1. Chapter 4:
Making Decisions

2. Outline Relational Operations If statement Logical operators Switch If
If/else statement
If/else if
Logical operators
Switch

3. Relational Operators 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

4. Relational Expressions
Boolean expressions – true or false
Examples:
12 > 5 is true
7 <= 5 is false
if x is 10, then
x == 10 is true,
x != 8 is true, and
x == 8 is false

5. 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

6. 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 true
y = x

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

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

9. Outline Relational Operations If statement Logical operators Switch If
If/else statement
If/else if
Logical operators
Switch

10. The if Statement
Allows statements to be conditionally executed or skipped over
Models the way we mentally evaluate situations:
"If it is raining, take an umbrella."
"If it is cold outside, wear a coat."

11. Flowchart for Evaluating a Decision

12. 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
expression
statement
FALSE

13. Flowchart for Evaluating a Decision

14. 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.

15. Example: Check a Car’s Mileage
#include <iostream>
int main ( void ) {
const double LIMIT = ; //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; }

16. Example: Results Output from first run:
Please enter the mileage recorded on the car:
This car is over the limit.
End of program.
Output from second run:
Please enter the mileage recorded on the car:

17. (Program Continues)

20. Flowchart for Lines 21 and 22

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

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)

23. Outline Relational Operations If statement Logical operators Switch If
If/else statement
If/else if
Logical operators
Switch

24. if-else provides two-way selection between executing one of 2 clauses: the if clause or the else clause
FALSE
TRUE
expression
else clause
if clause 24

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 block
else
statement2; // or block

26. if-else Syntax if ( expression ) statementA; else statementB;
NOTE: StatementA and StatementB each can be a single statement, a null statement, or a block.

27. if/else – what happens To evaluate: if (expression) statement1; else
If the expression is true, then statement1 is executed and statement2 is skipped.
If the expression is false, then statement1 is skipped and statement2 is executed.

28. Use of blocks recommended
if ( expression ) { }
else
“if clause”
“else clause”

29. A compound statement consists of
individual statements enclosed within braces.
Syntax:
if(expression) {
statement1;
statement2;
statement3; }
else
statement4;
statement5;
statement6;

30. Exercise: mail order
Assign value .25 to discount_rate and assign value to ship_cost if purchase is over
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;

31. These braces cannot be omitted
if ( purchase > ) {
discount_rate = .25 ;
ship_cost = ; }
else
discount_rate = .15 ;
ship_cost = ;
total_bill = purchase * (1.0 – discount_rate) + ship_cost ;

32. (Program Continues)

34. Outline Relational Operations If statement Logical operators Switch If
If/else statement
If/else if
Logical operators
Switch

35. Used when only one condition can be true
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 )
StatementN
else
Statement N+1
EXACTLY 1 block of these statements will be executed.
Used when only one condition can be true

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

37. Example int 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; }

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

39. Exercise: extended if statement (if-else chain):
Calculate the monthly income of a salesperson by using the following
commission schedule:
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

40. //Calculates salesperson's income
if(monthly_sales >= )
income = * monthly_sales;
else if(monthly_sales >= )
income = * monthly_sales;
else if(monthly_sales >= )
income = * monthly_sales;
else if(monthly_sales >= )
income = * monthly_sales;
else if(monthly_sales >= )
income = * monthly_sales;
else income = * monthly_sales;

41. 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 >= )
income = * monthly_sales;
else if (monthly_sales >= )
income = * monthly_sales;
else if (monthly_sales >= )
income = * monthly_sales;
else if (monthly_sales >= )
income = * monthly_sales;
else if (monthly_sales >= )
income = * monthly_sales;
else income = * 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; }

42. Outline Relational Operations If statement Logical operators Switch If
If/else statement
If/else if
Logical operators
Switch

43. Boolean Expressions
Simple boolean expression use relational and/or logical operators.
6 Relational (or comparison) Operators
< <= > >= == !=
3 Logical Operators
! && ||

44. Logical Operators
Used to create relational expressions from other relational expressions
Operators, meaning, and explanation:
&&
AND
New relational expression is true if both expressions are true || OR
New relational expression is true if either expression is true !
NOT
Reverses the value of an expression – true expression becomes false, and false becomes true

45. 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 if
both p and q are true.
It is false otherwise.
p || q p OR q p || q is true if either
p or q or both are true It is false otherwise.

46. Truth Table – Logical “AND”
X Y X && Y
TRUE
FALSE

47. Truth Table – Logical “AND”
X Y X && Y
TRUE
FALSE

48. Truth Table – Logical “OR”
X Y X || Y
TRUE
FALSE

49. Truth Table – Logical “OR”
X Y X || Y
TRUE
FALSE

50. Truth Table – Logical “not”
X !X
TRUE
FALSE

51. Truth Table – Logical “not”
X !X
TRUE
FALSE
If the Boolean expression is true, the combined expression is false.
If the Boolean expression is false, the combined expression is true.

52. Operator Meaning Associativity
! NOT Right
* / % Mult, Div, Modulus Left
+ , Add, Sub Left
< Less than Left
<= Less than or equal to Left
> Greater than Left
>= Greater than or equal to Left
== Is equal to Left
!= Is not equal to Left
&& AND Left
|| OR Left
= Assignment Right

53. 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)

54. Logical Operators - examples
int x = 12, y = 5, z = -4;
(x > y) && (y > z)
true
(x > y) && (z > y)
false
(x <= z) || (y == z)
(x <= z) || (y != z)
!(x >= z)

55. 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)

56. Example
The 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;

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
! && ||

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 < ) && !( 4 < 5 )
1 || (13 < 13) && !1
1 || && 0
1 || 1

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

60. because in their absence, C++ associates an else with the closest
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.

61. Outline Relational Operations If statement Logical operators Switch If
If/else statement
If/else if
Logical operators
Switch

62. Used when more than one condition can be true
Multi-alternative Selection
is 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

63. Slide 4- 63

64. switch Statement
Is a selection control structure for multi-way branching.
SYNTAX
switch ( IntegralExpression ) {
case constant1 :
statement(s); // optional
break;
case constant2 : .
default : // optional
statement(s); // optional }

65. Control in switch statement
control branches to the statement following the case label that matches the value of IntegralExpression. Control proceeds through all remaining statements, including the default, unless redirected with break
if no case label matches the value of IntegralExpression, control branches to the default label, if present--otherwise control passes to the statement following the entire switch statement
forgetting to use break can cause logical errors because after a branch is taken, control proceeds sequentially until either break or the end of the switch statement occurs

66. The switch Statement
Used to select among statements from several alternatives
In some cases, can be used instead of if/else if statements

67. switch statement format
switch (expression) //integer or character
{ // start of compound statement
case exp1: // terminate with a colon
statement1;
break;
case exp2: // terminate with a colon
statement2;
...
case expn: // terminate with a colon
statementn;
default: // terminate with a colon
statementn+1;
} //end of switch & compound statement

68. switch statement requirements
1) expression must be an integer variable or an expression that evaluates to an integer value
exp1 through expn must be constant integer expressions or literals, and must be unique in the switch statement
default is optional but highly recommended

69. switch statement – how it works
1) expression is evaluated
The value of expression is compared against exp1 through expn.
If expression matches value expi, the program branches to the statement following expi and continues to the end of the switch
If no matching value is found, the program branches to the statement after default:

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

72. Slide 4- 72

75. Multiple case values
switch (number) {
case 1:
cout << "Have a Good Morning" << endl;
break;
case 2:
cout << "Have a Happy Day" << endl;
case 3: case 4: case 5:
cout << "Have a Nice Evening" << endl; }
We can use multiple case values to refer to the same set of statements; the default label is optional

76. 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