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Chapter 4 - Selection Structures: if and switch Statements

Chapter 4 - Selection Structures: if and switch Statements. Jason C. H. Chen, Ph.D. Professor of MIS School of Business Administration Gonzaga University Spokane, WA 99258 chen@gonzaga.edu. Selection Statements.

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Chapter 4 - Selection Structures: if and switch Statements

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  1. Chapter 4 - Selection Structures:if and switch Statements Jason C. H. Chen, Ph.D. Professor of MIS School of Business Administration Gonzaga University Spokane, WA 99258 chen@gonzaga.edu

  2. Selection Statements • In this chapter we study statements that allow alternatives to straight sequential processing. In particular: • if statements (do this only if a condition is true) • if-else statements (do either this or that) • Logical expressions (evaluate to true or false) • Boolean operators (not: ! and: && or: ||)

  3. 4.1 Control Structures • Programs must often anticipate a variety of situations. • Consider an Automated Teller Machine: • ATMs must serve valid bank customers. They must also reject invalid PINs. • The code that controls an ATM must permit these different requests. • Software developers must implement code that anticipates all possible transactions.

  4. 4.2 Logical Expressions False False True True

  5. Boolean Variables • bool variable • Included with C++ compiler bool leapYear; leapYear = true; // Non zero return value leapYear = false; // Zero return value

  6. Boolean Expressions Examples (Write T for True, or F for False): int n1 = 55; int n2 = 75; n1 < n2 // _____ n1 > n2 // _____ (n1 + 35) > n2 // _____ (n1-n2) < 0.1 // _____ n1 == n2 // _____

  7. Logical Expressions • Logical expressions often use these relational operators: (Table 4.1) p.171

  8. Logical Operators • Logical operator (&& means AND) used in an if...else statement: ( (tryIt >= 0) && (tryIt <= 100) ) • Logical operator (| | means OR) used in an if...else statement: ( (tryIt >= 0) | | (tryIt <= 100) )

  9. Using && • Assume tryIt = 90, • Is tryIt within the range of 0 and 100 ? ( (tryIt >= 0) && (tryIt <= 100) ) ( ( 90 >= 0) && ( 90 <= 100) ) ( 1 && 1 ) 1

  10. Using || • Assume tryIt = 99 • Is tryIt outside the range of 0 and 100 ? ( (tryIt < 0) ¦¦ (tryIt > 100) ) ( ( 99 < 0) ¦¦ ( 99 > 100) ) ( 0 ¦¦ 0 ) 0

  11. Truth Tables for Boolean Operators • Truth tables Logical operators !, ¦¦, && • 1 is a symbol for true • 0 is a symbol for false See Table 4.3 – 4.5

  12. Precedence of Operators • Precedence: most operators are evaluated (grouped) in a left-to-right order: • a / b / c / d is equivalent to (((a/b)/c)/d) • Assignment operators group in a right-to-left order so the expression • x = y = z = 0.0 is equivalent to (x=(y=(z=0.0)))

  13. Precedence of Operators(Table 4.6; p.174) Do Example 4.2 (p.172)

  14. Boolean Assignment bool same; same = true; Form: variable = expression; Example: same = (x = = y); Exercise 4.2 (p.176)

  15. 4.3 Introduction to the if Dependent Control Statement • The if is the first statement that alters strict sequential control. • General form if (logical-expression) true-part; • logical-expression: any expression that evaluates to nonzero (true) or zero (false). • In C++, almost everything is true or false.

  16. if Control Statementswith Two Alternatives • The logical expression is evaluated. When true, the true-part is executed and the false-part is disregarded. When the logical expression is false, the false-part executes. • General Form if (logical-expression) true-part; else false-part;

  17. What happens when an if statement executes? Write in C++: • After the logical expression of the if statement evaluates, the true-part executes only if the logical expression was true. False gross >100.0 if (gross > 100.0) net =gross-tax; else net=gross; True net=gross-tax net=gross

  18. Programming Tip • Using = for = = is a common mistake. For example the following statements are legal: • int x = 25; Because assignment statements evaluate to the expression on the right of =, x = 1 is always 1, which is nonzero, which is true: if (x = 1) // should be (x == 1) Exercise 4.3 (p.182)

  19. 4.4 if Statements with Compound Alternatives • General form (also known as a block): { statement-1 ; statement-2 ; ... statement-N ; } • The compound statement groups together many statements that are treated as one.

  20. Writing Compound Statements if (transactionType == 'c') { // process check cout << "Check for $" << transactionAmount << endl; balance = balance - transactionAmount; } else { // process deposit cout << "Deposit of $" << transactionAmount << endl; balance = balance + transactionAmount; }

  21. Exercise 4.4 (p.186) 1. Debug syntax errors? If (x > y) { x = x + 10.0 cout << “x bigger” << endl; } else cout << “x smaller” << endl; cout << “y is: “ << y << endl; 2. What is the output when a) x is 5 y is 3 ; b) x is 6 y is 9

  22. Exercise 4.4 (p.186) - cont. What is the output when a) x is 5 y is 3 If (x > y) { x = x + 10.0; cout << “x bigger” << endl; } else { cout << “x smaller” << endl; cout << “y is: “ << y << endl; } b) x is 6 y is 9

  23. Break ! (Ch. 4 - Part I)

  24. 4.5 Decision Steps in Algorithms • Algorithm steps that select from a choice of actions are called decision steps. The algorithm in the following case contains decisions steps to compute an employee’s gross and net pay after deductions. The decision steps are coded as if statements. • Payroll Case Study (p.187)

  25. Decision Steps in Algorithms • Statement:  Your company pays its hourly workers once a week. An employee’s pay is based upon the number of hours worked (to the nearest half hour) and the employee’s hourly pay rate. Weekly hours exceeding 40 are paid at a rate of time and a half. Employees who earn over $100 a week must pay union dues of $15 per week. Write a payroll program that will determine the gross pay and net pay for an employee.

  26. Decision Steps in Algorithms • Analysis:  The problem data include the input data for hours worked and hourly pay and two required outputs, gross pay and net pay. There are also several constants: the union dues ($15), the minimum weekly earnings before dues must be paid ($100), the maximum hours before overtime must be paid (40), and the overtime rate (1.5 times the usual hourly rate). With this information, we can begin to write the data requirements for this problem. We can model all data using the money (see Section 3.7) and float data types.

  27. Decision Steps in Algorithms • Program Design:The problem solution requires that the program read the hours worked and the hourly rate before performing any computations. After reading these data, we need to compute and then display the gross pay and net pay. The structure chart for this problem (Figure 4.6) shows the decomposition of the original problem into five subproblems. We will write three of the subproblems as functions. For these three subproblems, the corresponding function name appears under its box in the structure chart.

  28. Decision Steps in Algorithms • Display user instructions (function instructUser). • Enter hours worked and hourly rate. • Compute gross pay (function computeGross). • Compute net pay (function computeNet). • Display gross pay and net pay.

  29. payrollFunction.cpp 5 Output display: gross salary, net salary 1 User input: #hours, hourly_rate Level - 0 Level - 1 2 3 4

  30. Payroll.cpp Program output This program computes gross and net salary. A dues amount of $15.00 is deducted for an employee who earns more than $100.00 Overtime is paid at the rate of 1.5 times the regular rate on hours worked over 40 Enter hours worked and hourly rate on separate lines after the prompts. Press <return> after typing each number.

  31. Payroll.cpp Program output Hours worked: 50 Hourly rate: 6 Gross salary is $330.00 Net salary is $315.00 Payroll Case Study: payroolFunctions.cpp (p.192)

  32. // File: payrollFunctions_money.cpp (p.192) // Computes and displays gross pay and net pay given an hourly // rate and number of hours worked. Deducts union dues of $15 // if gross salary exceeds $100; otherwise, deducts no dues. #include <iostream> #include "money.cpp” using namespace std; // Functions prototypes ... void instructUser(); money computeGross(float, money); money computeNet(money); //GLOBAL VARIABLES // max earnings before dues (dollars) const money MAX_NO_DUES = 100.00; // dues amount (dollars) const money dues = 15.00; // max hours before overtime const float MAX_NO_OVERTIME = 40.0; const float OVERTIME_RATE = 1.5; // overtime rate int main () { float hours; // input: hours worked float rate; // input: hourly pay rate (dollars) money gross; // output: gross pay (dollars) money net; // output: net pay (dollars) //Function Call // Display user instructions. instructUser(); // Enter hours and rate. cout << "Hours worked: "; cin >> hours; cout << "Hourly rate: "; cin >> rate; //Function Call // Compute gross salary. gross = computeGross(hours, rate); // Compute net salary. net = computeNet(gross); // Print gross and net. cout << "Gross salary is " << gross << endl; cout << "Net salary is " << net << endl; return 0; }

  33. Funciton: instructUser // Insert lower-level functions here. // ... // Displays user instructions void instructUser() { cout << "This program computes gross and net salary." << endl; cout << "A dues amount of " << dues << " is deducted for" << endl; cout << "an employee who earns more than " << MAX_NO_DUES << endl << endl; cout << "Overtime is paid at the rate of " << OVERTIME_RATE << endl; cout << "times the regular rate for hours worked over " << MAX_NO_OVERTIME << endl << endl; cout << "Enter hours worked and hourly rate" << endl; cout << "on separate lines after the prompts." << endl; cout << "Press <return> after typing each number." << endl << endl; } // end instructUser

  34. Two Functions: computeGross & computeNet //Function Definitions // FIND THE GROSS PAY money computeGross (float hours, // IN: number of hours worked money rate) // IN: hourly pay rate (dollars) { // Local data ... money gross; // RESULT: gross pay (dollars) money regularPay; // pay for first 40 hours money overtimePay; // pay for hours in excess of 40 // Compute gross pay. if (hours > MAX_NO_OVERTIME) { regularPay = MAX_NO_OVERTIME * rate; overtimePay = (hours - MAX_NO_OVERTIME) * OVERTIME_RATE * rate; gross = regularPay + overtimePay; } else gross = hours * rate; return gross; } // end computeGross // FIND THE NET PAY money computeNet (money gross) // IN: gross salary (dollars) { // Local data ... money net; // RESULT: net pay (dollars) // Compute net pay. if (gross > MAX_NO_DUES) net = gross - dues; // deduct dues amount else net = gross; // no deductions return net; } // end computeNet

  35. 4.6 Checking the Correctness of an Algorithm • Verifying the correctness of an algorithm is a critical step in algorithm design and often saves hours of coding and testing time. • We will now trace the execution of the refined algorithm for the payroll problem solved in the last section.

  36. Checking the Correctness of an Algorithm 1. Display user instructions. 2. Enter hours worked and hourly rate. 3. Compute gross pay. 3.1. If the hours worked exceed 40.0 (max hours before overtime) 3.1.1. Compute regularPay. 3.1.2. Compute overtimePay. 3.1.3. Add regularPay to overtimePay to get gross. else

  37. Checking the Correctness of an Algorithm 3.1.4. Compute gross as hours * rate. 4. Compute net pay. 4.1. If gross is larger than $100.00 4.1.1. Deduct the dues of $15.00 from gross pay. else 4.1.2. Deduct no dues. 5. Display gross and net pay.

  38. 4.7 Nested if Statements and Multiple Alternative Decisions • Nested logic is one control structure containing another similar control structure. • An if...else inside another if...else. e.g. (the 2nd if is placed on the same line as the 1st):

  39. Example of nested logic(p.203) if(x > 0) numPos = numPos + 1; else if (x < 0) numNeg = numNeg + 1; else // x equals 0 numZero = numZero + 1; Assume all variables initialized to 0 X numPos numNeg numZero 3.0 _______ _______ _______ -3.6 _______ _______ _______ 4.0 _______ _______ _______

  40. Writing a Nested if as a Multiple-Alternative Decision • Nested if statements can become quite complex. If there are more than three alternatives and indentation is not consistent, it may be difficult to determine the logical structure of the if statement.

  41. Order of Conditions (p.203) if (score >= 60) cout << "Grade is D " << endl; else if (score >= 70) cout << "Grade is C " << endl; else if (score >= 80) cout << "Grade is B " << endl; else if (score >= 90) cout << "Grade is A " << endl; else cout << "Grade is F " << endl; Is this a correct logic?

  42. Function displayGrade (p.202) void displayGrade ( int score) { if (score >= 90) cout << "Grade is A " << endl; else if (score >= 80) cout << "Grade is B " << endl; else if (score >= 70) cout << "Grade is C " << endl; else if (score >= 60) cout << "Grade is D " << endl; else cout << "Grade is F " << endl; }

  43. Short Circuit Evaluation (single = = ‘y’ && gender = = ‘m’ && age >= 18) //If single is false, gender and age are not evaluated (single = = ‘y’ || gender = = ‘m’ || age >= 18) //If single is true, gender and age are not evaluated

  44. 4.8 The switch Control Statement switch ( switch-expression ) { casevalue-1: statement(s)-1 break ; ... // many cases are allowed casevalue-n: statement(s)-n break ; default : default-statement(s) }

  45. Switch Control • When a switch statement is encountered, the switch-expression is evaluated. This value is compared to each case value until switch-expression == case value. All statements after the colon : are executed • It is important to include the break statement

  46. ? Example switch Statement: (p.209) • Show output when • watts = '?' ____________? • watts = ’40’ ____________? • watts = ’10' ____________? • watts = ’200' ____________? • watts = ’100' ____________? switch(watts) // Assume char option = '?’ { case 25: cout << " Life expectancy is 2500 hours. " << endl; break; case 40: case 60: cout << " Life expectancy is 1000 hours. " << endl; break; case 75: case 100: cout << " Life expectancy is 750 hours. " << endl; break; default: cout << "Invalid Bulb !!" << endl; } // end switch #1 Exercises 4.8 (p.211)

  47. 4.9 Common Programming Errors • Failing to use { and } if(Grade >= 3.5) // The true-part is the first cout only cout <<"You receive an A !!!"; cout <<"You made it by " << (Grade-3.5) << " points"; else; Any compilation error?

  48. 4.9 Common Programming Errors (cont.) // File: grade_if_else_syntax_error.cpp #include <iostream> using namespace std; int main() { float Grade; cout << "Please enter your grade (a numeric): "; cin >> Grade; if(Grade >= 3.5) // The true-part is the first cout only cout <<"You receive an A !!!"; cout <<"You made it by " << (Grade-3.5) << " points"; else cout << "Sorry, you missed an A. "; cout << "You missed it by " << 3.5-Grade << " points"; return 0; }//main

  49. 4.9 Common Programming Errors (cont.) // File: grade_if_else_logic_error.cpp #include <iostream> using namespace std; int main() { float Grade; cout << "Please enter your grade (a numeric): "; cin >> Grade; if(Grade >= 3.5) { // The true-part is the first cout only cout <<"You receive an A !!!\n"; cout <<"You made it by " << (Grade - 3.5) << " points\n"; } else cout << "Sorry, you missed an A.\n "; cout << "You missed it by " << (3.5 - Grade) << " points\n"; return 0; }//main With the above false part, you could get this confusing output (when Grade = 3.9): You received an A !!!. You made it by 0.4 points. You missed it by -0.4 points

  50. 4.9 Common Programming Errors (cont.) else { cout << "Sorry, you missed an A. \n"; cout << "You missed it by " << (3.5 - Grade) << " points\n"; } return 0; }//main // File: grade_if_else_correct.cpp #include <iostream> using namespace std; int main() { float Grade; cout << "Please enter your grade (a numeric): "; cin >> Grade; if(Grade >= 3.5) { cout <<"You receive an A !!!\n"; cout <<"You made it by " << (Grade - 3.5) << " points\n"; }

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