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Java Programming: From Problem Analysis to Program Design, Second Edition 1
Chapter 4: Selection Control
Structures
Relational and logical operators.
Logical (Boolean) expressions.
The selection control structures if,
if…else, and switch.
Java Programming: From Problem Analysis to Program Design, Second Edition 2
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 3
Control Structures
Java Programming: From Problem Analysis to Program Design, Second Edition 4
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 7
Relational Operators and Primitive Data Types
Java Programming: From Problem Analysis to Program Design, Second Edition 8
Comparing Strings
class String
Method compareTo
Method equals
Given string str1 and str
< <
0 str1 str
0 str1 str
0 str1 str str1.compareTo(str2) aninteger if string
ifstring isequaltostring
aninteger ifstring
Java Programming: From Problem Analysis to Program Design, Second Edition 9
Comparing Strings String str1 = "Hello"; String str2 = "Hi"; String str3 = "Air"; String str4 = "Bill"; String str5 = "Bigger";
Java Programming: From Problem Analysis to Program Design, Second Edition 10
Comparing Strings
Java Programming: From Problem Analysis to Program Design, Second Edition 13
Comparing Strings
Java Programming: From Problem Analysis to Program Design, Second Edition 14
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 15
Selection
One-way selection
Two-way selection
Compound (block of) statements
Multiple selections (nested if)
Conditional operator
switch structures
Java Programming: From Problem Analysis to Program Design, Second Edition 16
One-Way Selection
Syntax: if (expression) statement Expression referred to as decision maker. Statement referred to as action statement.
Java Programming: From Problem Analysis to Program Design, Second Edition 19
Two-Way Selection
Syntax:
if (expression)
statement
else
statement
else statement must be paired with an if.
Java Programming: From Problem Analysis to Program Design, Second Edition 20
Two-Way Selection
Java Programming: From Problem Analysis to Program Design, Second Edition 21
Two-Way Selection
Example 4-
if (hours > 40.0) wages = 40.0 * rate + 1.5 * rate * (hours - 40.0); else wages = hours * rate;
Java Programming: From Problem Analysis to Program Design, Second Edition 22
Example 4-
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
Java Programming: From Problem Analysis to Program Design, Second Edition 25
Conditional (? :) Operator
Ternary operator
Syntax:
expression1? expression2 : expression
If expression1 = true, then the result of the
condition is expression2.
Otherwise, the result of the condition is
expression3.
Java Programming: From Problem Analysis to Program Design, Second Edition 26
Multiple Selection: Nested if
Syntax:
if (expression1) statement else if (expression2) statement else statement
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 27
switch Structures
Expression is also
known as selector.
Expression can be an
identifier.
Value can only be
integral.
switch (expression) { case value1: statements break; case value2: statements break; ... case valuen: statementsn break; default: statements }
Java Programming: From Problem Analysis to Program Design, Second Edition 28
switch Structures
Java Programming: From Problem Analysis to Program Design, Second Edition 31
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.
Java Programming: From Problem Analysis to Program Design, Second Edition 32
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.
