Chapter 4: Control Structures II

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!"); 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). • 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)

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

Chapter 4: Control Structures II