Chapter 4. Making Decisions

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Chapter 4. Making Decisions

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Title: Chapter 4. Making Decisions

1
Chapter 4. Making Decisions
2
4.1 Relational Operators

Relational operators allow you to compare numeric
values and determine if one is greater than, less
than, equal to, or not equal to another.

3
Table 4-1
4
Table 4-2
5
The Value of a Relationship

Relational expressions are also know as a Boolean
expression
Warning! The equality operator is two equal
signs together

6
Table 4-3
7
Program 4-1

// This program displays the values of true and
false
// states.
include ltiostream.hgt
void main(void)
int trueValue, falseValue, x 5, y
10 trueValue X lt Y falseValue Y
X cout ltlt "True is " ltlt trueValue ltlt
endl cout ltlt "False is " ltlt falseValue ltlt endl
Program OutputTrue is 1False is 0

8
Table 4-4 (Assume x is 10, y is 7, a and b are
ints)
9
4.2 The if Statement

The if statement can cause other statements to
execute only under certain conditions.

10
Program 4-2

// This program averages 3 test scores
include ltiostream.hgt
void main(void)
int score1, score2, score3
float average
cout ltlt "Enter 3 test scores and I will average
them "
cin gtgt score1 gtgt score2 gtgt score3
average (score1 score2 score3) / 3.0
cout.precision(1)
cout.setf(iosshowpoint iosfixed)
cout ltlt "Your average is " ltlt average ltlt endl
if (average gt 95)
cout ltlt "Congratulations! That's a high
score!\n"

11
Program Output with Example Input

Enter 3 test scores and I will average them 80
90 70 Enter
Your average is 80.0
Program Output with Other Example Input
Enter 3 test scores and I will average them 100
100 100 Enter
Your average is 100.0
Congratulations! That's a high score!

12
Table 4-5
13
Be Careful With Semicolons

if (expression)
statement
Notice that the semicolon comes after the
statement that gets executed if the expression is
true the semicolon does NOT follow the expression

14
Program 4-3

// This program demonstrates how a misplaced
semicolon
// prematurely terminates an if statement.
include ltiostream.hgt
void main(void)
int x 0, y 10
cout ltlt x is " ltlt x ltlt " and y is " ltlt y ltlt
endl
if (x gt y) // misplaced semicolon!
cout ltlt x is greater than y\n" // Always
executed
Program Output
X is 0 and Y is 10
X is greater than Y

15
Programming Style and the if Statement

The conditionally executed statement should
appear on the line after the if statement.
The conditionally executed statement should be
indented one level from the if statement.
Note Each time you press the tab key, you are
indenting one level.

16
Comparing Floating Point Numbers

Round-off errors can cause problems when
comparing floating point numbers with the
equality operator ()

17
Program 4-4
// This program demonstrates how floating point
round-off // errors can make equality comparisons
unreliable. include ltiostream.hgt void
main(void) float result result 6.0
0.666666 // Round-off error if (result
4.0) cout ltlt "It's true!" else cout ltlt
"It's false!" Program Output It's false!
18
And Now Back to Truth

When a relational expression is true, it has the
value 1.
When a relational expression is false it has the
value 0.
An expression that has the value 0 is considered
false by the if statement.
An expression that has any value other than 0 is
considered true by the if statement.

19
Not All Operators Are Equal

Consider the following statement
if (x 2) // caution here!
cout ltlt It is True!
This statement does not determine if x is equal
to 2, it assigns x the value 2, therefore, this
expression will always be true because the value
of the expression is 2, a non-zero value

20
Program 4-5

// This program averages 3 test scores. The if
statement uses
// the operator, but the operator was
intended.
include ltiostream.hgt
void main(void)
int score1, score2, score3
float average
cout ltlt "Enter 3 test scores and I will average
them "
cin gtgt score1 gtgt score2 gtgt score3
average (score1 score2 score3) / 3.0
cout.precision(1)
cout.setf(iosshowpoint iosfixed)
cout ltlt "Your average is " ltlt average ltlt endl
if (average 100) // Wrong
cout ltlt "Congratulations! That's a high
score!\n"

21
Program 4-5 Output With Example Input

Program Output with Example Input
Enter your 3 test scores and I will average them
80 90 70Enter
Your average is 80.0
Congratulations! Thats a perfect score!

22
4.3 Flags

A flag is a variable, usually a boolean or an
integer, that signals when a condition exists.
If your compiler does not support the bool data
type, use int instead.

23
Program 4-6
// This program averages 3 test scores. It uses
the variable highScore as a flag. include
ltiostream.hgt void main(void) int score1,
score2, score3 float average bool highScore
false cout ltlt "Enter your 3 test scores and I
will average them " cin gtgt score1 gtgt score2 gtgt
score3 average (score1 score2 score3) /
3.0 if (average gt 95) highScore true //
Set the flag variable cout.precision(1) cout.se
tf(iosshowpoint iosfixed) cout ltlt "Your
average is " ltlt average ltlt endl if
(highScore) cout ltlt "Congratulations! That's a
high score!\n"\ Program Output with Example
Input Enter your 3 test scores and I will average
them 100 100 100 Enter Your average is
100.0 Congratulations! That's a high score!
24
4.4 Expanding the if Statement

The if statement can conditionally execute a
block of statement enclosed in braces.
if (expression)
statement
statement
// Place as many statements here as
necessary.

25
Program 4-7

// This program averages 3 test scores.
// It uses the variable highScore as a flag.
include ltiostream.hgt
void main(void)
int score1, score2, score3
float average
bool highScore false
cout ltlt "Enter 3 test scores and I will average
them "
cin gtgt score1 gtgt score2 gtgt score3
average (score1 score2 score3) / 3.0
if (average gt 95)
highScore true // Set the flag variable

Program continues on next slide
26
Program continued from previous slide

cout.precision(1)
cout.setf(iosshowpoint iosfixed)
cout ltlt "Your average is " ltlt average ltlt endl
if (highScore)
cout ltlt "Congratulations!\n"
cout ltlt "That's a high score.\n"
cout ltlt "You deserve a pat on the back!\n"

27
Program Output with Example Input

Enter your 3 test scores and I will average them
100 100 100 Enter
Your average is 100.0
Congratulations!
That's a high score.
You deserve a pat on the back!
Program Output with Different Example Input
Enter your 3 test scores and I will average them
80 90 70 Enter
Your average is 80.0

28
Dont Forget the Braces!

If you intend to execute a block of statements
with an if statement, dont forget the braces.
Without the braces, the if statement only
executes the very next statement.

29
Program 4-8

// This program averages 3 test scores.
// It uses the variable highScore as a flag.
include ltiostream.hgt
void main(void)
int score1, score2, score3
float average
bool highScore false
cout ltlt "Enter 3 test scores and I will average
them "
cin gtgt score1 gtgt score2 gtgt score3
average (score1 score2 score3) / 3.0
if (average gt 95)
highScore true // Set the flag variable

Program continues on next slide
30
Program continued from previous slide

cout.precision(1)
cout.setf(iosshowpoint iosfixed)
cout ltlt "Your average is " ltlt average ltlt endl
// The following if statement is
// missing its braces!
if (highScore)
cout ltlt "Congratulations!\n"
cout ltlt "That's a high score.\n"
cout ltlt "You deserve a pat on the back!\n"

31
Program Output with Example Input
Enter your 3 test scores and I will average them
100 100 100EnterYour average is
100Congratulations!Thats a high score.You
deserve a pat on the back! Program Output with
Different Example Input Enter your 3 test scores
and I will average them 80 90 70EnterYour
average is 100Congratulations!Thats a high
score.You deserve a pat on the back!

32
4.5 The if/else Statement

The if/else statement will execute one group of
statements if the expression is true, or another
group of statements if the expression is false.
if (expression)
statement or block of statements
else
statement or block of statements

33
Program 4-9

// This program uses the modulus operator to
determine
// if a number is odd or even. If the number is
evenly divided
// by 2, it is an even number. A remainder
indicates it is odd.
include ltiostream.hgt
void main(void)
int number
cout ltlt "Enter an integer and I will tell you if
it\n"
cout ltlt "is odd or even. "
cin gtgt number
if (number 2 0)
cout ltlt number ltlt " is even.\n"
else
cout ltlt number ltlt " is odd.\n"

34
Program Output with Example Input

Enter an integer and I will tell you if it
is odd or even. 17 Enter
17 is odd.

35
Program 4-10

// This program asks the user for two numbers,
num1 and num2.
// num1 is divided by num2 and the result is
displayed.
// Before the division operation, however, num2
is tested
// for the value 0. If it contains 0, the
division does not
// take place.
include ltiostream.hgt
void main(void)
float num1, num2, quotient
cout ltlt "Enter a number "
cin gtgt num1
cout ltlt "Enter another number "
cin gtgt num2

Program continues on next slide
36
Program continued from previous slide.

if (num2 0)
cout ltlt "Division by zero is not possible.\n"
cout ltlt "Please run the program again and
enter\n"
cout ltlt "a number besides zero.\n"
else
quotient num1 / num2
cout ltlt "The quotient of " ltlt num1 ltlt " divided
by "
cout ltlt num2 ltlt " is " ltlt quotient ltlt ".\n"

37
Program Output

(When the user enters 0 for num2)
Enter a number 10 Enter
Enter another number 0 Enter
Division by zero is not possible.
Please run the program again and enter
a number besides zero.

38
4.6 The if/else if Construct

The if/else if statement is a chain of if
statements. The perform their tests, one after
the other, until one of them is found to be true.
If (expression)
statement or block of statements
else if (expression)
statement or block of statements
// put as many else its as needed here
else if (expression)
statement or block of statements

39
Program 4-11

// This program uses an if/else if statement to
assign a
// letter grade (A, B, C, D, or F) to a numeric
test score.
include ltiostream.hgt
void main(void)
int testScore
char grade
cout ltlt "Enter your numeric test score and I
will\n"
cout ltlt "tell you the letter grade you earned
"
cin gtgt testScore

Program continues on next slide
40
Program continued from previous slide.

if (testScore lt 60)
grade 'F'
else if (testScore lt 70)
grade 'D'
else if (testScore lt 80)
grade 'C'
else if (testScore lt 90)
grade 'B'
else if (testScore lt 100)
grade 'A'
cout ltlt "Your grade is " ltlt grade ltlt ".\n"

41
Program Output with Example Input

Enter your test score and I will
tell you the letter grade you earned 88 Enter
Your grade is B.

42
Program 4-12

// This program uses independent if/else
statements to assign a
// letter grade (A, B, C, D, or F) to a numeric
test score.
// Do you think it will work?
include ltiostream.hgt
void main(void)
int testScore
char grade
cout ltlt "Enter your test score and I will tell
you\n"
cout ltlt "the letter grade you earned "
cin gtgt testScore

Program continues on next slide
43
Program continued from previous slide.

if (testScore lt 60)
grade 'F'
if (testScore lt 70)
grade 'D'
if (testScore lt 80)
grade 'C'
if (testScore lt 90)
grade 'B'
if (testScore lt 100)
grade 'A'
cout ltlt "Your grade is " ltlt grade ltlt ".\n"

44
Program Output with Example Input

Enter your test score and I will tell you
the letter grade you earned 40 Enter
Your grade is A.

45
Program 4-13

//This program uses an if/else if statement to
//assign a letter grade ( A, B, C, D, or F )
//to a numeric test score.
includeltiostream.hgt
void main(void)
int testScore
cout ltlt "Enter your test score and I will tell
you\n"
cout ltlt "the letter grade you earned "
cin gtgt testScore
if (testScore lt 60)
cout ltlt "Your grade is F.\n"
cout ltlt "This is a failing grade. Better see
your "
cout ltlt "instructor.\n"
else if (testScore lt 70)

Program continues on next slide
46
Program continued from previous slide.

else if (testScore lt 80)
cout ltlt "Your grade is C.\n"
cout ltlt "This is average.\n"
else if(testScore lt 90)
cout ltlt "Your grade is B.\n"
cout ltlt "This is an above average grade.\n"
else if (testScore lt 100)
cout ltlt "Your grade is A.\n"
cout ltlt "This is a superior grade. Good
work!\n"

47
Program Output with Example Input

Enter your test score and I will tell you
the letter grade you earned 94 Enter
Your grade is A.
This is a superior grade. Good work!

48
4.7 Using a Trailing else

A trailing else, placed at the end of an if/else
if statement, provides default action when none
of the ifs have true expressions

49
Program 4-14

// This program uses an if/else if statement to
assign a
// letter grade (A, B, C, D, or F) to a numeric
test score.
// A trailing else has been added to catch test
scores gt 100.
include ltiostream.hgt
void main(void)
int testScore
cout ltlt "Enter your test score and I will tell
you\n"
cout ltlt "the letter grade you earned "
cin gtgt testScore

Program continues on next slide
50
Program continued from previous slide.

if (testScore lt 60)
cout ltlt "Your grade is F.\n"
cout ltlt "This is a failing grade. Better see
your "
cout ltlt "instructor.\n"
else if (testScore lt 70)
cout ltlt "Your grade is D.\n"
cout ltlt "This is below average. You should get
"
cout ltlt "tutoring.\n"

51
Program continued from previous slide.

else if (testScore lt 80)
cout ltlt "Your grade is C.\n"
cout ltlt "This is average.\n"
else if (testScore lt 90)
cout ltlt "Your grade is B.\n"
cout ltlt "This is an above average grade.\n"

52
Program continued from previous slide.

else if (testScore lt 100)
cout ltlt "Your grade is A.\n"
cout ltlt "This is a superior grade. Good
work!\n"
else // Default action
cout ltlt testScore ltlt " is an invalid score.\n"
cout ltlt "Please enter scores no greater than
100.\n"

53
Program Output with Example Input

Enter your test score and I will tell you
the letter grade you earned 104 Enter
104 is an invalid score.
Please enter scores no greater than 100.

54
4.8 Focus on Software Engineering Menus

You can use the if/else if statement to create
menu-driven programs. A menu-driven program
allows the user to determine the course of action
by selecting it from a list of actions.

55
Program 4-15

// This program displays a menu and asks the user
to make a
// selection. An if/else if statement determines
which item
// the user has chosen.
include ltiostream.hgt
void main(void)
int choice, months
float charges
cout ltlt "\t\tHealth Club Membership Menu\n\n"
cout ltlt "1. Standard Adult Membership\n"
cout ltlt "2. Child Membership\n"
cout ltlt "3. Senior Citizen Membership\n"
cout ltlt "4. Quit the Program\n\n"

Program continues on next slide
56
Program continued from previous slide.

cout ltlt "Enter your choice "
cin gtgt choice
cout.setf(iosfixed iosshowpoint)
cout.precision(2)
if (choice 1)
cout ltlt "\nFor how many months? "
cin gtgt months
charges months 40.00
cout ltlt "The total charges are " ltlt charges ltlt
endl

57
Program continued from previous slide.

else if (choice 2)
cout ltlt "\nFor how many months? "
cin gtgt months
charges months 20.00
cout ltlt "The total charges are " ltlt charges ltlt
endl
else if (choice 3)
cout ltlt "\nFor how many months? "
cin gtgt months
charges months 30.00
cout ltlt "The total charges are " ltlt charges ltlt
endl

58
Program continued from previous slide.

else if (choice ! 4)
cout ltlt "The valid choices are 1 through 4. Run
the\n"
cout ltlt "program again and select one of
those.\n"

59
Program Output with Example Input

Health Club Membership Menu
1. Standard Adult Membership
2. Child Membership
3. Senior Citizen Membership
4. Quit the Program
Enter your choice 3 Enter
For how many months? 6 Enter
The total charges are 180.00

60
4.9 Focus on Software Engineering Nested if
Statements

A nested if statement is an if statement in the
conditionally-executed code of another if
statement.

61
Program 4-16

// This program demonstrates the nested if
statement.
include ltiostream.hgt
void main(void)
char employed, recentGrad
cout ltlt "Answer the following questions\n"
cout ltlt "with either Y for Yes or "
cout ltlt "N for No.\n"
cout ltlt "Are you employed? "
cin gtgt employed
cout ltlt "Have you graduated from college "
cout ltlt "in the past two years? "
cin gtgt recentGrad

Program continues on next slide
62
Program continued from previous slide.

if (employed 'Y')
if (recentGrad 'Y') // Nested if
cout ltlt "You qualify for the special "
cout ltlt "interest rate.\n"

63
Program Output with Example Input

Answer the following questionswith either Y
for Yes or N for No.Are you employed?
YEnterHave you graduated from college in the
past two years? YEnterYou qualify for the
special interest rate.

64
Program Output with Other Example Input

Answer the following questionswith either Y
for Yes or N for No.Are you employed?
YEnterHave you graduated from college in the
past two years? NEnter

65
4.10 Logical Operators

Logical operators connect two or more relational
expressions into one, or reverse the logic of an
expression.

66
Table 4-6
67
Table 4-7
68
Program 4-18

// This program demonstrates the logical
operator.
include ltiostream.hgt
void main(void)
char employed, recentGrad
cout ltlt "Answer the following questions\n"
cout ltlt "with either Y for Yes or "
cout ltlt "N for No.\n"
cout ltlt "Are you employed? "
cin gtgt employed

Program continues on next slide
69
Program continued from previous slide.

cout ltlt "Have you graduated from college "
cout ltlt "in the past two years? "
cin gtgt recentGrad
if (employed 'Y recentGrad 'Y') //
Operator
cout ltlt "You qualify for the special "
cout ltlt "interest rate.\n"
else
cout ltlt "You must be employed and have \n"
cout ltlt "graduated from college in the\n"
cout ltlt "past two years to qualify.\n"

70
Program Output with Example Input

Answer the following questionswith either Y
for Yes orN for No.Are you employed?
YEnterHave you graduated from college in the
past two years? NEnterYou must be employed and
havegraduated from college in thepast two years
to qualify.

71
Table 4-8
72
Program 4-19

// This program asks the user for their annual
income and
// the number of years they have been employed at
their current
// job. The operator is used in a if statement
that
// determines if the income is at least 35,000
or their time
// on the job is more than 5 years.
include ltiostream.hgt
void main(void)
float income
int years

Program continues on next slide
73
Program continues

cout ltlt "What is your annual income? "
cin gtgt income
cout ltlt "How many years have you worked at "
ltlt "your current job? "
cin gtgt years
if (income gt 35000 years gt 5) // Use
logical operator
cout ltlt "You qualify.\n"
else
cout ltlt "You must earn at least 35,000 or
have\n"
cout ltlt "been employed for more than 5
years.\n"

74
Program Output with Example Input

What is your annual income? 40000 Enter
How many years have you worked at your current
job? 2 Enter
You qualify.
Program Output with Example Input
What is your annual income? 20000 Enter
How many years have you worked at your current
job? 7 Enter
You qualify.

75
Table 4-9
76
Program 4-20

//This program asks the user for his annual
income and
//the number of years he has been employed at his
current job.
//The ! operator reverses the logic of the
expression in the if/else statement.
include ltiostream.hgt
void main(void)
float income
int years
cout ltlt "What is your annual income? "
cin gtgt income
cout ltlt "How many years have you worked at "
ltlt "your current job? "
cin gtgt years
if (!(income gt 35000 years gt 5)) // Uses
the ! Logical operator
cout ltlt "You must earn at least 35,000 or
have\n"
cout ltlt "been employed for more than 5
years.\n"

77
Precedence of Logical Operators

78
4.11 Checking Numeric Ranges With Logical
Operators

Logical operators are effective for determining
if a number is in or out of a range.

79
4.12 Focus on Software Engineering Validating
User Input

As long as the user of a program enters bad
input, the program will produce bad output.
Program should be written to filter out bad input.

80
Examples of validation

Numbers are check to ensure they are within a
range of possible values.
Values are check for their reasonableness.
Items selected from a menu or other set of
choices are check to ensure they are available
options.
Variables are check for values that might cause
problems, such as division by zero.

81
4.13 More About Variable Declarations and Scope

The scope of a variable is limited to the block
in which is is declared.
Variables declared inside a set of braces have
local scope or block scope.

82
Program 4-22A

//This program demonstrates late variable
declaration
include ltiostream.hgt
void main(void)
cout ltlt "What is your annual income? "
float income // variable declaration
cin gtgt income
cout ltlt "How many years have you worked at "
ltlt "your current job? "
int years // variable declaration
cin gtgt years
if (income gt 35000 years gt 5)
cout ltlt "You qualify.\n"
else
cout ltlt "You must earn at least 35,000 or
have\n"
cout ltlt "been employed for more than 5
years.\n"

83
Program 4-22B

//This program demonstrates late variable
declaration
include ltiostream.hgt
void main(void)
cout ltlt "What is your annual income? "
float income // variable declaration
cin gtgt income
cout ltlt "How many years have you worked at "
ltlt "your current job? "
int years // variable declaration
cin gtgt years
if (income gt 35000 years gt 5)
cout ltlt "You qualify.\n"
else
cout ltlt "You must earn at least 35,000 or
have\n"
cout ltlt "been employed for more than 5
years.\n"

84
Program 4-22C

//This program demonstrates late variable
declaration
include ltiostream.hgt
void main(void)
cout ltlt "What is your annual income? "
float income
cin gtgt income
int years
cout ltlt "How many years have you worked at "
ltlt "your current job? "
cin gtgt years
if (income gt 35000 years gt 5)
cout ltlt "You qualify.\n"
else
cout ltlt "You must earn at least 35,000 or
have\n"
cout ltlt "been employed for more than 5
years.\n"

85
Program 4-23

// This program demonstrates a variable declared
in an inner block.
include ltiostream.hgt
void main(void)
cout ltlt "What is your annual income? "
float income // variable declaration
cin gtgt income
if (income gt 35000)
int years // variable declaration
cout ltlt "How many years have you worked at "
ltlt "your current job? "
cin gtgt years
if (years gt 5)
cout ltlt "You qualify.\n"

Program continues on next slide
86
Program continued from previous slide.

else
cout ltlt "You must have been employed for\n"
cout ltlt "more than 5 years to qualify.\n"
else
cout ltlt "You must earn at least 35,000 to\n"
cout ltlt "qualify.\n"

87
Variables With the Same Name

When a block is nested inside another block, a
variable declared in the inner block may have the
same name as a variable declared in the outer
block. The variable in the inner block takes
precedence over the variable in the outer block.

88
Program 4-24