Java Programming: From Problem Analysis to Program Design, 4e Chapter 5 Control Structures II: Repetition

2Java Programming: From Problem Analysis to Program Design, 4e Chapter 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

3Java Programming: From Problem Analysis to Program Design, 4e 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

4Java Programming: From Problem Analysis to Program Design, 4e 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

5Java Programming: From Problem Analysis to Program Design, 4e The while Looping (Repetition) Structure (continued)

6Java Programming: From Problem Analysis to Program Design, 4e The while Looping (Repetition) Structure (continued) 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:

7Java Programming: From Problem Analysis to Program Design, 4e The while Looping (Repetition) Structure (continued) Typically, while loops are written in the following form:

8Java Programming: From Problem Analysis to Program Design, 4e Counter-Controlled while Loop Used when exact number of data or entry pieces is known General form:

9Java Programming: From Problem Analysis to Program Design, 4e Sentinel-Controlled while Loop Used when exact number of entry pieces is unknown but last entry (special/sentinel value) is known General form:

10Java Programming: From Problem Analysis to Program Design, 4e Flag-Controlled while Loop Boolean value used to control loop General form:

11Java Programming: From Problem Analysis to Program Design, 4e 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

12Java Programming: From Problem Analysis to Program Design, 4e EOF (End of File)-Controlled while Loop (continued) The expression console.hasNext() evaluates to true if there is an input in the input stream; otherwise it returns false Expressions such as console.nextInt() act as the loop condition A general form of the EOF-controlled while loop that uses the Scanner object console to input data is of the form:

13Java Programming: From Problem Analysis to Program Design, 4e EOF (End of File)-Controlled while Loop (continued) Suppose that inFile is a Scanner object initialized to the input file; in this case, the EOF- controlled while loop takes the following form:

14Java Programming: From Problem Analysis to Program Design, 4e

15Java Programming: From Problem Analysis to Program Design, 4e 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

16Java Programming: From Problem Analysis to Program Design, 4e 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

17Java Programming: From Problem Analysis to Program Design, 4e The for Looping (Repetition) Structure Specialized form of while loop Simplifies the writing of count-controlled loops Syntax for (initial expression; logical expression; update expression) statement

18Java Programming: From Problem Analysis to Program Design, 4e The for Looping (Repetition) Structure (continued) Execution –Initial statement executes –The loop condition is evaluated –If loop condition evaluates to true, execute for loop statement and execute update statement –Repeat until loop condition is false

19Java Programming: From Problem Analysis to Program Design, 4e The for Looping (Repetition) Structure (continued)

20Java Programming: From Problem Analysis to Program Design, 4e The for Looping (Repetition) Structure (continued) Example 5-9 The following for loop prints the first 10 nonnegative integers: for (i = 0; i < 10; i++) System.out.print(i + " "); System.out.println();

21Java Programming: From Problem Analysis to Program Design, 4e The for Looping (Repetition) Structure (continued) Example 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. for (i = 1; i <= 5; i++) System.out.println("Hello"); System.out.println("*"); This loop outputs the word Hello five times and the star only once

22Java Programming: From Problem Analysis to Program Design, 4e The for Looping (Repetition) Structure (continued) Does not execute if initial condition is false Update expression changes value of loop control variable, eventually making it false If logical expression is always true, result is an infinite loop Infinite loop can be specified by omitting all three control statements

The for Looping (Repetition) Structure (continued) If logical expression is omitted, it is assumed to be true for statement ending in semicolon is empty Java Programming: From Problem Analysis to Program Design, 4e23

Java Programming: From Problem Analysis to Program Design, 4e24

25Java Programming: From Problem Analysis to Program Design, 4e Programming Example: Classify Numbers Input: N integers (positive, negative, and zeros) int N = 20; //N easily modified Output: number of 0 s, number of even integers, number of odd integers

26Java Programming: From Problem Analysis to Program Design, 4e 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

27Java Programming: From Problem Analysis to Program Design, 4e Programming Example: Classify Numbers (Solution) (continued) The switch statement in Step 3c can also be written as an if... else statement as follows:

28Java Programming: From Problem Analysis to Program Design, 4e The do…while Loop (Repetition) Structure Syntax Statements executed first, and then logical expression evaluated Statement(s) executed at least once and then continued if logical expression is true

29Java Programming: From Problem Analysis to Program Design, 4e do…while Loop (Post-test Loop)

30Java Programming: From Problem Analysis to Program Design, 4e do…while Loop (Post-test Loop)

31Java Programming: From Problem Analysis to Program Design, 4e Choosing the Right Looping Structure All three loops have a place in Java If you know or the program can determine in advance the number of repetitions needed, the for loop is the correct choice If you do not know and the program cannot determine in advance the number of repetitions needed, and it could be zero, the while loop is the right choice If you do not know and the program cannot determine in advance the number of repetitions needed, and it is at least one, the do...while loop is the right choice

32Java Programming: From Problem Analysis to Program Design, 4e 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 exited immediately

33Java Programming: From Problem Analysis to Program Design, 4e 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 evaluated immediately after continue statement In a for structure, the update expression is executed after the continue statement; then the loop condition executes

34Java Programming: From Problem Analysis to Program Design, 4e Nested Control Structure 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

35Java Programming: From Problem Analysis to Program Design, 4e Nested Control Structure (Example) for (i = 1; i <= 5; i++) { for (j = 1; j <= i; j++) System.out.print(" *"); System.out.println(); } Output : * ** *** **** *****

36Java Programming: From Problem Analysis to Program Design, 4e Chapter 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