chapter 4: making decisions. outline relational operations if statement – if – if/else statement...

Chapter 4:

Making Decisions

Outline

• Relational Operations• If statement

– If – If/else statement– If/else if

• Logical operators• Switch

Slide 4- 3

Relational Operators

• Relational operations allow you to compare numeric and char values and determine whether one is greater, less, equal to, or not equal to another.

• Operators:> Greater than

< Less than

>= Greater than or equal to

<= Less than or equal to

== Equal to

!= Not equal to

Slide 4- 4

Relational Expressions

• Boolean expressions – true or false• Examples:

12 > 5 is true7 <= 5 is falseif x is 10, then x == 10 is true, x != 8 is true, and x == 8 is false

int x, y ;x = 4;y = 6;

EXPRESSION VALUE

x < y

x + 2 < y

x != y

x + 3 >= y

y == x

y == x+2

y = x + 3

int x, y ;x = 4;y = 6;

EXPRESSION VALUE

x < y true

x + 2 < y false

x != y true

x + 3 >= y true

y == x false

y == x+2 true

y = x + 3 7

Slide 4- 7

Relational Expressions

• Can be assigned to a variable:result = x <= y;

• By default, assigns 0 for false, 1 for true• Do not confuse = and ==• 0 is false; any other value is true

Slide 4- 8

Like all C++ expressions, relational expressions are evaluated to yield a numerical result. A condition that we would interpret as true evaluates to an integer value of 1; a false condition results in an integer value of 0.

Samples: cout << (3 < 4) << endl; cout << (2.0 > 3.0); Results: 1 0

Outline

• Relational Operations• If statement

– If – If/else statement– If/else if

• Logical operators• Switch

if statement

Select whether or not to execute a statement (which can be a single statement or an entire block) without the else clause

TRUE

FALSE statement

expression

Slide 4- 13

Flowchart for Evaluating a Decision

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The if Statement

• General Format:

if (expression)statement;

• If the expression is true, then statement is executed.

• If the expression is false, then statement is skipped.

#include <iostream>int main ( void ){ const double LIMIT = 30000.0; //set car mileage limit

double mileage = 0.0; //stores mileage entered by user

cout << "Please enter the mileage recorded on the car: "; cin >> mileage; if (mileage > LIMIT) cout << "This car is over the limit.";

cout << "\n\nEnd of program."; return 0;}

Example: Check a Car’s Mileage

Example: Results

Output from first run:Please enter the mileage recorded on the car: 35620.8This car is over the limit.

End of program.

Output from second run:Please enter the mileage recorded on the car: 25620.3

End of program.

Slide 4- 17

(Program Continues)

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Flowchart for Lines 21 and 22

Slide 4- 21

if statement notes

• Do not place ; after (expression)• Place statement; on a separate line after (expression), indented:if (score > 90)

grade = 'A';

• Be careful testing floats and doubles for equality

• 0 is false; any other value is true

Slide 4- 22

Expanding the if Statement

• To execute more than one statement as part of an if statement, enclose them in { }:

if (score > 90){

grade = 'A'; cout << "Good Job!\n";

} • { } creates a block of code (like your main function)

Outline

• Relational Operations• If statement

– If – If/else statement– If/else if

• Logical operators• Switch

if-else provides two-way selection between executing one of 2 clauses: the if clause or the else clause

TRUEFALSE

if clauseelse clause

expression

Slide 4- 25

The if/else Statement

• Provides two possible paths of execution• Performs one statement or block if the expression is true, otherwise performs another statement or block.

• General Format:if (expression)

statement1; // or blockelse

statement2; // or block

Use of blocks recommended

if ( expression ){ }else{

}

“if clause”

“else clause”

Slide 4- 29

A compound statement consists of individual statements enclosed within braces.

Syntax:if(expression){ statement1; statement2; statement3;}else{ statement4; statement5; statement6;}

Exercise: mail orderAssign value .25 to discount_rate and assign value 10.00 to ship_cost if purchase is over 100.00

Otherwise, assign value .15 to discount_rate and assign value 5.00 to ship_cost

Either way, calculate total_bill

if(expression){ statement1; statement2; statement3;}else{ statement4; statement5; statement6;}

These braces cannot be omitted

if ( purchase > 100.00 ){ discount_rate = .25 ;

ship_cost = 10.00 ;}else{

discount_rate = .15 ; ship_cost = 5.00 ;

}

total_bill = purchase * (1.0 – discount_rate) + ship_cost ;

(Program Continues)

Outline

• Relational Operations• If statement

– If – If/else statement– If/else if

• Logical operators• Switch

if/else if format (if-else chain or extended if-else)

Using nested if statements

if ( Expression1 ) Statement1

else if ( Expression2 )Statement2

.

.

.else if ( ExpressionN )

StatementNelse

Statement N+1

EXACTLY 1 block of these statements will be executed.

Used when only one condition can be true

Example

Marital Status Input Code Married M Single S Divorce D Widowed W

Exampleint main ( void ){ char marital_status; //marital status code entered by user //Prompt user for marital status code cout << "Enter a marital code: " << endl; cin >> marital_status; //Displays marital status message if (marital_status == 'M') cout <<"Individual is married." << endl; else if (marital_status == 'S')

cout <<"Individual is single." << endl; else if (marital_status == 'D')

cout <<"Individual is divorced." << endl; else if (marital_status == 'W')

cout <<"Individual is widowed." << endl; else

cout << "An invalid code was entered." << endl; //used as error message

return 0;}

Output (Test all possible paths): Enter a marital code: D Individual is divorced. Enter a marital code: S Individual is single. Enter a marital code: M Individual is married. Enter a marital code: W Individual is widowed. Enter a marital code: m An invalid code was entered. //results in an error message

Slide 4- 39

Monthly Sales Income

Greater than or equal to $50,000 $375 plus 16% of sales

Less than $50,000 but greater than or equal to $40,000 $350 plus 14% sales

Less than $40,000 but greater than or equal to $30,000 $325 plus 12% sales

Less than $30,000 but greater than or equal to $20,000 $300 plus 9% sales

Less than $20,000 but greater than or equal to $10,000 $250 plus 5% of sales

Less than $10,000 $200 plus 3% of sales

Exercise: extended if statement (if-else chain):

Calculate the monthly income of a salesperson by using the following commission schedule:

Slide 4- 40

//Calculates salesperson's income if(monthly_sales >= 50000.00)

income = 375.00 + .16 * monthly_sales;else if(monthly_sales >= 40000.00) income = 350.00 + .14 * monthly_sales;else if(monthly_sales >= 30000.00) income = 325.00 + .12 * monthly_sales;else if(monthly_sales >= 20000.00) income = 300.00 + .09 * monthly_sales;else if(monthly_sales >= 10000.00) income = 250.00 + .05 * monthly_sales; else income = 200.00 + .03 * monthly_sales;

int main ( void ){ //stores salesperson’s name, monthly sales and calculated income double monthly_sales = 0.0, income = 0.0; string salesperson_name;

//Prompts user for salesperson's name & sales cout << "Please enter the saleperson's name: "; getline(cin, salesperson_name); cout << "Enter the value of monthly sales: " << endl; cin >> monthly_sales;

//Calculates salesperson's income if (monthly_sales >= 50000.00)

income = 375.00 + .16 * monthly_sales;else if (monthly_sales >= 40000.00) income = 350.00 + .14 * monthly_sales;else if (monthly_sales >= 30000.00) income = 325.00 + .12 * monthly_sales;else if (monthly_sales >= 20000.00) income = 300.00 + .09 * monthly_sales;else if (monthly_sales >= 10000.00) income = 250.00 + .05 * monthly_sales; else income = 200.00 + .03 * monthly_sales;//Displays salesperson's name & income cout << setprecision(2) << fixed <<showpoint;

cout << salesperson_name << " has earned a total monthly income of $" << income << endl << " based on monthly sales of $" << monthly_sales;

return 0;}

Outline

• Relational Operations• If statement

– If – If/else statement– If/else if

• Logical operators• Switch

Simple boolean expression use relational and/or logical operators.

6 Relational (or comparison) Operators< <= > >= == !=

3 Logical Operators! && ||

Boolean 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 ifboth p and q are true. It is false otherwise.

p || q p OR q p || q is true if eitherp or q or both are true.

It is false otherwise.

Truth Table – Logical “AND”

TRUE TRUE

TRUE FALSE

FALSE TRUE

FALSE FALSE

X Y X && Y

Truth Table – Logical “AND”

TRUE TRUE TRUE

TRUE FALSE FALSE

FALSE TRUE FALSE

FALSE FALSE FALSE

X Y X && Y

Truth Table – Logical “OR”

TRUE TRUE

TRUE FALSE

FALSE TRUE

FALSE FALSE

X Y X || Y

Truth Table – Logical “OR”

TRUE TRUE TRUE

TRUE FALSE TRUE

FALSE TRUE TRUE

FALSE FALSE FALSE

X Y X || Y

Truth Table – Logical “not”

TRUE

FALSE

X !X

Truth Table – Logical “not”

TRUE FALSE

FALSE TRUE

X !X

If the Boolean expression is true, the combined expression is false.

If the Boolean expression is false, the combined expression is true.

Logical Operators - examples

int x = 12, y = 5, z = -4; (x > y) && (y > z)

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

(x <= z) || (y == z)

(x <= z) || (y != z)

!(x >= z)

Logical Operators - examples

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

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

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

(x <= z) || (y == z) false

(x <= z) || (y != z) true

!(x >= z) false

Write an expression for each• tax_rate is over 25% and income is less

than $20000(tax_rate > 0.25) && (income < 20000)

• temperature is less than or equal to 75 or humidity is less than 70%(temperature <= 75) || (humidity < .70)

• age is over 21 and age is less than 60(age > 21) && (age < 60)

• age is 21 or 22(age == 21) || (age == 22)

ExampleThe extended if statement should be as follows to allow the user to enter

either uppercase or lowercase marital codes:

//Displays marital status message if (marital_status == 'M' || marital_status == 'm') cout <<"Individual is married." << endl; else if (marital_status == 'S' || marital_status == 's')

cout <<"Individual is single." << endl; else if (marital_status == 'D' || marital_status == 'd')

cout <<"Individual is divorced." << endl; else if (marital_status == 'W' || marital_status == 'w')

cout <<"Individual is widowed." << endl; else

cout << "An invalid code was entered." << endl;

Slide 4- 57

Logical Operators - notes

• ! has highest precedence, followed by &&, then ||

• If the value of an expression can be determined by evaluating just the sub-expression on left side of a logical operator, then the sub-expression on the right side will not be evaluated (short circuit evaluation)

• 3 logical operators order of precedence

! && ||

Slide 4- 58

As with all expressions, parentheses can be used to alter the assigned operator priority and to improve the readability of relational expressions. By evaluating the expressions within the parentheses first, we find that the following compound condition is evaluated as shown:

(6 * 3 == 36 / 2) || (13 < 3 * 3 + 4) && !(6 – 2 < 5) (18 == 18) || ( 13 < 9 + 4 ) && !( 4 < 5 ) 1 || (13 < 13) && !1 1 || 0 && 0 1 || 0 1

Slide 4- 59

Nested if statements

An if-else statement can contain simple or compound statements. Any valid C++ statement can be used, including another if-else statement. Thus, one or more if-else statements can be included within either part of an if-else statement.

for statement1 in the if statement if ( hours < 9 ) statement1; else cout << “pop”; results in the nested if statement if ( hours < 9 ) { cout << “snap”;

if ( hours > 6 ) cout << “snap extra”; } else cout << “pop”;

Slide 4- 60

The braces around the inner one-way if are essential, because in their absence, C++ associates an else with the closest unpaired if. Thus, without the braces, the foregoing statement is equivalent to if ( hours < 9 ) if ( hours > 6 ) cout << “snap”; else cout << “pop”;

Here, the else is paired with the inner if, which destroys the meaning of the original if-else statement. Note also that the indentation is irrelevant as far as the computer is concerned. Whether the indentation exists or not, the statement is compiled by associating the last else with the closest unpaired if, unless braces are used to alter the default pairing.

The process of nesting if statements can be extended indefinitely, so the cout << “snap”; statement could itself be replaced by either a complete if-else statement or another one-way if statement.

Outline

• Relational Operations• If statement

– If – If/else statement– If/else if

• Logical operators• Switch

Slide 4- 62

Multi-alternative Selectionis also called multi-way branching, and

can be accomplished by one of two methods: Using sequential if statements

if ( Expression1 ) Statement1;

if ( Expression2 )Statement2;

.

.

.if ( ExpressionN )

StatementN;

One or more of these statements may be executed.

Used when more than one condition can be true

Slide 4- 63

Slide 4- 67

switch statement formatswitch (expression) //integer or character{ // start of compound statement

case exp1: // terminate with a colon

statement1; break;

case exp2: // terminate with a colon statement2; break;

...case expn: // terminate with a colon

statementn; break;

default: // terminate with a colon statementn+1;} //end of switch & compound

statement

Slide 4- 68

switch statement requirements

1) expression must be an integer variable or an expression that evaluates to an integer value

2) exp1 through expn must be constant integer expressions or literals, and must be unique in the switch statement

3) default is optional but highly recommended

Slide 4- 69

switch statement – how it works

1) expression is evaluated2) The value of expression is compared against exp1

through expn. 3) If expression matches value expi, the program

branches to the statement following expi and continues to the end of the switch

4) If no matching value is found, the program branches to the statement after default:

Slide 4- 70

break statement

• Used to exit a switch statement• Useful to execute a single case statement

without executing the statements following it• If it is left out, the program "falls through" the

remaining statements in the switch statement

• Used to stop execution in the current block

Slide 4- 71

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Multiple case values switch (number)

{ case 1: cout << "Have a Good Morning" << endl; break; case 2: cout << "Have a Happy Day" << endl; break; case 3: case 4: case 5: cout << "Have a Nice Evening" << endl; break; }

We can use multiple case values to refer to the same set of statements; the default label is optional

Character data types in “Switch” switch (choice) { case 'a': case 'e': case 'i': case 'o': case 'u': cout << "\nThe character in choice is a vowel." << endl; break; default: cout << "\nThe character in choice is not a vowel." << endl; break; //this break is optional } // end of switch statement

Because character data types are always converted to integers in an expression, a switch statement can also be used to "switch" on the basis of the value of a character expression