Chapter 4: Making Decisions

1. Chapter 4: Making Decisions

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

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:

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+2 true y = x + 3 7

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 0

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

10. ifstatement 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

11. Flowchart for Evaluating a Decision

12. The if Statement • General Format: if (expression) statement; • If the expression is true, then statementis executed. • If the expressionis false, then statementis skipped.

13. Example: Check a Car’s Mileage #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; }

14. Example: Results Output from first run: Please enter the mileage recorded on the car: 35620.8 This 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.

15. (Program Continues)

18. Flowchart for Lines 21 and 22

19. 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

20. 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)

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

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

23. 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

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

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

26. Exercise: mail order Assign 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; • }

27. 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 ;

28. (Program Continues)

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

31. EXACTLY 1 block of these statements will be executed. if/else ifformat (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 Used when only one condition can be true

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

33. 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; }

34. 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

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

36. //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;

37. 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; }

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

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

40. 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.

41. Truth Table – Logical “AND” X Y X && Y

42. Truth Table – Logical “AND” X Y X && Y

43. Truth Table – Logical “OR” X Y X || Y

44. Truth Table – Logical “OR” X Y X || Y

45. Truth Table – Logical “not” X !X

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

47. Logical Operators - examples int x = 12, y = 5, z = -4;

48. Logical Operators - examples int x = 12, y = 5, z = -4;

49. 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 andage is less than 60 (age > 21) && (age < 60) • age is 21 or 22 (age == 21) || (age == 22)

50. 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;