1. Chapter 3 Control Statements
选择是艰难的
谁能控制自己的命运？
Liang,Introduction to Java Programming,revised by Dai-kaiyu

2. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Objectives
To understand the flow of control in selection and loop statements (§ ).
To use Boolean expressions to control selection statements and loop statements (§ ).
To implement selection control using if and nested if statements (§3.2).
To implement selection control using switch statements (§3.2).
To write expressions using the conditional operator (§3.2).
To use while, do-while, and for loop statements to control the repetition of statements (§3.4).
To write nested loops (§3.4).
To know the similarities and differences of three types of loops (§3.5).
To implement program control with break and continue (§3.6).
Liang,Introduction to Java Programming,revised by Dai-kaiyu

3. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Algorithms
Algorithm
Series of actions in specific order
The actions executed
The order in which actions execute
Program control
Specifying the order in which actions execute
Control structures help specify this order
Liang,Introduction to Java Programming,revised by Dai-kaiyu

4. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Pseudocode
Pseudocode
Informal language for developing algorithms
Not executed on computers
Helps developers “think out” algorithms
Normally describes only executable statements
if student’s grade is greater then or equal to 60
Print " Passed"
if(grade>=60)
System.out.println(“Passed");
Liang,Introduction to Java Programming,revised by Dai-kaiyu

5. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Control Structures
Sequential execution
Program statements execute one after the other
Transfer of control
Three control statements can specify order of statements
Sequence structure (built in Java)
Selection structure
Repetition structure
Flowchart
Graphical representation of algorithm
Flowlines indicate order in which actions execute
Liang,Introduction to Java Programming,revised by Dai-kaiyu

6. Liang,Introduction to Java Programming,revised by Dai-kaiyu
add grade to total
total = total + grade;
add 1 to counter
counter = counter + 1 ;
Flowcharting Java’s sequence structure.
Flowlines
Action Symbols
Connector Symbols
Liang,Introduction to Java Programming,revised by Dai-kaiyu

7. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Decision Symbol
true
grade >= 60
print “Passed”
false
Flowcharting the single-selection if structure.
Liang,Introduction to Java Programming,revised by Dai-kaiyu

8. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Selection Statements
if Statements
switch Statements
Conditional Operators
Liang,Introduction to Java Programming,revised by Dai-kaiyu

9. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Simple if Statements
if (radius >= 0) {
area = radius * radius * PI;
System.out.println("The area" +
“ for the circle of radius "
+ radius + " is " + area); }
if (booleanExpression) {
statement(s); }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

10. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Note
Liang,Introduction to Java Programming,revised by Dai-kaiyu

11. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Caution
Adding a semicolon at the end of an if clause is a common mistake.
if (radius >= 0); {
area = radius*radius*PI;
System.out.println(
"The area for the circle of radius " +
radius + " is " + area); }
logic error.
This error often occurs when you use the next-line block style.
Wrong
Liang,Introduction to Java Programming,revised by Dai-kaiyu

12. Liang,Introduction to Java Programming,revised by Dai-kaiyu
The if...else Statement
Can’t use 0,1
if (booleanExpression) {
statement(s)-for-the-true-case; }
else {
statement(s)-for-the-false-case;
What if there is no else statement, the difference
Liang,Introduction to Java Programming,revised by Dai-kaiyu

13. Liang,Introduction to Java Programming,revised by Dai-kaiyu
if...else Example
if (radius >= 0) {
area = radius * radius * ;
System.out.println("The area for the “
+ “circle of radius " + radius +
" is " + area); }
else {
System.out.println("Negative input");
Liang,Introduction to Java Programming,revised by Dai-kaiyu

14. Multiple Alternative if Statements
better
Liang,Introduction to Java Programming,revised by Dai-kaiyu

15. Trace if-else statement
Suppose score is 70.0
The condition is false
if (score >= 90.0)
grade = 'A';
else if (score >= 80.0)
grade = 'B';
else if (score >= 70.0)
grade = 'C';
else if (score >= 60.0)
grade = 'D';
else
grade = 'F';
Liang,Introduction to Java Programming,revised by Dai-kaiyu

16. Trace if-else statement
Suppose score is 70.0
The condition is false
if (score >= 90.0)
grade = 'A';
else if (score >= 80.0)
grade = 'B';
else if (score >= 70.0)
grade = 'C';
else if (score >= 60.0)
grade = 'D';
else
grade = 'F';
Liang,Introduction to Java Programming,revised by Dai-kaiyu

17. Trace if-else statement
Suppose score is 70.0
The condition is true
if (score >= 90.0)
grade = 'A';
else if (score >= 80.0)
grade = 'B';
else if (score >= 70.0)
grade = 'C';
else if (score >= 60.0)
grade = 'D';
else
grade = 'F';
Liang,Introduction to Java Programming,revised by Dai-kaiyu

18. Trace if-else statement
Suppose score is 70.0
grade is C
if (score >= 90.0)
grade = 'A';
else if (score >= 80.0)
grade = 'B';
else if (score >= 70.0)
grade = 'C';
else if (score >= 60.0)
grade = 'D';
else
grade = 'F';
Liang,Introduction to Java Programming,revised by Dai-kaiyu

19. Trace if-else statement
Suppose score is 70.0
Exit the if statement
if (score >= 90.0)
grade = 'A';
else if (score >= 80.0)
grade = 'B';
else if (score >= 70.0)
grade = 'C';
else if (score >= 60.0)
grade = 'D';
else
grade = 'F';
Liang,Introduction to Java Programming,revised by Dai-kaiyu

20. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Note
The else clause matches the most recent if clause in the same block.
Liang,Introduction to Java Programming,revised by Dai-kaiyu

21. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Note, cont.
To force the else clause to match the first if clause:
int i = 1;
int j = 2;
int k = 3;
if (i > j) {
if (i > k)
System.out.println("A"); }
else
System.out.println("B");
This statement prints B.
Liang,Introduction to Java Programming,revised by Dai-kaiyu

22. Liang,Introduction to Java Programming,revised by Dai-kaiyu
TIP
Liang,Introduction to Java Programming,revised by Dai-kaiyu

23. Liang,Introduction to Java Programming,revised by Dai-kaiyu
CAUTION
better
if (even = true)
statement;
compare
Liang,Introduction to Java Programming,revised by Dai-kaiyu

24. Can we omit this statement?
1 // Fig. 2.20: Comparison.java
2 // Compare integers using if structures, relational operators
3 // and equality operators. 4
5 // Java extension packages
6 import javax.swing.JOptionPane; 7
8 public class Comparison { 9
// main method begins execution of Java application
public static void main( String args[] ) {
String firstNumber; // first string entered by user
String secondNumber; // second string entered by user
String result; // a string containing the output
int number1; // first number to compare
int number2; // second number to compare 18
// read first number from user as a string
firstNumber =
JOptionPane.showInputDialog( "Enter first integer:" ); 22
// read second number from user as a string
secondNumber =
JOptionPane.showInputDialog( "Enter second integer:" ); 26
// convert numbers from type String to type int
number1 = Integer.parseInt( firstNumber );
number2 = Integer.parseInt( secondNumber ); 30
// initialize result to empty String
result = ""; 33
Can we omit this statement?
Liang,Introduction to Java Programming,revised by Dai-kaiyu

25. Liang,Introduction to Java Programming,revised by Dai-kaiyu
if ( number1 == number2 )
result = number1 + " == " + number2; 36
if ( number1 != number2 )
result = number1 + " != " + number2; 39
if ( number1 < number2 )
result = result + "\n" + number1 + " < " + number2; 42
if ( number1 > number2 )
result = result + "\n" + number1 + " > " + number2; 45
if ( number1 <= number2 )
result = result + "\n" + number1 + " <= " + number2; 48
if ( number1 >= number2 )
result = result + "\n" + number1 + " >= " + number2; 51
// Display results
JOptionPane.showMessageDialog(
null, result, "Comparison Results",
JOptionPane.INFORMATION_MESSAGE ); 56
System.exit( 0 ); // terminate application 58
} // end method main 60
61 } // end class Comparison
Test for equality, create new string, assign to result.
Notice use of JOptionPane.INFORMATION_MESSAGE
Liang,Introduction to Java Programming,revised by Dai-kaiyu

26. Liang,Introduction to Java Programming,revised by Dai-kaiyu
lengthy statement split after comma, operator,…,indent
JOptionPane.showMessageDialog(
null, result, “Comparison Results　Ｗ　　ＡＳ　”+
“ASASA DA DSA DAS DAS “,
JOptionPane.INFORMATION_MESSAGE );
In main method, we always do not put such trivial statements.
Liang,Introduction to Java Programming,revised by Dai-kaiyu

27. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Program Output
Liang,Introduction to Java Programming,revised by Dai-kaiyu

28. Example 3.1 Computing Taxes
The US federal personal income tax is calculated based on the filing status and taxable income. There are four filing statuses: single filers, married filing jointly, married filing separately, and head of household. The tax rates for 2002 are shown in Table 3.1.
Liang,Introduction to Java Programming,revised by Dai-kaiyu

29. Example 3.1 Computing Taxes, cont.
if (status == 0) {
// Compute tax for single filers }
else if (status == 1) {
// Compute tax for married file jointly
else if (status == 2) {
// Compute tax for married file separately
else if (status == 3) {
// Compute tax for head of household
else {
// Display wrong status
Computer may think none of the branch will excute. Thus cause errors
Common Error Not initializing before using a variable in method
Compute TaxWithSelectionStatement
Liang,Introduction to Java Programming,revised by Dai-kaiyu

30. Liang,Introduction to Java Programming,revised by Dai-kaiyu
switch Statements
switch (status) {
case 0: compute taxes for single filers;
break;
case 1: compute taxes for married file jointly;
case 2: compute taxes for married file separately;
case 3: compute taxes for head of household;
default: System.out.println("Errors: invalid status");
System.exit(0); }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

31. switch Statement Flow Chart
Liang,Introduction to Java Programming,revised by Dai-kaiyu

32. switch Statement Rules
The switch-expression must yield a value of char, byte, short, or int type and must always be enclosed in parentheses.
switch (switch-expression) {
case value1: statement(s)1;
break;
case value2: statement(s)2; …
case valueN: statement(s)N;
default: statement(s)-for-default; }
The value1, ..., and valueN must have the same data type as the value of the switch-expression. The resulting statements in the case statement are executed when the value in the case statement matches the value of the switch-expression. Note that value1, ..., and valueN are constant expressions, meaning that they cannot contain variables in the expression, such as 1 + x.
Liang,Introduction to Java Programming,revised by Dai-kaiyu

33. switch Statement Rules
The keyword break is optional, but it should be used at the end of each case in order to terminate the remainder of the switch statement. If the break statement is not present, the next case statement will be executed.
switch (switch-expression) {
case value1: statement(s)1;
break;
case value2: statement(s)2; …
case valueN: statement(s)N;
default: statement(s)-for-default; }
The default case, which is optional, can be used to perform actions when none of the specified cases matches the switch-expression.
The case statements are executed in sequential order, but the order of the cases (including the default case) does not matter. However, it is good programming style to follow the logical sequence of the cases and place the default case at the end.
Liang,Introduction to Java Programming,revised by Dai-kaiyu

34. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Switch 语句的落空
下面程序的输出结果是什么
TestSwitch.java
在switch语句中，你通常在每一种case情况后都应使用break语句，否则，第一个相等情况后面所有的语句都会被执行，这种情况叫做落空
TestSwitch1.java
TestSwitch2.java
TestSwitch3.java
Liang,Introduction to Java Programming,revised by Dai-kaiyu

35. Conditional operator (?:)
Java’s only ternary operator-takes three operands(Ternary ;Binary ;Unary )
Grammer
Variable = booleanexpression ? expression1 : expression2
If (booleanexpression)
variable = expression1;
else
variable = expression2;
Liang,Introduction to Java Programming,revised by Dai-kaiyu

36. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Conditional Operator
if (x > 0)
y = 1
else
y = -1;
is equivalent to
y = (x > 0) ? 1 : -1;
(booleanExpression) ? expression1 : expression2
Liang,Introduction to Java Programming,revised by Dai-kaiyu

37. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Conditional Operator
if (num % 2 == 0)
System.out.println(num + “is even”);
else
System.out.println(num + “is odd”);
System.out.println(
(num % 2 == 0)? num + “is even” :
num + “is odd”);
Liang,Introduction to Java Programming,revised by Dai-kaiyu

38. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Repetitions
while Loops
do-while Loops
for Loops
break and continue
Liang,Introduction to Java Programming,revised by Dai-kaiyu

39. Liang,Introduction to Java Programming,revised by Dai-kaiyu
while Loop Flow Chart
int count = 0;
while (count < 100) {
System.out.println("Welcome to Java!");
count++; }
while (loop-continuation-condition) {
// loop-body;
Statement(s); }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

40. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Trace while Loop
Initialize count
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

41. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Trace while Loop, cont.
(count < 2) is true
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

42. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Trace while Loop, cont.
Print Welcome to Java
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

43. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Trace while Loop, cont.
Increase count by 1
count is 1 now
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

44. Trace while Loop, cont. int count = 0; while (count < 2) {
(count < 2) is still true since count is 1
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

45. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Trace while Loop, cont.
Print Welcome to Java
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

46. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Trace while Loop, cont.
Increase count by 1
count is 2 now
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

47. Trace while Loop, cont. int count = 0; while (count < 2) {
(count < 2) is false since count is 2 now
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

48. Trace while Loop int count = 0; while (count < 2) {
The loop exits. Execute the next statement after the loop.
int count = 0;
while (count < 2) {
System.out.println("Welcome to Java!");
count++; }
Avoid infinite loops
Liang,Introduction to Java Programming,revised by Dai-kaiyu

49. Example 3.2: Using while Loops
Problem: Write a program that reads and calculates the sum of an unspecified number of integers. The input 0 signifies the end of the input.
TestWhile
Liang,Introduction to Java Programming,revised by Dai-kaiyu

50. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Caution
Don’t use floating-point values for equality checking in a loop control.
// data should be zero
double data = Math.pow(Math.sqrt(2), 2) - 2;
if (data == 0)
System.out.println("data is zero");
else
System.out.println("data is not zero");
Demo FloatNotPrecise.java
Liang,Introduction to Java Programming,revised by Dai-kaiyu

51. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Formulating Algorithms with Top-Down, Stepwise Refinement (Sentinel-Controlled Repetition)
Sentinel value
Used to indicated the end of data entry
Also called a signal value, flag value
Average2.java has indefinite repetition
Indefinite repetition: the number of repetitions is not known before the loop begins executing.
User enters sentinel value (-1) to end repetition
Liang,Introduction to Java Programming,revised by Dai-kaiyu

52. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Initialize total to zero Initialize counter to zero Input the first grade (possibly the sentinel) While the user has not as yet entered the sentinel Add this grade into the running total Add one to the grade counter Input the next grade (possibly the sentinel) If the counter is not equal to zero Set the average to the total divided by the counter Print the average else Print “No grades were entered”
Pseudocode algorithm that uses sentinel-controlled repetition to solve the class-average problem.
Determine the class average for the quiz
Initialize variables
Input,sum up and count the quiz grades
Calculate and print the class average
Liang,Introduction to Java Programming,revised by Dai-kaiyu

53. Liang,Introduction to Java Programming,revised by Dai-kaiyu
1 // Fig. 4.9: Average2.java
2 // Class average program with sentinel-controlled repetition. 3
4 // Java core packages
5 import java.text.DecimalFormat; 6
7 // Java extension packages
8 import javax.swing.JOptionPane; 9
10 public class Average2 { 11
// main method begins execution of Java application
public static void main( String args[] ) {
int gradeCounter, // number of grades entered
gradeValue, // grade value
total; // sum of grades
double average; // average of all grades
String input; // grade typed by user 20
// Initialization phase
total = 0; // clear total
gradeCounter = 0; // prepare to loop 24
// Processing phase
// prompt for input and read grade from user
input = JOptionPane.showInputDialog(
"Enter Integer Grade, -1 to Quit:" ); 29
// convert grade from a String to an integer
gradeValue = Integer.parseInt( input ); 32
Average2.java
Liang,Introduction to Java Programming,revised by Dai-kaiyu

54. loop until gradeValue equals sentinel value (-1)
while ( gradeValue != -1 ) { 34
// add gradeValue to total
total = total + gradeValue; 37
// add 1 to gradeCounter
gradeCounter = gradeCounter + 1; 40
// prompt for input and read grade from user
input = JOptionPane.showInputDialog(
"Enter Integer Grade, -1 to Quit:" ); 44
// convert grade from a String to an integer
gradeValue = Integer.parseInt( input ); } 48
// Termination phase
DecimalFormat twoDigits = new DecimalFormat( "0.00" ); 51
if ( gradeCounter != 0 ) {
average = (double) total / gradeCounter; 54
// display average of exam grades
JOptionPane.showMessageDialog( null,
"Class average is " + twoDigits.format( average ),
"Class Average", JOptionPane.INFORMATION_MESSAGE ); }
else
JOptionPane.showMessageDialog( null,
"No grades were entered", "Class Average",
JOptionPane.INFORMATION_MESSAGE ); 64
System.exit( 0 ); // terminate application 66
loop until gradeValue equals sentinel value (-1)
Format numbers to nearest hundredth
Return a String
Liang,Introduction to Java Programming,revised by Dai-kaiyu

55. 67 } // end method main 68 69 } // end class Average2
Average2.java
Liang,Introduction to Java Programming,revised by Dai-kaiyu

56. Liang,Introduction to Java Programming,revised by Dai-kaiyu
double x= ; DecimalFormat y = new DecimalFormat("0.000"); System.out.println(y.format( x ));
New operator
Creates an object as the program executes by obtaining enough memory to store an object of the type specified to the right of new
Dynamic memory allocation operator
Object of Class String is instantiated automatically
DecimalFormat objects format number
70.898
70.8970.890
Liang,Introduction to Java Programming,revised by Dai-kaiyu

57. Liang,Introduction to Java Programming,revised by Dai-kaiyu
average = (double) total / gradeCounter;
Explicit conversion
The value stored in total is still an integer
Temporary double version of total created
Promotion(implicit conversion)
Cast operators
Parentheses around the name of a data type
Float-points number are not always 100% precise
Liang,Introduction to Java Programming,revised by Dai-kaiyu

58. Liang,Introduction to Java Programming,revised by Dai-kaiyu
do-while Loop
do {
// Loop body;
Statement(s);
} while (loop-continuation-condition);
Liang,Introduction to Java Programming,revised by Dai-kaiyu

59. Liang,Introduction to Java Programming,revised by Dai-kaiyu
for Loops
Components of a typical for structure header.
Liang,Introduction to Java Programming,revised by Dai-kaiyu

60. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Can be any statement
for Loops
for (initial-action; loop-continuation-condition; action-after-each-iteration) {
// loop body;
Statement(s); }
int i;
for (i = 0; i < 100; i++) {
System.out.println(
"Welcome to Java!"); }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

61. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Trace for Loop
Initialize count
int i;
for (i = 0; i < 2; i++) {
System.out.println(
"Welcome to Java!"); }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

62. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Trace for Loop, cont.
Execute initializer
i is now 0
int i;
for (i = 0; i < 2; i++) {
System.out.println(
"Welcome to Java!"); }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

63. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Trace for Loop, cont.
(i < 2) is true
since i is 0
int i;
for (i = 0; i < 2; i++) {
System.out.println(
"Welcome to Java!"); }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

64. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Trace for Loop, cont.
Print Welcome to Java
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!"); }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

65. Trace for Loop, cont. int i; for (i = 0; i < 2; i++) {
Execute adjustment statement
i now is 1
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!"); }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

66. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Trace for Loop, cont.
(i < 2) is still true
since i is 1
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!"); }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

67. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Trace for Loop, cont.
Print Welcome to Java
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!"); }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

68. Trace for Loop, cont. int i; for (i = 0; i < 2; i++) {
Execute adjustment statement
i now is 2
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!"); }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

69. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Trace for Loop, cont.
(i < 2) is false
since i is 2
int i;
for (i = 0; i < 2; i++) {
System.out.println("Welcome to Java!"); }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

70. Trace for Loop, cont. int i; for (i= 0; i < 2; i++) {
Exit the loop. Execute the next statement after the loop
int i;
for (i= 0; i < 2; i++) {
System.out.println("Welcome to Java!"); }
What if here is 2
Control variable’s scope: i can’t be used after the body of the structure,if variable i is defined inside the for structure
Three parts of the for structure can be omitted, but the semicolon can’t
Liang,Introduction to Java Programming,revised by Dai-kaiyu

71. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Note
The initial-action in a for loop can be a list of zero or more comma-separated expressions.
The action-after-each-iteration in a for loop can be a list of zero or more comma-separated statements.
for (int i = 1; i < 100; System.out.println(i++));
for (int i = 0, j = 0; (i + j < 10); i++, j++) {
// Do something }
Liang,Introduction to Java Programming,revised by Dai-kaiyu

72. (b) Is recommended to avoid confusion
Note
If the loop-continuation-condition in a for loop is omitted, it is implicitly true.
(b) Is recommended to avoid confusion
Liang,Introduction to Java Programming,revised by Dai-kaiyu

73. Example 3.3 Using for Loops
Problem: Write a program that sums a series that starts with 0.01 and ends with 1.0. The numbers in the series will increment by 0.01, as follows: and so on.
TestSum
Liang,Introduction to Java Programming,revised by Dai-kaiyu

74. Example 3.4 Displaying the Multiplication Table
Problem: Write a program that uses nested for loops to print a multiplication table.
TestMultiplicationTable
Liang,Introduction to Java Programming,revised by Dai-kaiyu

75. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Which Loop to Use?
The three forms of loop statements, while, do-while, and for, are expressively equivalent; that is, you can write a loop in any of these three forms.
Liang,Introduction to Java Programming,revised by Dai-kaiyu

76. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Recommendations
use the one that is most intuitive and comfortable for you.
In general, a for loop may be used if the number of repetitions is known, as, for example, when you need to print a message 100 times.
A while loop may be used if the number of repetitions is not known, as in the case of reading the numbers until the input is 0.
A do-while loop can be used to replace a while loop if the loop body has to be executed before testing the continuation condition.
Liang,Introduction to Java Programming,revised by Dai-kaiyu

77. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Caution
Adding a semicolon at the end of the for clause before the loop body is a common mistake, as shown below:
for (int i=0; i<10; i++); {
System.out.println("i is " + i); }
Logic Error
Liang,Introduction to Java Programming,revised by Dai-kaiyu

78. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Caution, cont.
Similarly, the following loop is also wrong:
int i=0;
while (i < 10); {
System.out.println("i is " + i);
i++; }
In the case of the do loop, the following semicolon is needed to end the loop.
do {
} while (i<10);
Logic Error
Correct
Liang,Introduction to Java Programming,revised by Dai-kaiyu

79. Statements break and continue
break/continue
Alter flow of control
break statement
Causes immediate exit from control structure
Used in while, for, do/while or switch statements
continue statement
Skips remaining statements in loop body
Proceeds to next iteration
Used in while, for or do/while statements
Liang,Introduction to Java Programming,revised by Dai-kaiyu

80. Using the Keywords break and continue
Liang,Introduction to Java Programming,revised by Dai-kaiyu

81. Liang,Introduction to Java Programming,revised by Dai-kaiyu
The continue Keyword
Liang,Introduction to Java Programming,revised by Dai-kaiyu

82. Using break and continue
Examples for using the break and continue keywords:
Example 3.5: TestBreak.java
TestBreak
Example 3.6: TestContinue.java
TestContinue
Liang,Introduction to Java Programming,revised by Dai-kaiyu

83. Labeled break and continue Statements
Every statement in Java can have an optional label
Labeled block
Set of statements enclosed by {}
Preceded by a label
Labeled break statement
Exit from nested control structures
Proceeds to end of specified labeled block
Labeled continue statement
Skips remaining statements in nested-loop body
Proceeds to beginning of specified labeled block
Liang,Introduction to Java Programming,revised by Dai-kaiyu

84. stop is the labeled block
1 // Fig. 5.13: BreakLabelTest.java
2 // Using the break statement with a label 3
4 // Java extension packages
5 import javax.swing.JOptionPane; 6
7 public class BreakLabelTest { 8
// main method begins execution of Java application
public static void main( String args[] ) {
String output = ""; 13
stop: { // labeled block 15
// count 10 rows
for ( int row = 1; row <= 10; row++ ) { 18
// count 5 columns
for ( int column = 1; column <= 5 ; column++ ) { 21
// if row is 5, jump to end of "stop" block
if ( row == 5 )
break stop; // jump to end of stop block 25
output += "* "; 27
} // end inner for structure 29
output += "\n"; 31
} // end outer for structure 33
// the following line is skipped
output += "\nLoops terminated normally";
stop is the labeled block
Loop 10 times
Nested loop 5 times
Exit to line 37 (next slide)
Liang,Introduction to Java Programming,revised by Dai-kaiyu

85. Liang,Introduction to Java Programming,revised by Dai-kaiyu36
} // end labeled block 38
JOptionPane.showMessageDialog(
null, output,"Testing break with a label",
JOptionPane.INFORMATION_MESSAGE ); 42
System.exit( 0 ); // terminate application 44
} // end method main 46
47 } // end class BreakLabelTest
Liang,Introduction to Java Programming,revised by Dai-kaiyu

86. nextRow is the labeled block
1 // Fig. 5.14: ContinueLabelTest.java
2 // Using the continue statement with a label 3
4 // Java extension packages
5 import javax.swing.JOptionPane; 6
7 public class ContinueLabelTest { 8
// main method begins execution of Java application
public static void main( String args[] ) {
String output = ""; 13
nextRow: // target label of continue statement 15
// count 5 rows
for ( int row = 1; row <= 5; row++ ) {
output += "\n"; 19
// count 10 columns per row
for ( int column = 1; column <= 10; column++ ) { 22
// if column greater than row, start next row
if ( column > row )
continue nextRow; // next iteration of
// labeled loop 27
output += "* "; 29
} // end inner for structure 31
} // end outer for structure 33
nextRow is the labeled block
Loop 5 times
Nested loop 10 times
continue to line 14 (nextRow)
Liang,Introduction to Java Programming,revised by Dai-kaiyu

87. Liang,Introduction to Java Programming,revised by Dai-kaiyu
JOptionPane.showMessageDialog(
null, output,"Testing continue with a label",
JOptionPane.INFORMATION_MESSAGE ); 37
System.exit( 0 ); // terminate application 39
} // end method main 41
42 } // end class ContinueLabelTest
Liang,Introduction to Java Programming,revised by Dai-kaiyu

88. Example 3.7 Finding the Greatest Common Divisor
Problem: Write a program that prompts the user to enter two positive integers and finds their greatest common divisor.
Solution: Suppose you enter two integers 4 and 2, their greatest common divisor is 2. Suppose you enter two integers 16 and 24, their greatest common divisor is 8. So, how do you find the greatest common divisor? Let the two input integers be n1 and n2. You know number 1 is a common divisor, but it may not be the greatest commons divisor. So you can check whether k (for k = 2, 3, 4, and so on) is a common divisor for n1 and n2, until k is greater than n1 or n2.
GreatestCommonDivisor
Run
Liang,Introduction to Java Programming,revised by Dai-kaiyu

89. Example 3.8 Finding the Sales Amount
Problem: You have just started a sales job in a department store. Your pay consists of a base salary and a commission. The base salary is $5,000. The scheme shown below is used to determine the commission rate.
Sales Amount Commission Rate
$0.01–$5, percent
$5,000.01–$10, percent
$10, and above 12 percent
Your goal is to earn $30,000 in a year. Write a program that will find out the minimum amount of sales you have to generate in order to make $25,000.
FindSalesAmount
Run
Liang,Introduction to Java Programming,revised by Dai-kaiyu

90. Example 3.9 Displaying a Pyramid of Numbers
Problem: Write a program that prompts the user to enter an integer from 1 to 15 and displays a pyramid. For example, if the input integer is 12, the output is shown below.
Three parts
(numberOfLines-row)*3
PrintPyramid
Run
Liang,Introduction to Java Programming,revised by Dai-kaiyu

91. Example 3.10 Displaying Prime Numbers
Problem: Write a program that displays the first 50 prime numbers in five lines, each of which contains 10 numbers. An integer greater than 1 is prime if its only positive divisor is 1 or itself. For example, 2, 3, 5, and 7 are prime numbers, but 4, 6, 8, and 9 are not.
Solution: The problem can be broken into the following tasks:
For number = 2, 3, 4, 5, 6, ..., test whether the number is prime.
Determine whether a given number is prime.
Count the prime numbers.
Print each prime number, and print 10 numbers per line.
PrimeNumber
Run
Liang,Introduction to Java Programming,revised by Dai-kaiyu

92. Liang,Introduction to Java Programming,revised by Dai-kaiyu
QUIZ
Identify and correct the errors in each of the following.
if ( age >= 65 );
System.out.println( "Age greater than or equal to 65" );
else
System.out.println( "Age is less than 65 )";
b. int x = 1, total; while ( x <= 10 ) {
total += x; ++x; }
While ( x <= 100 )
total += x; ++x;
while ( y > 0 ) { System.out.println( y ); ++y;
Liang,Introduction to Java Programming,revised by Dai-kaiyu

93. Liang,Introduction to Java Programming,revised by Dai-kaiyu
a.Semicolon at the end of the if condition should be removed. The closing double quote of the second System.out.println should be inside of the closing parenthesis.
Answer
if ( age >= 65 );
System.out.println( "Age greater than or equal to 65" );
else
System.out.println( "Age is less than 65 )";
b. int x = 1, total; while ( x <= 10 ) {
total += x; ++x; }
c. While ( x <= 100 )
total += x; ++x;
d. int y = 5;
while ( y > 0 ) { System.out.println( y ); ++y;
b. The variable total should be initialized to zero.
c. The W in While should be lowercase. The two statements should be enclosed in curly braces to properly group them into the body of the while; otherwise the loop will be an infinite loop
d. The ++ operator should be changed to --. The closing curly brace for the while loop is missing.
Liang,Introduction to Java Programming,revised by Dai-kaiyu

94. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Find the error in each of the following. [Note: There may be more than one error.]
For ( x = 100, x >= 1, x++ )   System.out.println( x );
The following code should print whether integer value is odd or even: switch ( value % 2 ) {   case 0:     System.out.println( "Even integer" );   case 1:     System.out.println( "Odd integer" ); }
The following code should output the odd integers from 19 to 1: for ( x = 19; x >= 1; x += 2 )   System.out.println( x );
The following code should output the even integers from 2 to 100: counter = 2; do {   System.out.println( counter );   counter += 2; } While ( counter < 100 );
Liang,Introduction to Java Programming,revised by Dai-kaiyu

95. Liang,Introduction to Java Programming,revised by Dai-kaiyu
Find the error in each of the following. [Note: There may be more than one error.]
For ( x = 100, x >= 1, x++ )   System.out.println( x );
The following code should print whether integer value is odd or even: switch ( value % 2 ) {   case 0:     System.out.println( "Even integer" );   case 1:     System.out.println( "Odd integer" ); }
The following code should output the odd integers from 19 to 1: for ( x = 19; x >= 1; x += 2 )   System.out.println( x );
The following code should output the even integers from 2 to 100: counter = 2; do {   System.out.println( counter );   counter += 2; } While ( counter < 100 );
The F in for should be lowercase. Semicolons should be used in the for header instead of commas. ++ should be --
A break statement should be placed in case 0
+= should be -=
The W in While should be lowercase. < should be <=
Liang,Introduction to Java Programming,revised by Dai-kaiyu