8/10/2019 Chapter 04 c++

1/42

INTRODUCTION TOC++

Chapter 04: Making Decision

1

8/10/2019 Chapter 04 c++

2/42

Topics

4.1 Relational Operators

4.2 The ifStatement

4.3 The if/elseStatement

4.4 The if/elseifStatement

4.5 Menu-Driven Programs

4.6 Logical Operators4.7 Validating User Input4.8 More about Variable Definitions and Scope4.9 Comparing Characters and Strings4.10 The Conditional Operator4.11 The switchStatement

4-2

8/10/2019 Chapter 04 c++

3/42

4.1 Relational Operators

Used to compare numbers to determine

relative order

Operators:

4-3

> Greater than

< Less than

>= Greater than or equal to

8/10/2019 Chapter 04 c++

4/42

Relational Expressions

Relational expressions are Boolean (i.e.,evaluate to trueor false)

Examples:12 > 5is true

7

8/10/2019 Chapter 04 c++

5/42

Relational Expressions

Can be assigned to a variable

boolresult = (x

8/10/2019 Chapter 04 c++

6/42

4.2 The ifStatement

Allows statements to be conditionally

executed or skipped over

Models the way we mentally evaluate

situations

If it is cold outside,

wear a coat and wear a hat.

4-6

8/10/2019 Chapter 04 c++

7/42

Format of the ifStatement

if (condition){

statement1;statement2;

statementn;

}The block inside the braces is called the bodyof the ifstatement. If there is only 1 statementin the body, the {}may be omitted.

4-7

No ;

goes here

;goes here

8/10/2019 Chapter 04 c++

8/42

How the ifStatement Works

If (condition)is true, then thestatement(s)in the body areexecuted.

If (condition)is false, then thestatement(s)are skipped.

4-8

8/10/2019 Chapter 04 c++

9/42

ifStatement Flow of Control

4-9

condition

1 or more

statements

true false

8/10/2019 Chapter 04 c++

10/42

Example ifStatements

if (score >= 60)

cout = 90)

{

grade = 'A';

cout

8/10/2019 Chapter 04 c++

11/42

ifStatement Notes

Do not place ;after (condition)

Don't forget the { } around a multi-statement

body

Place each statement;on a separate lineafter (condition), indented

0 is false;any other value is true

4-11

8/10/2019 Chapter 04 c++

12/42

What is trueand false?

An expression whose value is 0 is consideredfalse.

An expression whose value is non-zero isconsidered true.

An expression need not be a comparisonit

can be a single variable or a mathematical

expression.

4-12

8/10/2019 Chapter 04 c++

13/42

Flag

A variable that signals a condition

Usually implemented as abool

Meaning:

true: the condition exists

false: the condition does not exist

The flag value can be both set and tested withifstatements

4-13

8/10/2019 Chapter 04 c++

14/42

Flag Example

Example:bool validMonths = true;

if (months < 0)validMonths = false;

if (validMonths)

moPayment = total / months;

4-14

8/10/2019 Chapter 04 c++

15/42

Comparisons with floating-point

numbers

It is difficult to test for equality when working

with floating point numbers.

It is better to use greater than, less than tests, or test to see if value is very close to a given value

4-15

8/10/2019 Chapter 04 c++

16/42

4.3 The if/elseStatement

Allows a choice between statementsdepending on whether (condition)istrueor false

Format: if (condition){

statement set 1;}

else{statement set 2;

}

4-16

8/10/2019 Chapter 04 c++

17/42

How the if/elseWorks

If (condition)is true, statementset1is executed and statementset2is skipped.

If (condition)is false, statementset1is skipped and statementset2is executed.

4-17

8/10/2019 Chapter 04 c++

18/42

if/elseFlow of Control

4-18

condition

statement

set 1

true false

statement

set 2

8/10/2019 Chapter 04 c++

19/42

Example if/elseStatements

if (score >= 60)cout

8/10/2019 Chapter 04 c++

20/42

4.4 The if/elseifStatement

Chain of ifstatements that test in orderuntil one is found to be true

Also models thought processes

If it is raining, take an umbrella,

else, if it is windy, take a hat,

else, if it is sunny, take sunglasses.

4-20

8/10/2019 Chapter 04 c++

21/42

if/elseifFormat

if (condition 1)

{ statement set 1;

}

else if (condition 2)

{ statement set 2;

}

else if (condition n)

{ statement set n;

}

4-21

8/10/2019 Chapter 04 c++

22/42

Using a Trailing else

Used with if/elseifstatement when all ofthe conditions are false

Provides a default statement or action Can be used to catch invalid values or handle

other exceptional situations

4-22

8/10/2019 Chapter 04 c++

23/42

8/10/2019 Chapter 04 c++

24/42

4.5 Menu-Driven Program

Menu: list of choices presented to the user on

the computer screen

Menu-driven program: program executioncontrolled by user selecting from a list of

actions

Menu can be implemented using if/elseif

statements

4-24

8/10/2019 Chapter 04 c++

25/42

Menu-driven Program

Organization

Display list of numbered or lettered choices for

actions.

Input users selection of number or letter

Test user selection in (condition)

if a match, then execute code to carry out desired

action if not, then test with next (condition)

4-25

8/10/2019 Chapter 04 c++

26/42

4.6 Logical Operators

Used to create relational expressions fromother relational expressions

Operators, Meaning, and Explanation

4-26

&& AND New relational expression is true ifboth expressions are true

|| OR New relational expression is true if

either expression is true

! NOTReverses the value of an expression;true expression becomes false, falseexpression becomes true

8/10/2019 Chapter 04 c++

27/42

Logical Operator Examples

int x = 12, y = 5, z = -4;

(x > y) && (y > z) true

(x > y) && (z > y) false

(x

8/10/2019 Chapter 04 c++

28/42

Logical Precedence

Highest !&&

Lowest ||

Example:(2 < 3) || (5 > 6) && (7 > 8)

is true because AND is evaluated before OR

4-28

8/10/2019 Chapter 04 c++

29/42

8/10/2019 Chapter 04 c++

30/42

8/10/2019 Chapter 04 c++

31/42

4.7 Validating User Input

Input validation: inspecting input data to

determine if it is acceptable

Want to avoid accepting bad input

Can perform various tests

Range

Reasonableness

Valid menu choice Zero as a divisor

4-31

4 8 M Ab t V i bl D fi iti

8/10/2019 Chapter 04 c++

32/42

4.8 More About Variable Definitions

and Scope

Scopeof a variable is the block in which it is

defined, from the point of definition to the end of

the block

Variables are usually defined at beginning of

function

They may instead be defined close to first use

4-32

M Ab t V i bl D fi iti

8/10/2019 Chapter 04 c++

33/42

More About Variable Definitions

and Scope

Variables defined inside {}have localorblock scope

When in a block that is nested inside anotherblock, you can define variables with the samename as in the outer block.

When the program is executing in the innerblock, the outer definition is not available

This is generally not a good idea

4-33

4 9 Comparing Characters and

8/10/2019 Chapter 04 c++

34/42

4.9 Comparing Characters and

Strings

Can use relational operators with characters andstring objects

if (menuChoice == 'A')

if (firstName == "Beth")

Comparing characters is really comparing ASCIIvalues of characters

Comparing string objects is comparing the ASCII

values of the characters in the strings. Comparisonis character-by-character

Cannot compare C-style strings with relationaloperators

4-34

8/10/2019 Chapter 04 c++

35/42

4.10 The Conditional Operator

Can use to create short if/elsestatements

Format: expr ? expr : expr;

4-35

8/10/2019 Chapter 04 c++

36/42

4.11 The switchStatement

Used to select among statements from severalalternatives

May sometimes be used instead of if/elseifstatements

4-36

8/10/2019 Chapter 04 c++

37/42

it h Statement

8/10/2019 Chapter 04 c++

38/42

switchStatement

Requirements

1) IntExpressionmust be a charor an

integer variable or an expression that

evaluates to an integer value

2) exp1through expnmust be constant

integer type expressions and must be uniquein the switchstatement

3) defaultis optional but recommended

4-38

8/10/2019 Chapter 04 c++

39/42

How the switchStatement Works

1) IntExpressionis evaluated

2) The value of intExpressionis compared

against exp1through expn.3) If IntExpressionmatches value expi, the

program branches to the statement(s)following expiand continues to the end of the

switch4) If no matching value is found, the program

branches to the statement after default:

4-39

8/10/2019 Chapter 04 c++

40/42

ThebreakStatement

Used to stop execution in the current block

Also used to exit a switchstatement

Useful to execute a single casestatementwithout executing statements following it

4-40

8/10/2019 Chapter 04 c++

41/42

Example switchStatement

switch (gender) {

case 'f': cout

8/10/2019 Chapter 04 c++

42/42

Using switchwith a Menu

switchstatement is a natural choice for

menu-driven program

display menu

get user input use user input as IntExpressionin switch

statement

use menu choices as expto test against in the

casestatements