Lecture 4 Objectives

1. Lecture 4 Objectives • Learn about repetition (looping) control structures. • Explore how to construct and use count-controlled, sentinel-controlled, flag-controlled, and EOF-controlled repetition structures. • Examine break and continue statements. • Discover how to form and use nested control structures. Java Programming: From Problem Analysis to Program Design, Second Edition

2. 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. Java Programming: From Problem Analysis to Program Design, Second Edition

3. The while Looping (Repetition) Structure • Syntax: while (expression) statement • Expression is always true in an infinite loop. • Statements must change value of expression to false. Java Programming: From Problem Analysis to Program Design, Second Edition

4. The while Looping (Repetition) Structure Example 5-1 i = 0; //Line 1 while (i <= 20) //Line 2 { System.out.print(i + " "); //Line 3 i = i + 5; //Line 4 } System.out.println(); //Line 5 Output 0 5 10 15 20 Java Programming: From Problem Analysis to Program Design, Second Edition

5. 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) . . . } Java Programming: From Problem Analysis to Program Design, Second Edition

6. 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++; . . . } Java Programming: From Problem Analysis to Program Design, Second Edition

7. 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; . . . } Java Programming: From Problem Analysis to Program Design, Second Edition

8. Flag-Controlled while Loop • Boolean value used to control loop. • General form: boolean found = false; while (!found) { . . . if (expression) found = true; . . . } Java Programming: From Problem Analysis to Program Design, Second Edition

9. EOF(End of File)-Controlled while Loop • Used when input is from files. • Sentinel value is not always appropriate. • In an EOF-controlled while loop that uses the Scanner object console to input data, console acts at the loop control variable. • The method hasNext, of the classScanner, returns true if there is an input in the input stream; otherwise, it returns false. • The expression console.hasNext() acts as the loop condition. • Expressions such as console.nextInt() update the value of the loop condition. Java Programming: From Problem Analysis to Program Design, Second Edition

10. EOF-Controlled while Loop • A general form of the EOF-controlled while loop that uses the Scanner object console to input data is: while (console.hasNext()) { //Get the next input and store in an //appropriate variable //Process data } Java Programming: From Problem Analysis to Program Design, Second Edition

11. EOF-Controlled while Loop • Suppose that inFile is a Scanner object initialized to the input file. In this case, the EOF-controlled while loop takes the following form: while (inFile.hasNext()) { //Get the next input and store in an //appropriate variable //Process data } Java Programming: From Problem Analysis to Program Design, Second Edition

12. Programming Example: Checking Account Balance • Input file: Customer’s account number, account balance at beginning of month, transaction type (withdrawal, deposit, interest), transaction amount. • Output: Account number, beginning balance, ending balance, total interest paid, total amount deposited, number of deposits, total amount withdrawn, number of withdrawals. Java Programming: From Problem Analysis to Program Design, Second Edition

13. Programming Example: Checking Account Balance • Solution: • Read data. • EOF-controlled loop. • switch structure of transaction types. • Determine action (add to balance or subtract from balance depending on transaction type). Java Programming: From Problem Analysis to Program Design, Second Edition

14. 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. Java Programming: From Problem Analysis to Program Design, Second Edition

15. Programming Example: Fibonacci Number (Solution) 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. Java Programming: From Problem Analysis to Program Design, Second Edition

16. The for Looping (Repetition) Structure • Specialized form of while loop. • Simplifies the writing of count-controlled loops. • Syntax: for (initial statement; loop condition; update statement) statement Java Programming: From Problem Analysis to Program Design, Second Edition

17. 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 until loop condition is false. Java Programming: From Problem Analysis to Program Design, Second Edition

18. The for Looping (Repetition) Structure Example 5-8 The following for loop prints the first 10 nonnegative integers: for (i = 0; i < 10; i++) System.out.print(i + " "); System.out.println(); Java Programming: From Problem Analysis to Program Design, Second Edition

19. The for Looping (Repetition) Structure Example 5-9 • 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("*"); Java Programming: From Problem Analysis to Program Design, Second Edition

20. The for Looping (Repetition) Structure • Does not execute if initial condition is 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. • for statement ending in semicolon is empty. Java Programming: From Problem Analysis to Program Design, Second Edition

21. 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. Java Programming: From Problem Analysis to Program Design, Second Edition

22. 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 Java Programming: From Problem Analysis to Program Design, Second Edition

23. 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. Java Programming: From Problem Analysis to Program Design, Second Edition

24. do…while Loop (Post-Test Loop) Java Programming: From Problem Analysis to Program Design, Second Edition

25. break Statements • Used to exit early from a loop. • Used to skip remainder of switch structure. • Can be placed within if statement of a loop. • If condition is met, loop is exited immediately. Java Programming: From Problem Analysis to Program Design, Second Edition

26. 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. Java Programming: From Problem Analysis to Program Design, Second Edition

27. 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. Java Programming: From Problem Analysis to Program Design, Second Edition

28. Nested Control Structures (Example) 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, Second Edition

29. Lecture 4 Summary • Looping mechanisms: • Counter-controlled while loop • Sentinel-controlled while loop • Flag-controlled while loop • EOF-controlled while loop • for loop • do…while loop • break statements • continue statements • Nested control structures Java Programming: From Problem Analysis to Program Design, Second Edition