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Control Structures

Chapters 4, 5, & 6 Overview

Today’s Lesson Plan

• Discuss labs, quizzes, and TDA topics (10 min)
  • Discussion posts should cite sources
• Class Activities – covers Chapter 4, 5, & 6 (120 min)
  • Discuss the 7 Java control Structures. Start with the sequence

structure. (30 min)

• Discuss the three selection structures – single selection = if; double

selection = if/else; multiple selection = switch. (30 min)

• Discuss the three repetition structures – while ; do/while; for. (

min)

• Discuss data manipulation, validation and formatting. (30 min)
• Coding Exercises & break (90 min)
• Code Ex 1 – p 249 – # 5.2.8 (a & b)
• Code Ex 2 – p 323 – # 6.4.
• Code Ex 3 – p 225 – # 4.5.

Structured-Programming Summary

• All programs broken down into

– Sequence

– Selection

• if , if/else , or switch
  • Any selection can be rewritten as an if statement

– Repetition

• while , do/while , or for
  • Any repetition structure can be rewritten as a while statement

Flow of Control

• the order in which program

statements are executed

WHAT ARE THE POSSIBILITIES...

Flow of Control

• is Sequential unless a “control structure” is used to change

that

• there are 2 general types of control structures that change

a sequential flow:

Selection (also called branching)

Repetition (also called looping)

Java control structures

• Selection

if

if... else

switch

• Repetition

for loop

while loop

do... while loop

Selection Statements

• if-then-else statement: implements a

decision structure for two alternatives

Syntax:

if (condition) statement executed if condition is true; else statement executed if condition is false;

The if-then-else Statement

 if-then-else performs instructions based on

the result of a comparison  Place statements on separate lines for readability

Syntax:

if-then-else provides two-way

selection

between executing one of 2 clauses (the if

clause or the else clause)

TRUE (^) FALSE

if clause else clause

condition

If-Then-Else Syntax

if ( Condition )

StatementA

else

StatementB

NOTE: StatementA and StatementB each can be a

single statement, a null statement, or a block.

Use of blocks is recommended

if ( Condition ) {

} else {

}

“if clause”

“else clause”

The if-then-else Flowchart

The if-

then-else

flowchart.

Example (Multiple Lines)

int carDoors, driverAge ; float premium, monthlyPayment ;

... if ( (carDoors == 4 ) && (driverAge > 24) ) { premium = 650.00 ; System.out.println(“ LOW RISK “ ); } else { premium = 1200.00 ; System.out.println(“ HIGH RISK ”) ; }

monthlyPayment = premium / 12.0 + 5.00 ;

What happens if you omit braces?

if ( (carDoors == 4 ) && (driverAge > 24) ) premium = 650.00 ; System.out.println(“ LOW RISK “) ; else premium = 1200.00 ; System.out.println(“ HIGH RISK ”) ;

monthlyPayment = premium / 12.0 + 5.00 ;

COMPILE ERROR OCCURS. The “if clause” is the single statement following the if.

Braces can only be omitted when

each clause is a single statement

if ( lastInitial <= ‘K’ )

volume = 1;

else

volume = 2;

System.out.println(“Look it up in volume # “

• volume + “ of NYC phone book”);

The Selection Condition

• The condition is evaluated to its boolean

value:

– Can ONLY use boolean values

• The else portion is optional

– It is executed only if the condition is false

• The condition may be any valid Java

expression that has a boolean value

Relational Expressions

• Relational expression: compares two

operands or expressions using relational

operators

– Evaluates to true or false

Logical Operators

 Logical operators

 AND ( && ): condition is true only if both expressions are true  OR ( || ): condition is true if either one or both of the expressions is true  NOT (! ): changes an expression to its opposite state; true becomes false, false becomes true

Logical Expressions

LOGICAL

EXPRESSION MEANING DESCRIPTION

! p NOT p! p is false if p is true ! p is true if p is false

p && q p AND q p && q is true if both p and q are true. It is false otherwise.

p || q p OR q p || q is true if either p or q or both are true. It is false otherwise.

Caveats! (obscure)

 Comparing single and double precision values for

equality ( == ) can lead to errors because values are

stored in binary

 Reason is that numbers like 0.1 cannot be

represented exactly in binary

 Thus, testing for exact equality of such numbers

can fail

 Instead, test that the absolute value of the

difference is within an acceptable range, epsilon (ε)

 Example:

abs(operandOne – operandTwo)<0.

What is the value?

int age ;

boolean isSenior, hasFever ;

float temperature ;

age = 20;

temperature = 102.0 ;

isSenior = (age >= 55) ; // isSenior is false

hasFever = (temperature > 98.6) ; // hasFever is true

EXPRESSION VALUE

isSenior && hasFever?

isSenior || hasFever?

! isSenior?

! hasFever?

The value is…?

int age, height;

age = 25;

height = 70;

EXPRESSION VALUE

! (age < 10)?

! (height > 60)?

Write an expression for each

taxRate is over 25% and income is less than $20000

temperature is less than or equal to 75 or humidity is less than

70%

age is over 21 and age is less than 60

age is 21 or 22

Some Answers

(taxRate > .25) && (income < 20000)

(temperature <= 75) || (humidity < .70)

(age > 21) && (age < 60)

(age == 21) || (age == 22)

WARNING about Expressions in Java

• “Boolean expression” means an expression whose value is

true or false

• an expression is any valid combination of operators and

operands

• each expression has a value
• this can lead to UNEXPECTED RESULTS
• construct your expressions CAREFULLY
• use of parentheses is encouraged
• otherwise, use precedence chart to determine order