1. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 01360972001 Chapter 3 Selections

2. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 2 Motivations If you assigned a negative value for radius in Listing 2.1, ComputeArea.cpp, the program would print an invalid result. If the radius is negative, you don't want the program to compute the area. How can you deal with this situation?

3. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 01360972003 Objectives F To declare bool type and write Boolean expressions using comparison operators (§3.2). F To implement selection control using one-way if statements (§3.3) F To program the GuessBirthDate game using one-way if statements (§3.4). F To implement selection control using two-way if statements (§3.5). F To implement selection control using nested if statements (§3.6). F To avoid common errors in if statements (§3.7). F To generate random numbers using the rand function and set a seed using the srand function (§3.10). F To combine conditions using logical operators (&&, ||, and !) (§3.11). F To implement selection control using switch statements (§3.14). F To write expressions using the conditional operator (§3.15). // skip F To format output using the stream manipulators (§3.16). F To examine the rules governing operator precedence and operator associativity (§3.17).

4. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 4 The bool Type and Operators Often in a program you need to compare two values, such as whether i is greater than j. C++ provides six relational operators (also known as comparison operators) in Table 3.1 that can be used to compare two values.

5. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 5 Comparison Operators

6. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 6 One-way if Statements if (booleanExpression) { statement(s); } if (radius >= 0) { area = radius * radius * PI; cout << "The area for the circle of " << " radius " << radius << " is " << area; } <- These are Flow Charts

7. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 7 Note

8. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 8 Examples Listing 3.1 gives a program that checks whether a number is even or odd. The program prompts the user to enter an integer (line 9) and displays “number is even” if it is even (lines 11-12) and “number is odd” if it is odd (lines 14-15). TestBoolean Run

9. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 9 Caution Adding a semicolon at the end of an if clause is a common mistake. This mistake is hard to find, because it is not a compilation error or a runtime error, it is a logic error. This error often occurs when you use the next-line block style.

10. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 10 The if...else Statement if (booleanExpression) { statement(s)-for-the-true-case; } else { statement(s)-for-the-false-case; }

11. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 11 Nested if Statements if (i > k) { if (j > k) cout << "i and j are greater than k"; } else cout << "i is less than or equal to k";

12. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 12 Multiple Alternative if Statements

13. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 13 Trace if-else statement if (score >= 90.0) grade = 'A'; else if (score >= 80.0) grade = 'B'; else if (score >= 70.0) grade = 'C'; else if (score >= 60.0) grade = 'D'; else grade = 'F'; Suppose score is 70.0The condition is false animation

14. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 14 Trace if-else statement if (score >= 90.0) grade = 'A'; else if (score >= 80.0) grade = 'B'; else if (score >= 70.0) grade = 'C'; else if (score >= 60.0) grade = 'D'; else grade = 'F'; Suppose score is 70.0The condition is false animation

15. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 15 Trace if-else statement if (score >= 90.0) grade = 'A'; else if (score >= 80.0) grade = 'B'; else if (score >= 70.0) grade = 'C'; else if (score >= 60.0) grade = 'D'; else grade = 'F'; Suppose score is 70.0The condition is true animation

16. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 16 Trace if-else statement if (score >= 90.0) grade = 'A'; else if (score >= 80.0) grade = 'B'; else if (score >= 70.0) grade = 'C'; else if (score >= 60.0) grade = 'D'; else grade = 'F'; Suppose score is 70.0grade is C animation

17. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 17 Trace if-else statement if (score >= 90.0) grade = 'A'; else if (score >= 80.0) grade = 'B'; else if (score >= 70.0) grade = 'C'; else if (score >= 60.0) grade = 'D'; else grade = 'F'; Suppose score is 70.0Exit the if statement animation

18. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 18 Note The else clause matches the most recent if clause in the same block. Note visual studio indents automatically

19. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 19 Note, cont. Nothing is printed from the preceding statement. To force the else clause to match the first if clause, you must add a pair of braces: int i = 1; int j = 2; int k = 3; if (i > j) { if (i > k) cout << "A"; } else cout << "B"; This statement prints B. Note – much better way to do this using &&

20. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 20 TIP

21. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 21 CAUTION i.e., Department of redundancy department

22. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 22 Common Errors in Selection Statements Common Error 1: Forgetting Necessary Braces

23. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 23 Common Errors in Selection Statements Common Error 2: Wrong Semicolon at the if Line

24. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 24 Common Errors in Selection Statements Common Error 3: Mistakenly Using = for == if (count = 1) cout << "count is zero" << endl; else cout << "count is not zero" << endl; Most programming languages will blow up with this C/C++ does unexpected things – yet another example of C++’s various sharp corners.

25. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 25 Common Errors in Selection Statements Common Error 4: Redundant Testing of Boolean Values

26. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 26 Example: Computing Taxes The US federal personal income tax is calculated based on the filing status and taxable income. There are four filing statuses: single filers, married filing jointly, married filing separately, and head of household. The tax rates for 2002 are shown in Table 3.6.

27. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 27 Example: Computing Taxes, cont. ComputeTaxWithSelectionStatement if (status == 0) { // Compute tax for single filers } else if (status == 1) { // Compute tax for married file jointly } else if (status == 2) { // Compute tax for married file separately } else if (status == 3) { // Compute tax for head of household } else { // Display wrong status }

28. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 28 Logical Operators

29. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 29 Truth Table for Operator !

30. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 30 Truth Table for Operator &&

31. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 31 Truth Table for Operator ||

32. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 32 Examples Listing 3.3 gives a program that checks whether a number is divisible by 2 and 3, whether a number is divisible by 2 or 3, and whether a number is divisible by 2 or 3 but not both: TestBooleanOperators Run

33. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 33 Short-Circuit Operator When evaluating p1 && p2, C++ first evaluates p1 and then evaluates p2 if p1 is true; if p1 is false, it does not evaluate p2. When evaluating p1 || p2, C++ first evaluates p1 and then evaluates p2 if p1 is false; if p1 is true, it does not evaluate p2. Therefore, && is referred to as the conditional or short-circuit AND operator, and || is referred to as the conditional or short-circuit OR operator.

34. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 34 Examples Write a program that lets the user enter a year and checks whether it is a leap year. A year is a leap year if it is divisible by 4 but not by 100 or if it is divisible by 400. So you can use the following Boolean expression to check whether a year is a leap year: (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0) LeapYear Run

35. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 35 switch Statements switch (status) { case 0: compute taxes for single filers; break; case 1: compute taxes for married file jointly; break; case 2: compute taxes for married file separately; break; case 3: compute taxes for head of household; break; default: System.out.println("Errors: invalid status"); System.exit(0); }

36. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 36 switch Statement Flow Chart

37. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 37 switch Statement Rules switch (switch-expression) { case value1: statement(s)1; break; case value2: statement(s)2; break; … case valueN: statement(s)N; break; default: statement(s)-for-default; } The switch-expression must yield a value of char, byte, short, or int type and must always be enclosed in parentheses. The value1,..., and valueN must have the same data type as the value of the switch-expression. The resulting statements in the case statement are executed when the value in the case statement matches the value of the switch- expression. Note that value1,..., and valueN are constant expressions, meaning that they cannot contain variables in the expression, such as 1 + x.

38. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 38 switch Statement Rules The keyword break is optional, but it should be used at the end of each case in order to terminate the remainder of the switch statement. If the break statement is not present, the next case statement will be executed. switch (switch-expression) { case value1: statement(s)1; break; case value2: statement(s)2; break; … case valueN: statement(s)N; break; default: statement(s)-for-default; } The default case, which is optional, can be used to perform actions when none of the specified cases matches the switch-expression. The case statements are executed in sequential order, but the order of the cases (including the default case) does not matter. However, it is good programming style to follow the logical sequence of the cases and place the default case at the end.

39. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 39 Trace switch statement switch (ch) { case 'a': cout << ch; case 'b': cout << ch; case 'c': cout << ch; } Suppose ch is 'a': animation

40. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 40 Trace switch statement switch (ch) { case 'a': cout << ch; case 'b': cout << ch; case 'c': cout << ch; } ch is 'a': animation

41. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 41 Trace switch statement switch (ch) { case 'a': cout << ch; case 'b': cout << ch; case 'c': cout << ch; } Execute this line animation

42. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 42 Trace switch statement switch (ch) { case 'a': cout << ch; case 'b': cout << ch; case 'c': cout << ch; } animation Execute this line

43. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 43 Trace switch statement switch (ch) { case 'a': cout << ch; case 'b': cout << ch; case 'c': cout << ch; } animation Execute this line

44. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 44 Trace switch statement switch (ch) { case 'a': cout << ch; case 'b': cout << ch; case 'c': cout << ch; } Next statement; animation Execute next statement

45. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 45 Trace switch statement switch (ch) { case 'a': cout << ch; break; case 'b': cout << ch ; break ; case 'c': cout << ch ; } Suppose ch is 'a': animation

46. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 46 Trace switch statement switch (ch) { case 'a': cout << ch ; break; case 'b': cout << ch ; break ; case 'c': cout << ch; } ch is 'a': animation

47. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 47 Trace switch statement switch (ch) { case 'a': cout << ch ; break; case 'b': cout << ch ; break ; case 'c': cout << ch ; } Execute this line animation

48. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 48 Trace switch statement switch (ch) { case 'a': cout << ch ; break; case 'b': cout << ch ; break ; case 'c': cout << ch ; } Execute this line animation

49. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 49 Trace switch statement switch (ch) { case 'a': cout << ch ; break; case 'b': cout << ch ; break ; case 'c': cout << ch ; } Next statement; Execute next statement animation

50. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 50 Formatting Output

51. Liang, Introduction to Programming with C++, Second Edition, (c) 2010 Pearson Education, Inc. All rights reserved. 0136097200 51 Operator Precedence How to evaluate 3 + 4 * 4 > 5 * (4 + 3) – 1?