibonacci numbers in sequence, position in sequence of desired Fibonacci number (n).– int previous1 = Fibonacci number 1– int previous2 = Fibonacci number 2– int nthFibonacci = Position of nth Fibonacci number

• Output: nth Fibonacci number.


While Loop Programming Example: Fibonacci Number

if (nthFibonacci == 1) current = previous1; else if (nthFibonacci == 2) current = previous2; else { counter = 3; while (counter <= nthFibonacci) { current = previous2 + previous1; previous1 = previous2; previous2 = current; counter++; } }

• Final result found in last value of current.


The for Looping (Repetition) Structure

• Specialized form of while loop.

• Its primary purpose is to simplify the writing of counter-controlled loops. For this reason, the for loop is typically called a counted or indexed for loop. .

• Syntax:

for (initial statement; loop condition; update statement)

statement



• Execution:1. Initial statement executes. 2. Loop condition is evaluated.3. If loop condition evaluates to true,

1. execute for loop statement and 2. execute update statement.

4. Repeat step 2 until loop condition is false.



Example 5-9

The following for loop prints the first 10 nonnegative integers:

for (i = 0; i < 10; i++)

System.out.print(i + " ");



Example 5-101. The following for loop outputs the word Hello and a star (on

separate lines) five times:

for (i = 1; i <= 5; i++){

System.out.println("Hello"); System.out.println("*");

}2. The following for loop outputs the word Hello five times and

the star only once:

for (i = 1; i <= 5; i++) System.out.println("Hello"); System.out.println("*");



• Does not execute if loop condition is initially false.• Update expression changes value of loop control

variable, eventually making it false.

• If loop condition is always true, result is an infinite loop.

• Infinite loop can be specified by omitting all three control statements.

• If loop condition is omitted, it is assumed to be true.

• Action of for loop ending in semicolon is empty.


For Loop Programming Example: Classify Numbers

• Input: N integers (positive, negative, and zeros).

int N = 20; //N easily modified

• Output: Number of 0s, number of even integers, number of odd integers.


For Loop Programming Example: Classify Numbers (solution)

for (counter = 1; counter <= N; counter++){ number = console.nextInt(); System.out.print(number + " "); switch (number % 2) { case 0: evens++; if (number == 0) zeros++; break; case 1: case -1: odds++; } //end switch} //end for loop


The do…while Loop (Repetition) Structure

• Syntax:

do

statementwhile (expression);

• Statements are executed first and then expression is evaluated.

• Statements are executed at least once and then continued if expression is true.


do…while Loop (Post-Test Loop)


do…while Loop (Post-Test Loop)

Example :i = 0 ;do {

System.out.print(i + “ “ ) ; i = i + 5 ;

} while ( i <= 30 ) ;

output : 0 5 10 15 20 25 30


break Statements

• Used to – exit early from a loop. (while, for, and do...while)– skip remainder of switch structure.

• Can be placed within if statement of a loop.– If condition is met, loop is exited immediately.

• After the break statement executes, the program continues to execute with the first statement after the structure


break StatementsExample :

int count ;

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

{ if ( count == 5)

break ;

System.out.print(count + “ ” );

}

Output

1 2 3 4


continue Statements

• Used in while, for, and do...while structures.• When executed in a loop, the remaining statements

in the loop are skipped; proceeds with the next iteration of the loop.

• When executed in a while/do…while structure, expression is evaluated immediately after continue statement.

• In a for structure, the update statement is executed after the continue statement; the loop condition then executes.


continue Statements

Example :

int count ;

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

{ if ( count == 5)

continue;

System.out.print(count + “ ” );

}

Output1 2 3 4 6 7 8 9 10


Nested Control Structures

• Provides new power, subtlety, and complexity.• if, if…else, and switch structures can be

placed within while loops.

• for loops can be found within other for loops.


Nested Control Structures (Example 5-18)

for (int i = 1; i <= 5; i++){ for (int j = 1; j <= i; j++)

System.out.print(" *"); System.out.println();}

Output:***************

Java Programming: From Problem Analysis to Program Design, D.S. Malik

Nested Control Structures (Example 5-19)

//printing a multiplication tablefor (i = 1; i <= 5; i++)

{ for (j = 1; j <= 10; j++) System.out.printf("%3d", i*j); System.out.println(); }

Output

1 2 3 4 5 6 7 8 9 102 4 6 8 10 12 14 16 18 203 6 9 12 15 18 21 24 27 304 8 12 16 20 24 28 32 36 405 10 15 20 25 30 35 40 45 50


Chapter Summary

• Looping mechanisms:– Counter-controlled while loop– Sentinel-controlled while loop– Flag-controlled while loop– for loop– do…while loop

• break statements• continue statements• Nested control structures

Java Programming: From Problem Analysis to Program Design, D.S. Malik Chapter 4: Control Structures...

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