Control structures i

Chapter 4: Control
Structures I
JAVA PROGRAMMING:
FROM PROBLEM ANALYSIS TO PROGRAM DESIGN,
SECOND EDITION

2
Chapter Objectives
 Learn about control structures.
 Examine relational and logical operators.
 Explore how to form and evaluate logical
(Boolean) expressions.
 Learn how to use the selection control
structures if, if…else, and switch in
a program.

3
Control Structures
 Three methods of processing a program:
 In sequence
 Branching
 Looping
 Branch: Altering the flow of program
execution by making a selection or choice.
 Loop: Altering the flow of program
execution by repeating statements.

5
Relational Operators
 Relational operator:
 Allows you to make comparisons in a program.
 Binary operator.
 Condition is represented by a logical
expression in Java.
 Logical expression: An expression that has
a value of either true or false.

7
Relational Operators and
Primitive Data Types
 Can be used with integral and floating-point
data types.
 Can be used with the char data type.
 Unicode Collating Sequence.

8
Relational Operators and Primitive Data Types

9
Comparing Strings
 class String
 Method compareTo
 Method equals
 Given string str1 and str2





=
>>
<<
str2str10
str2str10
str2str10
reTo(str2)str1.compa
stringifintegeran
stringtoequalisstringif
stringifintegeran

10
Comparing Strings
String str1 = "Hello";
String str2 = "Hi";
String str3 = "Air";
String str4 = "Bill";
String str5 = "Bigger";

15
Short-Circuit Evaluation
 A process in which the computer evaluates
a logical expression from left to right and
stops as soon as the value of the expression
is known.

16
Selection
 One-way selection
 Two-way selection
 Compound (block of) statements
 Multiple selections (nested if)
 Conditional operator
 switch structures

17
One-Way Selection
 Syntax:
if (expression)
statement
 Expression referred to as decision maker.
 Statement referred to as action statement.

18
Example 4-11
//Determine the absolute value of an integer
import javax.swing.JOptionPane;
public class AbsoluteValue
{
public static void main(String[] args)
{
int number;
int temp;
String numString;
numString =
JOptionPane.showInputDialog
("Enter an integer:"); //Line 1
number = Integer.parseInt(numString); //Line 2
temp = number; //Line 3
One-Way Selection

19
if (number < 0) //Line 4
number = -number; //Line 5
JOptionPane.showMessageDialog(null,
"The absolute value of " + temp
+ " is " + number,
"Absolute Value",
JOptionPane.INFORMATION_MESSAGE); //Line 6
System.exit(0);
}
One-Way Selection

20
Two-Way Selection
 Syntax:
if (expression)
statement1
else
statement2
 else statement must be paired with an if.

22
Two-Way Selection
Example 4-14
if (hours > 40.0)
wages = 40.0 * rate +
1.5 * rate * (hours - 40.0);
else
wages = hours * rate;

23
Example 4-15
if (hours > 40.0); //Line 1
wages = 40.0 * rate +
1.5 * rate * (hours - 40.0); //Line 2
else //Line 3
wages = hours * rate; //Line 4
Because a semicolon follows the closing parenthesis of the if
statement (Line 1), the else statement stands alone. The
semicolon at the end of the if statement (see Line 1) ends the
if statement, so the statement at Line 2 separates the else
clause from the if statement. That is, else is by itself.
Because there is no separate else statement in Java, this code
generates a syntax error.
Two-Way Selection

24
Compound (Block of) Statements
Syntax:
{
statement1
statement2
.
.
.
statementn
}

25
Compound (Block of) Statements
if (age > 18)
{
System.out.println("Eligible to vote.");
System.out.println("No longer a minor.");
}
else
{
System.out.println("Not eligible to vote.");
System.out.println("Still a minor.");
}

26
Multiple Selection: Nested if
 Syntax:
if (expression1)
statement1
else
if (expression2)
statement2
else
statement3
 Else is associated with the
most recent incomplete if.
 Multiple if statements can
be used in place of if…
else statements.
 May take longer to
evaluate.

27
Conditional (? :) Operator
 Ternary operator
 Syntax:
expression1 ? expression2 :
expression3
 If expression1 = true, then the result of the
condition is expression2.
Otherwise, the result of the condition is
expression3.

28
switch Structures
 Expression is also
known as selector.
 Expression can be an
identifier.
 Value can only be
integral.
switch (expression)
{
case value1: statements1
break;
case value2: statements2
break;
...
case valuen: statementsn
break;
default: statements
}

30
Example 4-24
switch (grade)
{
case 'A': System.out.println("The grade is A.");
break;
case 'B': System.out.println("The grade is B.");
break;
case 'C': System.out.println("The grade is C.");
break;
case 'D': System.out.println("The grade is D.");
break;
case 'F': System.out.println("The grade is F.");
break;
default: System.out.println("The grade is
invalid.");
}
switch Structures

31
Programming Example:
Cable Company Billing
 Input: Customer’s account number,
customer code, number of premium
channels to which customer subscribes,
number of basic service connections (in the
case of business customers).
 Output: Customer’s account number and the
billing amount.

32
Programming Example:
Cable Company Billing
Solution:
1. Prompt user for information.
2. Use switch statements based on customer’s
type.
3. Use an if statement nested within a switch
statement to determine the amount due by
each customer.

33
Chapter Summary
 Control structures are used to process programs.
 Logical expressions and order of precedence of
operators are used in expressions.
 Compare strings.
 If statements.
 if…else statements.
 switch structures.
 Proper syntax for using control statements.

Control structures i

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