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Chapter 3 Selections

Motivations

If you assigned a negative value for radius in Listing 2.1, ComputeArea.cpp, the program would print an invalid result. If the radius is negative, you don't want the program to compute the area. How can you deal with this situation?

Objectives

To declare bool variables and write Boolean expressions using relational operators (§3.2).
To implement selection control using one-way if statements (§3.3).
To program using one-way if statements (GuessBirthday) (§3.4).
To implement selection control using two-way if statements (§3.5).
To implement selection control using nested if and multi-way if-else statements (§3.6).
To avoid common errors and pitfalls in if statements (§3.7).
To program using selection statements for a variety of examples (BMI, ComputeTax, SubtractionQuiz) (§§3.8–3.10).
To generate random numbers using the rand function and set a seed using the srand function (§3.9).
To combine conditions using logical operators (&&, ||, and !) (§3.10).
To program using selection statements with combined conditions (LeapYear, Lottery) (§§3.11–3.12).
To implement selection control using switch statements (§3.13).
To write expressions using the conditional operator (§3.14).
To examine the rules governing operator precedence and operator associativity (§3.15).
To debug errors (§3.16).

The bool Type and Operators

Often in a program you need to compare two values, such as whether i is greater than j. C++ provides six relational operators (also known as comparison operators) in Table 3.1 that can be used to compare two values.

Relational Operators

Operator Name Example Result

< less than 1 < 2 true

<= less than or equal to 1 <= 2 true

> greater than 1 > 2 false

>= greater than or equal to 1 >= 2 false == equal to 1 == 2 false

!= not equal to 1 != 2 true

One-way if Statements

if (booleanExpression)

{

statement(s);

}

if (radius >= 0)

{

area = radius * radius * PI;

cout << "The area for the circle of " <<

" radius " << radius << " is " << area;

}

Boolean

Expression

true

Statement(s)

false

area = radius * radius * PI;

cout << "The area for the circle of " <<

" radius " << radius << " is " << area;

true

(radius >= 0)

(a)

(b)

Note

(a)

if ((i > 0) && (i < 10))

{

cout << "i is an " <<

"integer between 0 and 10";

}

if ((i > 0) && (i < 10))

cout << "i is an " <<

"integer between 0 and 10";

Equivalent

Braces can be omitted if the block contains a single statement

(b)

Outer parentheses required

Examples

Listing 3.1 gives a program that checks whether a number is even or odd. The program prompts the user to enter an integer (line 9) and displays “number is even” if it is even (lines 11-12) and “number is odd” if it is odd (lines 14-15).

TestBoolean

Run

Caution

Adding a semicolon at the end of an if clause is a common mistake.

This mistake is hard to find, because it is not a compilation error or a runtime error, it is a logic error.

This error often occurs when you use the next-line block style.

(a)

if (radius >= 0);

{

area = radius * radius * PI;

cout << "The area "

<< " is " << area;

}

if (radius >= 0) { };

{

area = radius * radius * PI;

cout << "The area "

<< " is " << area;

}

Equivalent

Empty Body

(b)

Logic Error

Lab #1 – Hand In

C++ Program

Write a program that accepts as input first one integer into a variable named intValue1.

Next input a character into a variable named chrValue1.

If the intValue1 is even, then output to the screen “Integer is even”.

If the chrValue1 is greater than ‘G’, then output to the screen “Character is greater than G”.

Make sure you contain at least 2 comments

Variables are named correctly

The use of the if statement is done twice

The if...else Statement

if (booleanExpression)

{

statement(s)-for-the-true-case;

}

else

{

statement(s)-for-the-false-case;

}

false

Boolean

Expression

Statement(s) for the true case

true

Statement(s) for the false case

Nested if Statements

if (i > k)

{

if (j > k)

cout << "i and j are greater than k";

}

else

cout << "i is less than or equal to k";

Multiple Alternative if Statements

(a)

if (score >= 90.0)

cout << "Grade is A";

else

if (score >= 80.0)

cout << "Grade is B";

else

if (score >= 70.0)

cout << "Grade is C";

else

if (score >= 60.0)

cout << "Grade is D";

else

cout << "Grade is F";

(b)

Equivalent

This is better

if (score >= 90.0)

cout << "Grade is A";

else if (score >= 80.0)

cout << "Grade is B";

else if (score >= 70.0)

cout << "Grade is C";

else if (score >= 60.0)

cout << "Grade is D";

else

cout << "Grade is F";

Trace if-else statement

if (score >= 90.0)

cout << “Grade is A";

else if (score >= 80.0)

cout << “Grade is B";

else if (score >= 70.0)

cout << “Grade is C";

else if (score >= 60.0)

cout << “Grade is D";

else

cout << “Grade is F";

Suppose score is 70.0

The condition is false

animation

Trace if-else statement

if (score >= 90.0)

cout << “Grade is A";

else if (score >= 80.0)

cout << “Grade is B";

else if (score >= 70.0)

cout << “Grade is C";

else if (score >= 60.0)

cout << “Grade is D";

else

cout << “Grade is F";

Suppose score is 70.0

The condition is false

animation

Trace if-else statement

if (score >= 90.0)

cout << “Grade is A";

else if (score >= 80.0)

cout << “Grade is B";

else if (score >= 70.0)

cout << “Grade is C";

else if (score >= 60.0)

cout << “Grade is D";

else

cout << “Grade is F";

Suppose score is 70.0

The condition is true

animation

Trace if-else statement

if (score >= 90.0)

cout << “Grade is A";

else if (score >= 80.0)

cout << “Grade is B";

else if (score >= 70.0)

cout << “Grade is C";

else if (score >= 60.0)

cout << “Grade is D";

else

cout << “Grade is F";

Suppose score is 70.0

grade is C

animation

Trace if-else statement

if (score >= 90.0)

cout << “Grade is A";

else if (score >= 80.0)

cout << “Grade is B";

else if (score >= 70.0)

cout << “Grade is C";

else if (score >= 60.0)

cout << “Grade is D";

else

cout << “Grade is F";

Suppose score is 70.0

Exit the if statement

animation

Note

The else clause matches the most recent if clause in the same block.

(a)

int i = 1;

int j = 2;

int k = 3;

if (i > j)

if (i > k)

cout << "A";

else

cout << "B";

(b)

Equivalent

int i = 1;

int j = 2;

int k = 3;

if (i > j)

if (i > k)

cout << "A";

else

cout << "B";

This is better with correct indentation

Note, cont.

Nothing is printed from the preceding statement. To force the else clause to match the first if clause, you must add a pair of braces:

int i = 1; int j = 2; int k = 3;

if (i > j)

{

if (i > k)

cout << "A";

}

else

cout << "B";

This statement prints B.

TIP

(a)

if (number % 2 == 0)

even = true;

else

even = false;

(b)

Equivalent

This is better

bool even

= number % 2 == 0;

CAUTION

(a)

if (even == true)

cout <<"It is even.";

(b)

Equivalent

if (even)

cout << "It is even.";

This is better

Common Errors in Selection Statements

Common Error 1: Forgetting Necessary Braces

(a) Wrong

if (radius >= 0)

area = radius * radius * PI;

cout << "The area "

<< " is " << area;

if (radius >= 0)

{

area = radius * radius * PI;

cout << "The area "

<< " is " << area;

}

(b) Correct

Common Errors in Selection Statements

Common Error 2: Wrong Semicolon at the if Line

(a)

if (radius >= 0);

{

area = radius * radius * PI;

cout << "The area "

<< " is " << area;

}

if (radius >= 0) { };

{

area = radius * radius * PI;

cout << "The area "

<< " is " << area;

}

Equivalent

Empty Body

(b)

Logic Error

Common Errors in Selection Statements

Common Error 3: Mistakenly Using = for ==

if (count = 1)

cout << "count is zero" << endl;

else

cout << "count is not zero" << endl;

Common Errors in Selection Statements

Common Error 4: Redundant Testing of Boolean Values

(a)

if (even == true)

cout <<"It is even.";

(b)

Equivalent

if (even)

cout << "It is even.";

This is better

Problem: Body Mass Index

Body Mass Index (BMI) is a measure of health on weight. It can be calculated by taking your weight in kilograms and dividing by the square of your height in meters. The interpretation of BMI for people 16 years or older is as follows:

ComputeBMI

BMI Interpretation

below 16 serious underweight

underweight

normal weight

overweight

29-35 seriously overweight

above 35 gravely overweight

Example: Computing Taxes

The US federal personal income tax is calculated based on the filing status and taxable income. There are four filing statuses: single filers, married filing jointly, married filing separately, and head of household. The tax rates for 2002 are shown in Table 3.6.

Example: Computing Taxes, cont.

ComputeTax

if (status == 0)

{

// Compute tax for single filers

}

else if (status == 1)

{

// Compute tax for married file jointly

}

else if (status == 2)

{

// Compute tax for married file separately

}

else if (status == 3)

{

// Compute tax for head of household

}

else

{

// Display wrong status

}

Example: A Simple Math Learning Tool

SubtractionQuiz

Run

This example creates a program for a first grader to practice subtractions. The program randomly generates two single-digit integers number1 and number2 with number1 >= number2 and displays a question such as “What is 9 – 2?” to the student, as shown in the sample output. After the student types the answer, the program displays a message to indicate whether the answer is correct.

C++ Programming: From Problem Analysis to Program Design, Fourth Edition

Lab #2 – Hand In

C++ Program

Write a program that accepts as input first one integer into a variable named intValue1.

Accept a second integer from input into a variable named intValue2.

If the intValue1 is even, then output to the screen “Value 1 is even”, otherwise say “Value 1 is odd”.

If the intValue2 is even, then output to the screen “Value 2 is even”, otherwise say “Value 2 is odd”.

If intValue2 is greater than intValue1, than output to the screen “Value2 is greater than Value1” and then also output another line saying “If part is true”, otherwise output to the screen “Value1 is greater than Value2” and then also output another line saying “else part is true”.

Make sure you contain at least 2 comments

Variables are named correctly

The use of the if-else statement is used 3 times.

C++ Programming: From Problem Analysis to Program Design, Fourth Edition

Logical Operators

Operator Name Description

! not logical negation

&& and logical conjunction

|| or logical disjunction

Truth Table for Operator !

p !p

true false

false true

Example (assume age = 24, weight = 140)

!(age > 18) is false, because (age > 18) is true.

!( weight == 150) is true, because (weight == 150) is false.

Truth Table for Operator &&

Example (assume age = 24, weight = 140)

(age > 28) && (weight < 140) is false, because (age > 28) and (weight < 140) are both false.

(age > 18) && (weight >= 140) is true, because both (age > 18) and (weight >= 140) are true.

p1 p2 p1 && p2

false false false

false true false

true false false

true true true

Truth Table for Operator ||

Example (assume age = 24, weight = 140)

(age > 34) || (weight < 140) is false, because both (age > 34) and (weight < 140) are false.

(age > 18) || (weight >= 150) is false, because (age > 18) is true.

p1 p2 p1 || p2

false false false

false true true

true false true

true true true

Examples

Listing 3.3 gives a program that checks whether a number is divisible by 2 and 3, whether a number is divisible by 2 or 3, and whether a number is divisible by 2 or 3 but not both:

TestBooleanOperators

Run

Short-Circuit Operator

When evaluating p1 && p2, C++ first evaluates p1 and then evaluates p2 if p1 is true; if p1 is false, it does not evaluate p2. When evaluating p1 || p2, C++ first evaluates p1 and then evaluates p2 if p1 is false; if p1 is true, it does not evaluate p2. Therefore, && is referred to as the conditional or short-circuit AND operator, and || is referred to as the conditional or short-circuit OR operator.

Examples

Write a program that lets the user enter a year and checks whether it is a leap year.

A year is a leap year if it is divisible by 4 but not by 100 or if it is divisible by 400. So you can use the following Boolean expression to check whether a year is a leap year:

(year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)

LeapYear

Run

Problem: Lottery

Randomly generates a lottery of a two-digit number, prompts the user to enter a two-digit number, and determines whether the user wins according to the following rule:

Lottery

If the user input matches the lottery in exact order, the award is $10,000.
If the user input matches the lottery, the award is $3,000.
If one digit in the user input matches a digit in the lottery, the award is $1,000.

C++ Programming: From Problem Analysis to Program Design, Fourth Edition

Lab #3 – Hand In

C++ Program

Write a program that accepts as input first one integer into a variable named intValue1.

Accept a second integer from input into a variable named intValue2.

If the intValue1 is even, then output to the screen “Value 1 is even”.

If intValue1 is not even, then test to see if the intValue2 is even, then output to the screen “Value 2 is even”, otherwise say “Both values is odd”.

Make sure you contain at least 2 comments

Variables are named correctly

The use of the if-else statement is used 3 times.

C++ Programming: From Problem Analysis to Program Design, Fourth Edition

switch Statements

switch (status)

{

case 0: compute taxes for single filers;

break;

case 1: compute taxes for married file jointly;

break;

case 2: compute taxes for married file separately;

break;

case 3: compute taxes for head of household;

break;

default: cout << "Errors: invalid status“ << endl;

}

switch Statement Flow Chart

status is 0

Compute tax for single filers

break

Compute tax for married file separately

break

Compute tax for married file jointly

status is 1

status is 2

break

Compute tax for married file separatly

status is 3

break

Compute tax for head of household

default

Next Statement

Default actions

switch Statement Rules

switch (switch-expression) {

case value1: statement(s)1;

break;

case value2: statement(s)2;

break;

…

case valueN: statement(s)N;

break;

default: statement(s)-for-default;

}

The switch-expression must yield a value of char, byte, short, or int type and must always be enclosed in parentheses.

The value1, ..., and valueN must have the same data type as the value of the switch-expression. The resulting statements in the case statement are executed when the value in the case statement matches the value of the switch-expression. Note that value1, ..., and valueN are constant expressions, meaning that they cannot contain variables in the expression, such as 1 + x.

switch Statement Rules

The keyword break is optional, but it should be used at the end of each case in order to terminate the remainder of the switch statement. If the break statement is not present, the next case statement will be executed.

switch (switch-expression) {

case value1: statement(s)1;

break;

case value2: statement(s)2;

break;

…

case valueN: statement(s)N;

break;

default: statement(s)-for-default;

}

When the value in a case statement matches the value of the switch-expression, the statements starting from this case are executed until either a break statement or the end of the switch statement is reached.

The default case, which is optional, can be used to perform actions when none of the specified cases matches the switch-expression.

Trace switch statement

switch (day)

{

case 1: // Fall to through to the next case

case 2: // Fall to through to the next case

case 3: // Fall to through to the next case

case 4: // Fall to through to the next case

case 5: cout << "Weekday"; break;

case 0: // Fall to through to the next case

case 6: cout << "Weekend";

}

Suppose day is 3:

animation

Trace switch statement

switch (day)

{

case 1: // Fall to through to the next case

case 2: // Fall to through to the next case

case 3: // Fall to through to the next case

case 4: // Fall to through to the next case

case 5: cout << "Weekday"; break;

case 0: // Fall to through to the next case

case 6: cout << "Weekend";

}

Suppose day is 3:

animation

Trace switch statement

switch (day)

{

case 1: // Fall to through to the next case

case 2: // Fall to through to the next case

case 3: // Fall to through to the next case

case 4: // Fall to through to the next case

case 5: cout << "Weekday"; break;

case 0: // Fall to through to the next case

case 6: cout << "Weekend";

}

Suppose day is 3:

animation

Trace switch statement

switch (day)

{

case 1: // Fall to through to the next case

case 2: // Fall to through to the next case

case 3: // Fall to through to the next case

case 4: // Fall to through to the next case

case 5: cout << "Weekday"; break;

case 0: // Fall to through to the next case

case 6: cout << "Weekend";

}

Suppose day is 3:

animation

Trace switch statement

switch (day)

{

case 1: // Fall to through to the next case

case 2: // Fall to through to the next case

case 3: // Fall to through to the next case

case 4: // Fall to through to the next case

case 5: cout << "Weekday"; break;

case 0: // Fall to through to the next case

case 6: cout << "Weekend";

}

Suppose day is 3:

animation

Problem: Chinese Zodiac

Write a program that prompts the user to enter a year and displays the animal for the year.

ChineseZodiac

rabbit

tiger

rat

dragon

snake

horse

sheep

monkey

rooster

dog

pig

0: monkey

1: rooster

2: dog

3: pig

4: rat

5: ox

6: tiger

7: rabbit

8: dragon

9: snake

10: horse

11: sheep

year % 12 =

Conditional Operator

if (x > 0)

y = 1

else

y = -1;

is equivalent to

y = (x > 0) ? 1 : -1;

(booleanExpression) ? expression1 : expression2

Ternary operator

Binary operator

Unary operator

Conditional Operator

cout << ((num % 2 == 0) ? "num is even" : "num is odd");

Conditional Operator, cont.

(booleanExp) ? exp1 : exp2

C++ Programming: From Problem Analysis to Program Design, Fourth Edition

Lab #4 – Hand In

C++ Program

Write a program that accepts as input a character variable called chrValue1.

If chrValue1 is ‘G’ or ‘g’ then write out to the screen “You entered a g”, if chrValue1 is ‘T’ or ‘t’ then write out to the screen “You entered a t”, if chrValue1 is ‘P’ or ‘p’ then write out to the screen “You entered a p”, otherwise output “You tricked me”.

Make sure you contain at least 3 comments

Variables are named correctly

Make sure that you use the switch statement and do not use any if statements.

C++ Programming: From Problem Analysis to Program Design, Fourth Edition

Operator Precedence

How to evaluate 3 + 4 * 4 > 5 * (4 + 3) – 1?

Operator Precedence

var++, var--
+, - (Unary plus and minus), ++var,--var
(type) Casting
! (Not)
*, /, % (Multiplication, division, and remainder)
+, - (Binary addition and subtraction)
<, <=, >, >= (Comparison)
==, !=; (Equality)
&& (Conditional AND) Short-circuit AND
|| (Conditional OR) Short-circuit OR
=, +=, -=, *=, /=, %= (Assignment operator)

Enumerated Types

enum Day {MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY};

Once a type is defined, you can declare a variable of that type:

Day day;

The variable day can hold one of the values defined in the enumerated type. For example, the following statement assigns enumerated value MONDAY to variable day:

day = MONDAY;

Companion Website

Enumerated Types

As with any other type, you can declare and initialize a variable in one statement:

Day day = MONDAY;

Furthermore, C++ allows you to declare an enumerated type and variable in one statement. For example,

enum Day {MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY} day = MONDAY;

TestEnumeratedType

Run

Companion Website

Operator Name Example Result

< less than 1 < 2 true

<= less than or equal to 1 <= 2 true

> greater than 1 > 2 false

>= greater than or equal to 1 >= 2 false

== equal to 1 == 2 false

!= not equal to 1 != 2 true

Boolean

Expression

true

Statement(s)

false

(radius >= 0)

true

area = radius * radius * PI;

cout <<

"The area for the circle of "

" radius "

radius

" is "

<< area

;

false

(

)

(

)

(

(i >

) && (i <

)

{

cout <<

"i is an "

"integer between 0 and 10"

;

}

(

Equivalent

(b)

(

(i >

) && (i <

)

cout <<

"i is an "

"integer between 0 and 10"

;

Outer parentheses required

Braces can be omitted if the block contains a single

statement

(radius >=

)

;

{

area = radius * radius * PI;

cout <<

"The area "

" is "

area;

}

)

Equivalent

Logic Error

(radius >=

)

{ }

;

{

area = radius * radius * PI;

cout <<

"The area "

" is "

area;

}

(b)

Empty Body

Boolean

Expression

false

true

Statement(s) for the false case

Statement(s) for the true case

(score >=

90.0

)

cout <<

"Grade is

;

else

(score >=

80.0

)

cout <<

"Grade is

;

else

(score >=

70.0

)

cout <<

"Grade is

;

else

(score >=

60.0

)

cout <<

"Grade is

;

else

cout

"Grade is

;

)

Equivalent

(score >=

90.0

)

cout <<

"Grade is

;

else if

(score >=

80.0

)

cout <<

"Grade is

;

else if

(score >=

70.0

)

cout <<

"Grade is

;

else if

(score >=

60.0

)

cout <<

"Grade is

;

else

cou

t <<

"Grade is

;

)

This is better

int

i =

;

int

j =

;

int

k =

;

(i > j)

(i > k)

cout <<

"A"

;

else

cout <<

"B"

;

)

Equivalent

(

)

int

i =

;

int

j =

;

int

k =

;

(i > j)

(i > k)

cout <<

"A"

;

else

cout <<

"B"

;

This i

s better

with correct

indentation

(number %

)

even =

true

;

else

even =

false

;

)

Equivalent

bool

even

= number %

;

(

)

This is better

(even ==

true

)

cout <<

"It is even."

;

(

)

Equivalent

(even)

cout <<

"It is even."

;

(b)

This is better

(radius >=

)

area = radius * radius * PI;

cout <<

"The area "

" is "

area;

)

Wrong

(radius >=

)

{

area = radius * radius * PI;

cout <<

"The area "

" is "

area;

}

(b)

Correct

Run

BMI

Interpretation

below 16

serious underweight

underweight

normal weight

verweight

seriously overweight

above 35

gravely overweight

Operator Name Description

! not logical negation

&& and logical conjunction

|| or logical disjunction

p !p

true false

false true

Example

(

assume

age = 24,

weight = 140

)

age > 18

)

false

, because

(

age

)

true

weight =

150

)

true

, because

(

weight

= 150

)

false

p1 p2 p1 && p2

false false false

false true false

true false false

true true true

Example

(assume age = 24,

weight = 140

)

(

age

) && (

weight <

140

)

false

, because

(age

and

(

weight <

140

)

are bo

false

(age > 18) && (

weight >

140

)

true

, because

both (age > 18) and

(

weight >

140

)

are true

p1 p2 p1 || p2

false false false

false true true

true false true

true true true

Example (assume age = 24,

weight = 140

)

(age > 34) || (

weight <

140

)

false

, because

both

(age

> 34) and

(

weight <

140

)

are false

(age >

) ||

(

weight >= 1

)

false

, because

(age >

)

is true

status is 0

Compute tax for single filers

break

Compute tax for married file jointly

break

status is 1

Compute tax for married file separatly

break

status is 2

Compute tax for head of household

break

atus is 3

Default actions

default

Next Statement

rat

tiger

rabbit

dragon

snake

horse

sheep

monkey

rooster

dog

pig

0: monkey

1: rooster

: dog

: pig

: rat

: ox

: tiger

: rabbit

: dragon

: snake

: horse

11: sheep

year % 12