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Chapter 4: Making Decisions. Outline Relational Operations If statement – If – If/else statement – If/else if Logical operators Switch.

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Presentation on theme: "Chapter 4: Making Decisions. Outline Relational Operations If statement – If – If/else statement – If/else if Logical operators Switch."— Presentation transcript:

1 Chapter 4: Making Decisions

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

3 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

4 Slide 4- 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; EXPRESSIONVALUE 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; EXPRESSIONVALUE 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 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

8 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 3.0); Results: 1 0

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

10 Slide 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 Slide 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 FALSE statement expression

13 Slide Flowchart for Evaluating a Decision

14 Slide 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 #include 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; } Example: Check a Car’s Mileage

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: End of program.

17 Slide (Program Continues)

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20 Slide Flowchart for Lines 21 and 22

21 Slide 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 float s and double s for equality 0 is false ; any other value is true

22 Slide 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 – 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 TRUEFALSE if clauseelse clause expression

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

27 Slide if/else – what happens To evaluate: if (expression) statement1; else statement2; 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 Slide 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 = 5.00 ; } total_bill = purchase * (1.0 – discount_rate) + ship_cost ;

32 (Program Continues)

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

35 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. 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

39 Slide Monthly SalesIncome 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:

40 Slide //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 <

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

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

44 Slide Logical Operators Used to create relational expressions from other relational expressions Operators, meaning, and explanation: && ANDNew relational expression is true if both expressions are true || ORNew relational expression is true if either expression is true ! NOTReverses 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 qp || q is true if either p or q or both are true. It is false otherwise.

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

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

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

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

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

51 Truth Table – Logical “not” TRUEFALSE 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.

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

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 Slide 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 Slide 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 && 0 1 || 0 1

59 Slide 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”;

60 Slide 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 – If – If/else statement – If/else if Logical operators Switch

62 Slide 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

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64 switch Statement Is a selection control structure for multi-way branching. SYNTAX switch ( IntegralExpression ) { case constant1 : statement(s); // optional break; case constant2 : statement(s); // optional break;. 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 Slide The switch Statement Used to select among statements from several alternatives In some cases, can be used instead of if/else if statements

67 Slide 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; break;... case expn: // terminate with a colon statementn; break; default: // terminate with a colon statementn+1; } //end of switch & compound statement

68 Slide 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

69 Slide switch statement – how it works 1) expression is evaluated 2)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:

70 Slide 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

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75 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

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


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