Chapter 4: Control Structures II

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Chapter 4: Control Structures II. 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.

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### Chapter 4: Control Structures II

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 program

int counter = 0;

while (counter < N)

{

.

.

.

counter++;

.

.

}

Counter-Controlled while Loop-Example 5-3

//Counter-controlled while loopimport java.util.*;publicclass CounterControlledWhileLoop{static Scanner console = new Scanner(System.in);publicstaticvoid main(String[] args) {int limit; //store the number of items//in the listint number; //variable to store the numberint sum; //variable to store the sumint 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 Integers

2 1 5 8

The 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 loopimport java.util.*;publicclass SentinelControlledWhileLoop{static Scanner console = new Scanner(System.in);staticfinalint SENTINEL = -999;publicstaticvoid main (String[] args) {int number; //variable to store the numberint sum = 0; //variable to store the sumint 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.*;publicclass TelephoneDigit{static Scanner input = new Scanner (System.in);publicstaticvoid 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);

Sentinel-Controlled while LoopExample 5-5 (continued)

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;

Sentinel-Controlled while LoopExample 5-5 (continued)

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;

Sentinel-Controlled while LoopExample 5-5 (continued)

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.*;publicclass FlagControlledLoop{static Scanner console = new Scanner(System.in);publicstaticvoid main (String[] args) {//declare the variablesint num; //variable to store the random numberint guess; //variable to store the number//guessed by the userboolean 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; }

elseif (guess < num)

System.out.println("Your guess is " + "lower than " + "the number.\n" + "Guess again!");

+ "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).
• intprevious1 = Fibonacci number 1
• intprevious2 = Fibonacci number 2
• intnthFibonacci = 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 forloop is typically called a counted or indexed for loop. .
• Syntax:

for (initial statement; loop condition; update statement)

statement

The for Looping (Repetition) Structure
• Execution:
• Initial statement executes.
• Loop condition is evaluated.
• If loop condition evaluates to true,
• execute for loop statement and
• execute update statement.
• Repeat step 2 until loop condition is false.
The for Looping (Repetition) Structure

Example 5-9

The following for loop prints the first 10 nonnegative integers:

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

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

The for Looping (Repetition) Structure

Example 5-10

• 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("*");

The for Looping (Repetition) Structure
• 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: Nintegers (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

statement

while (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)

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 Statements

Example :

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 + “ ” );

}

Output

1 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:

*

**

***

****

*****

Nested Control Structures (Example 5-19)

//printing a multiplication table

for (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