Flow of Control: Branching - Computer Science and Programming | IT 111, Lab Reports of Information Technology

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L E C T U R E 1 0

New Mexico Tech: CSE/IT 111

Flow of Control:

Branching

Class Maintenance

y New Mexico Tech: CSE/IT 111

Lab 2 will be posted by tomorrow morning

No prelab

y

HW 2 will be posted by Wednesday

Due: 2/18 (Wed.)

Flow of Control

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Flow of control

The order in which a program performs actions

y

Control structures

are used to regulate flow of control

Branching statement (Chapter 3)^ Ù

A statement that chooses one action from a list of two or more possibleactions Ù

Also called

decision structures

Loop statement (Chapter 4)^ Ù

A statement that repeats an action again and again until some stoppingcondition is met

T H E B A S I C

i f - e l s e

S T A T E M E N T

B O O L E A N E X P R E S S I O N S

C O M P A R I N G S T R I N G S

N E S T E D

i f - e l s e

S T A T E M E N T S

M U L T I B R A N C H

i f - e l s e

S T A T E M E N T S

T H E C O N D I T I O N A L O P E R A T O R

T H E

e x i t

M E T H O D

New Mexico Tech: CSE/IT 111

The

if-else

Statement

Putting together the

if-else

Statement

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Boolean expression

An expression composed of variables, constants, boolean operators, and/orcomparison operators whose value is either

true

or

false

Ù

Based on Boolean algebra developed by George Boole (

th

century)

Ù

Example:

balance

y

Compound statement

One or more programming-language statements enclosed in a pair of braces andoften used as a part of a branching statement or a loop statement^ Ù

Allows us to include more than one statement (enclosed in

) in each branch

if (balance >= 0) {

System.out.println("Good for you. You earned interest."); balance = balance + (INTEREST_RATE * balance) / 12; } else {

System.out.println("You will be charged a

penalty.");

balance = balance - OVERDRAWN_PENALTY; }

Semantics of the

if-else

Statement

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The basic

if-else

statement

Syntax^ Ù

if

(

Boolean_Expression

)

Statement_

else

Statement_

If the expression Boolean_Expression istrue, Statement_1 is executed; otherwise, Statement_2 is executed

y

An

if

statement without an

else

Program

skips

Statement_

when the

if

statement’s expression is

false In other words:

the statement either performs an action or

does not

Evaluate

Boolean_Expression

Execute

Statement_

Execute

Statement_

Start

False

True

Boolean Expressions: Logical Operators

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Logical operator

One of the operators

&&

,^

||

, and

!

used to

combine boolean expressions

to

form a larger boolean expression^ Ù

Sub_Expression_

Sub_Expression_

True

if and only if both

Sub_Expression_

and

Sub_Expression_

are true

Ù

Sub_Expression_

Sub_Expression_

True

if either

Sub_Expression_

or

Sub_Expression_

is true

or both

are true

Ù

Boolean_Expression

True if

Boolean_Expression

is

false

Name

Java Notation

Java Examples

Logical

and

(sum > min) && (sum < max)

Logical

or

(answer == ‘y’) || (answer == ‘Y’)

Logical

not

!(number < 0)

Comparing Strings: Equality

New Mexico Tech: CSE/IT 111 y

Equality operator

The operator

==

is used to

compare two primitive values(such as two numbers) Has a different meaning whenapplied to objects (such asstrings)^ Ù

Tests whether the objects arestored in the

same memory

location

y

Instead, use the method equals

to compare strings

Returns true if strings have equal values^ String

.equals(

Other_String

)

if

(

s1.equals(s

))

System.out.println(“Equal"); else

System.out.println(“Not equal.");

y

Can also use the method equalsIgnoreCase

String

.equalsIgnoreCase(

Other_String

)

if

(

“Hello”.equalsIgnoreCase(“hello”

))

System.out.println(“Equal");

Nested

if-else

Statements

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Nest

: to place a statement within a statement of the same kind if (balance >= 0) {

if (INTEREST_RATE >= 0) {

balance = balance + (INTEREST_RATE * balance) / 12;

} else {

System.out.println(“Cannot have a negative interest.");

else {

balance = balance - OVERDRAWN_PENALTY; }

y

Each

else

is paired with the nearest preceding unmatched

if

To clarify your intent, use indentation that is consistent with the meaning of the statement^ Ù

REMEMBER: compiler ignores indentation

Use braces to make the statement’s meaning EXPLICIT

Multibranch

if-else

Statements

New Mexico Tech: CSE/IT 111

if (balance >= 0) {

System.out.println(“Positive”);

} else {

if (balance < 0) {

System.out.println(“Negative”);

} else {

if (balance == 0) {

System.out.println(“Zero”);

} }

y

Multibranch

if-else

statement

A style of nested

if-else

statements that

provide a choice among several actions^ if (balance >= 0)^ {

System.out.println(“Positive”);

} else if (balance < 0) {

System.out.println(“Negative”);

} else

// Default action

System.out.println(“Zero”);

} At execution, tested in order from top Ù

First true condition is executed; subsequentconditions ignored Ù

Important if conditions are not

mutually

exclusive

New Mexico Tech: CSE/IT 111

The

switch

Statement

The

switch

Statement: Syntax

New Mexico Tech: CSE/IT 111 y

Switch statement

A Java statement that provides a choice of severaloutcomes according to the value of a

controlling

expression

y

Controlling expression

An integer or character expression whose value isused to determine the outcome of a

switch

statement

y

Case label

A value associated with the keyword case within a switch

statement

that is a possible value of the

controlling expression

y

Break statement

A statement used either to indicate the end of acase within a

switch

statement

or to end the

iteration within a loop

y

Default case

An optional case within a

switch

statement

that

indicates the action taken when the value of the controlling expression

does not match any of

the

case labels

switch

Controlling_Expression

case

Case_Label

Statement

... Statement

break

case

Case_Label

Statement

... Statement

break

default

Statement

... Statement

break

The

break

Statement

New Mexico Tech: CSE/IT 111

y

When execution reaches a

break

statement, the

switch

statement’s

execution ends

y

If a case has no

break

statement,

execution continues on with the nextcase until a

break

statement is

encountered or the

switch

statement

ends

Will execute statements in followingcases, even if the value of thecontrolling expression does notmatch the case

y

Omitting a

break

statement can be

useful when you have multiple casesthat produce the same case action

switch

(numberOfBabies)

{

case

1:

System.out.println("Congrats."); break

;

case

2:

System.out.println("Wow.

Twins.");

break

;

/*

If

we

omit

the

previous

break,

---

any

time

the

statements

in

case

2

---

are

executed,

the

statements

in

---

case

3

will

also

be

executed.

*/

case

3:

System.out.println("Wow.

Triplets.");

break

;

case

4:

//

case

with

no

break

statement

case

5:

//

cases

4

&

5

produce

same

action

System.out.print("Unbelievable;

");

System.out.println(numberOfBabies

“

babies.");

break

;

default

:

//

executes

when

no

case

matches

System.out.println("I

don't

believe

you.");

break

;

}

Next Time

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Java

: Ch. 3.

y

CSI

: Continue Ch. 3