1. Java Programming: From Problem Analysis to Program Design, Second Edition1 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.

2. Java Programming: From Problem Analysis to Program Design, Second Edition2 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.

3. Java Programming: From Problem Analysis to Program Design, Second Edition3 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.

4. Java Programming: From Problem Analysis to Program Design, Second Edition4 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

5. Java Programming: From Problem Analysis to Program Design, Second Edition5 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). }

6. Java Programming: From Problem Analysis to Program Design, Second Edition6 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++;. }

7. Java Programming: From Problem Analysis to Program Design, Second Edition7 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;. }

8. Java Programming: From Problem Analysis to Program Design, Second Edition8 Flag-Controlled while Loop  Boolean value used to control loop.  General form: boolean found = false; while (!found) {. if (expression) found = true;. }

9. Java Programming: From Problem Analysis to Program Design, Second Edition9 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 class Scanner, 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.

10. Java Programming: From Problem Analysis to Program Design, Second Edition10 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 }

11. Java Programming: From Problem Analysis to Program Design, Second Edition11 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 }

12. Java Programming: From Problem Analysis to Program Design, Second Edition12 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.

13. Java Programming: From Problem Analysis to Program Design, Second Edition13 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).

14. Java Programming: From Problem Analysis to Program Design, Second Edition14 Programming Example: Fibonacci Number  Fibonacci formula for any Fibonacci sequence: a n = a n-1 + a n-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.

15. Java Programming: From Problem Analysis to Program Design, Second Edition15 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.

16. Java Programming: From Problem Analysis to Program Design, Second Edition16 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

17. Java Programming: From Problem Analysis to Program Design, Second Edition17 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.

18. Java Programming: From Problem Analysis to Program Design, Second Edition18 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();

19. Java Programming: From Problem Analysis to Program Design, Second Edition19 The for Looping (Repetition) Structure Example 5-9 1.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("*");

20. Java Programming: From Problem Analysis to Program Design, Second Edition20 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.

21. Java Programming: From Problem Analysis to Program Design, Second Edition21 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.

22. Java Programming: From Problem Analysis to Program Design, Second Edition22 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

23. Java Programming: From Problem Analysis to Program Design, Second Edition23 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.

24. Java Programming: From Problem Analysis to Program Design, Second Edition24 do…while Loop (Post-Test Loop)

25. Java Programming: From Problem Analysis to Program Design, Second Edition25 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.

26. Java Programming: From Problem Analysis to Program Design, Second Edition26 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.

27. Java Programming: From Problem Analysis to Program Design, Second Edition27 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.

28. Java Programming: From Problem Analysis to Program Design, Second Edition28 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: * ** *** **** *****

29. Java Programming: From Problem Analysis to Program Design, Second Edition29 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