Programming Logic and Design Fifth Edition, Comprehensive

1. Programming Logic and DesignFifth Edition, Comprehensive Chapter 4 Making Decisions

2. Objectives • Evaluate Boolean expressions to make comparisons • Use the relational comparison operators • Learn about AND logic • Learn about OR logic • Make selections within ranges Programming Logic and Design, Fifth Edition, Comprehensive

3. Objectives (continued) • Learn about precedence when combining AND and OR selections • Learn more about the case structure • Use a decision table Programming Logic and Design, Fifth Edition, Comprehensive

4. Evaluating Boolean Expressions to Make Comparisons • Dual-alternative (or binary) selection structure: • Provides an action for each of two possible outcomes Figure 4-1 The dual-alternative selection structure Programming Logic and Design, Fifth Edition, Comprehensive

5. Evaluating Boolean Expressions to Make Comparisons (continued) • Single-alternative (or unary) selection structure • Action is provided for only one outcome Figure 4-2 The single-alternative selection structure Programming Logic and Design, Fifth Edition, Comprehensive

6. Evaluating Boolean Expressions to Make Comparisons (continued) Figure 4-3 Flowchart and pseudocode for overtime payroll program Programming Logic and Design, Fifth Edition, Comprehensive

7. Evaluating Boolean Expressions to Make Comparisons (continued) Figure 4-4 Pseudocode for payroll program with dental insurance determination Programming Logic and Design, Fifth Edition, Comprehensive

8. Evaluating Boolean Expressions to Make Comparisons (continued) • Boolean expression • Represents only one of two states • Evaluates to true or false • Every decision in a computer program involves evaluating a Boolean expression • Computer circuitry consists of two-state on-off switches • Represented by 1 or 0 • Every computer decision yields a true-false result Programming Logic and Design, Fifth Edition, Comprehensive

9. Using the Relational Comparison Operators • For any two values, three types of comparisons: • Two values are equal • First value greater than the second value • First value less than the second value • Relational comparison operators • Express Boolean tests • Different languages use different symbols Programming Logic and Design, Fifth Edition, Comprehensive

10. Using the Relational Comparison Operators (continued) • Any logical situation can be expressed with only three types of comparisons: =, >, and < • Operators >= and <= are not necessary, but make code more readable • “Not equal” operator • Most confusing of comparisons • Most likely to be different in different languages Programming Logic and Design, Fifth Edition, Comprehensive

11. Using the Relational Comparison Operators (continued) Figure 4-5 Using a negative comparison Programming Logic and Design, Fifth Edition, Comprehensive

12. Using the Relational Comparison Operators (continued) Figure 4-6 Using the positive equivalent of the negative comparison in Figure 4-5 Programming Logic and Design, Fifth Edition, Comprehensive

13. Using the Relational Comparison Operators (continued) Table 4-1 Relational comparisons Programming Logic and Design, Fifth Edition, Comprehensive

14. Understanding AND Logic • Compound condition • Asks multiple questions before an outcome is determined • AND decision • Requires that both of two tests evaluate to True • Requires a nested decision (nested if) • Using nested if statements • Second selection structure is contained entirely within one side of first structure • else clause paired with last if Programming Logic and Design, Fifth Edition, Comprehensive

15. Understanding AND Logic (continued) Figure 4-8 Flowchart and pseudocode for selection process in revised bonus-determining program Programming Logic and Design, Fifth Edition, Comprehensive

16. Nesting AND Decisions for Efficiency • When nesting decisions, either selection can come first • Performance time can be improved by asking questions in the proper order • In an AND decision, first ask the question that is less likely to be true • Eliminates as many instances of the second decision as possible • Speeds up processing time Programming Logic and Design, Fifth Edition, Comprehensive

17. Combining Decisions in an AND Selection • Conditional AND operator allows you to ask two or more questions in a single comparison • Each Boolean expression must be true for entire expression to evaluate to true • Truth tables describe the truth of an entire expression based on the truth of its parts • Short-circuit evaluation: expression evaluated only as far as necessary to determine truth Programming Logic and Design, Fifth Edition, Comprehensive

18. Combining Decisions in an AND Selection (continued) Table 4-2 Truth table for the AND operator Programming Logic and Design, Fifth Edition, Comprehensive

19. Combining Decisions in an AND Selection (continued) Figure 4-10 Using an AND operator and the logic behind it Programming Logic and Design, Fifth Edition, Comprehensive

20. Avoiding Common Errors in an AND Selection • Second decision must be made entirely within the first decision • Range of values – every value between low and high limits • In most programming languages logical AND is a binary operator • Requires complete Boolean expression on both sides Programming Logic and Design, Fifth Edition, Comprehensive

21. Understanding OR Logic • When you want to take action when one or the other of two conditions is true • Example: • Salespeople get bonus when they have achieved one of two goals: • Sell at least five items • Sell at least $2,000 in merchandise • itemsSold >= ITEMS_MIN? • If true, assign $300 bonus Programming Logic and Design, Fifth Edition, Comprehensive

22. Writing OR Decisions for Efficiency • May ask either question first • Both produce the same output, but vary widely in number of questions asked • If first question is true, no need to ask second • In an OR decision first ask the question that is more likely to be true • Eliminates as many repetitions as possible of second decision Programming Logic and Design, Fifth Edition, Comprehensive

23. Combining Decisions in an OR Selection • Conditional OR operator allows you to ask two or more questions in a single comparison • Only one Boolean expression in an OR selection must be true to produce a result of true • Question placed first will be asked first, so consider efficiency • Computer can ask only one question at a time Programming Logic and Design, Fifth Edition, Comprehensive

24. Combining Decisions in an OR Selection (continued) Table 4-3 Truth table for the OR operator Programming Logic and Design, Fifth Edition, Comprehensive

25. Combining Decisions in an OR Selection (continued) Figure 4-14 Using an OR operator and the logic behind it Programming Logic and Design, Fifth Edition, Comprehensive

26. Avoiding Common Errors in an OR Selection • Second question must be self-contained structure with one entry and exit point • Request for A and B in English often translates to a request for A or B logically • Example: • “Give a bonus to anyone who has sold at least three items and to anyone who has sold $2000” • “Give a bonus to anyone who has sold at least three items or $2000” Programming Logic and Design, Fifth Edition, Comprehensive

27. Avoiding Common Errors in an OR Selection (continued) Figure 4-15 Unstructured flowchart for determining bonuses Programming Logic and Design, Fifth Edition, Comprehensive

28. Avoiding Common Errors in an OR Selection (continued) Figure4-16 Incorrect logic that attempts to provide a discount for young and old movie patrons Programming Logic and Design, Fifth Edition, Comprehensive

29. Avoiding Common Errors in an OR Selection (continued) Figure 4-17 Correct logic that provides a discount for young and old movie patrons Programming Logic and Design, Fifth Edition, Comprehensive

30. Avoiding Common Errors in an OR Selection (continued) Figure 4-18 Incorrect logic that attempts to charge full price for patrons over 12 and under 65 Programming Logic and Design, Fifth Edition, Comprehensive

31. Avoiding Common Errors in an OR Selection (continued) Figure 4-19 Correct logic that charges full price for patrons over 12 and under 65 Programming Logic and Design, Fifth Edition, Comprehensive

32. Making Selections within Ranges • Rangecheck: compare a variable to a series of values between limits • Use the lowest or highest value in each range • Adjust the question logic when using highest versus lowest values • Should end points of the range be included? • Yes: use >= or <= • No: use < or > Programming Logic and Design, Fifth Edition, Comprehensive

33. Making Selections within Ranges (continued) Figure 4-20 Proposed meeting schedule Programming Logic and Design, Fifth Edition, Comprehensive

34. Making Selections within Ranges (continued) Figure 4-22 Program that prints the employee memo using high-end values in a range check Programming Logic and Design, Fifth Edition, Comprehensive

35. Understanding Common Errors Using Range Checks • Avoid dead or unreachable paths • Don’t check for values that can never occur • Requires some prior knowledge of the data • Never ask a question if there is only one possible outcome • Avoid asking a question when the logic has already determined the outcome Programming Logic and Design, Fifth Edition, Comprehensive

36. Understanding Precedence When Combining AND and OR Selections • Combine multiple AND and OR operators in an expression • When multiple conditions must all be true, use multiple ANDs if score1 >= 75 AND score2 >= 75 AND score 3 >= 75 then classGrade = “Pass” else classGrade = “Fail” endif Programming Logic and Design, Fifth Edition, Comprehensive

37. Understanding Precedence When Combining AND and OR Selections (continued) • When only one of multiple conditions must be true, use multiple ORs if score1 >= 75 OR score2 >= 75 OR score3 >= 75 then classGrade = “Pass” else classGrade = “Fail” endif Programming Logic and Design, Fifth Edition, Comprehensive

38. Understanding Precedence When Combining AND and OR Selections (continued) • When AND and OR operators are combined in the same statement, AND operators are evaluated first if age <= 12 OR age >= 65 AND rating = “G” • Use parentheses to correct logic and force evaluations to occur in the order desired if (age <= 12 OR age >= 65) AND rating = “G” Programming Logic and Design, Fifth Edition, Comprehensive

39. Understanding Precedence When Combining AND and OR Selections (continued) • Mixing AND and OR operators makes logic more complicated • Can avoid mixing AND and OR decisions by nesting if statements Programming Logic and Design, Fifth Edition, Comprehensive

40. Understanding Precedence When Combining AND and OR Selections (continued) Figure 4-25 Nested decisions that determine movie patron discount Programming Logic and Design, Fifth Edition, Comprehensive

41. The Case Structure • Provides a series of alternatives based on the value of a single variable • Replaces a series of chained if-else statements • Makes code easier to read Programming Logic and Design, Fifth Edition, Comprehensive

42. The Case Structure (continued) Figure 4-27 Flowchart and pseudocode that determines base price for house based on model number using the case structure Programming Logic and Design, Fifth Edition, Comprehensive

43. Using Decision Tables • Managing multiple possible outcomes of multiple decisions can be difficult • Decision table: four-part problem-analysis tool • Conditions • Possible combinations of Boolean values for each condition • Possible actions based on the conditions • Specific actions that correspond to each Boolean value of each condition Programming Logic and Design, Fifth Edition, Comprehensive

44. Using Decision Tables (continued) Figure 4-28 Sample residence hall assignments report Programming Logic and Design, Fifth Edition, Comprehensive

45. Using Decision Tables (continued) • Rules for assigning residence halls • Students under age 21 who request a hall with quiet study hours: Addams Hall • Students under age 21 who do not request a hall with quiet study hours: Grant Hall • Students age 21 and over who request a hall with quiet study hours: Lincoln Hall • Students age 21 and over who do not request a hall with quiet study hours: Lincoln Hall Programming Logic and Design, Fifth Edition, Comprehensive

46. Using Decision Tables (continued) • To create a decision table: • List all possible conditions • Determine the possible Boolean value combinations for each condition • # combinations = 2 (number of conditions) Table 4-4 Conditions and possible values for residence hall determination Programming Logic and Design, Fifth Edition, Comprehensive

47. Using Decision Tables (continued) • To create a decision table (continued): • Add rows to list possible outcome actions • Choose one required outcome for each combination Table 4-6 Completed decision table for residence hall selection Programming Logic and Design, Fifth Edition, Comprehensive

48. Using Decision Tables (continued) Figure 4-29 Program that assigns a student to a residence hall Programming Logic and Design, Fifth Edition, Comprehensive

49. Summary • Decisions involve evaluating Boolean expressions • Use relational operators to compare values • AND decision requires that both conditions be true to produce a true result • In an AND decision, first ask the question that is less likely to be true • OR decision requires that either of the conditions be true to produce a true result Programming Logic and Design, Fifth Edition, Comprehensive

50. Summary (continued) • In an OR decision, first ask the question that is more likely to be true • For a range check, make comparisons with the highest or lowest values in each range • Eliminate unnecessary or previously answered questions • Case structure allows a series of alternative actions based on the value in a single variable • Decision table aids in program design analysis to manage multiple conditions and decisions Programming Logic and Design, Fifth Edition, Comprehensive