1. Repetition Chapter 6 12/06/16 & 12/07/16 1 1
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 1 1

2. Chapter Contents The Logic of a Loop The while Statement
A Problem Solved: A Guessing Game
Errors in Loops
Off-by-One Errors
Infinite Loops
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 2 2

3. The Logic of a Loop Iteration  repeat a group of statements
Use a controlling statement
Figure 12-1, Steps of a typical loop
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 3 3

4. The while Statement Controls the execution of a while loop Syntax
Logic of a while statement
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 4 4

5. The while Statement
Consider – exactly what numbers are printed by this example?
Note often used style of incrementing counter = counter + 1; // and counter++; // have same result
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 5 5

6. The while Statement Counting loops Sentinel controlled loops
Loop must run specific number of times
Variable incremented/decremented required number of times
Sentinel controlled loops
Task must be repeated until a certain condition is true
That condition is the sentinel
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 6 6

7. A Guessing Game One person (computer) thinks of a number
Other person tries to guess the number
First person responds
“too high”
“too low”
“that’s it”
Process repeats until guess is correct
We seek a Java program to play the game
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 7 7

8. A Guessing Game Pseudocode solution Let's write the code. 8 8
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 8 8

9. Off by One Errors Common mistake in loop design
Incorrect condition is tested
Possible misuse of ≤ vs <
Incorrect initial value
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 9 9

10. Off by One Errors Placement of the increment statement
//This main() is suppose to count by 10 from 10 to 100
public static void main(String[] args) {
int n = 10;
while(n <= 100){
n = n + 10;
System.out.println(n); }
See: MisplacedInc.java
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 10 10

11. public class ErrorMisplaceInc {
//This program is suppose to count by 10 from 10 to 100
//It still has problems.
public static void main(String[] args) {
int n = 10;
while(n != 99) {
System.out.println(n);
n = n + 10; } 11

12. Infinite Loops More mistakes in loop design
Again, incorrect condition tested
Check for equality that can never happen
Checking for equality with reals
Should check for “closeness”
Missing increment/decrement statement
Extra semicolon
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 12 12

13. Question: What is the ouput?
int num =0, max = 20;
while (num < max){
System.out.println(num);
num = num + 4; } 13 13

14. Participation
Imagine a program that read the population of a city using the following statements:
System.out.println(“Enter the population:”);
Int population = scan.nextInt();
Write a while loop after these statements that ensures the population is positive. If the user enters a population that is either negative or zero, ask the user to enter a non-negative value. Keep asking until a non-negative is entered.
Echo output the number that is finally stored. 14 14

15. Strings charAt(n) – returns char at position n. Line.charAt(4) →'g'

16. Participation
Work with a partner. Write a program that inputs a word. Use a while loop and the charAt() method to output the word spelled backwards.
Sample execution:
Input:desserts
Output:stressed 16 16

17. Estimation of Square Root
(Skip)
Can not discuss finding roots as the text does because it requires calculus.
Can use an algorithm for finding the square root of a number that is based on the same principle.
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 17 17

18. Estimation of Square Start with an estimate.
Get another estimate based on that estimate.
Repeat this until the two estimates are within the require error tolerance.
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 18 18

19. Estimation of Square Input s, tolerance
x = s //First estimate for x is the input
newX = ½(x+ s/x) // Second estimate
While( |newX – x| < tolerance)
X = newX //Keep new est.
newX = ½(x+ s/x) //Find next est.
5)Output x “is the sqrt of “ s
Try it with 7.
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 19 19

20. Nested Loops(Skip)
Consider code to generate a two dimensional pattern of asterisks
Nested loop solution
Inner loop displays each star in a row
Outer loop drives the creation of the rows
View code to create pattern of stars, Listing 12-2
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21. Scope of a Variable(skip)
Scope  portion of program where variable can be used
Figure Scope of Variables
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22. Repetition
Chapter 6
Imagine! Java: Programming Concepts in Context by Frank M. Carrano, (c) Pearson Education - Prentice Hall, 2010 22 22