1. Programming Logic and Design Fourth Edition, Introductory
Chapter 5
Making Decisions

2. Objectives Evaluate Boolean expressions to make comparisons
Use the relational comparison operators
Understand AND logic
Understand OR logic
Use selections within ranges
Programming Logic and Design, Introductory, Fourth Edition

3. Objectives (continued)
Understand precedence when combining AND and OR selections
Understand the case structure
Use decision tables
Programming Logic and Design, Introductory, Fourth Edition

4. Evaluating Boolean Expressions to Make Comparisons
Dual-alternative (or binary) selection structure:
Provides an action for each of two possible outcomes
Programming Logic and Design, Introductory, Fourth Edition

5. Evaluating Boolean Expressions to Make Comparisons (continued)
Single-alternative (or unary) selection structure
Action is provided for only one outcome
Programming Logic and Design, Introductory, Fourth Edition

6. Evaluating Boolean Expressions to Make Comparisons (continued)
Dual-alternative (or binary) selection structure:
Also called an if-then-else structure
Programming Logic and Design, Introductory, Fourth Edition

7. Evaluating Boolean Expressions to Make Comparisons (continued)
Single-alternative (or unary) selection structure
Also called an if-then structure
Programming Logic and Design, Introductory, Fourth Edition

8. Evaluating Boolean Expressions to Make Comparisons (continued)
Represents only one of two states
Expression evaluates to either true or false
Expressions with relational operators produce Boolean results:
hours worked > 40
Programming Logic and Design, Introductory, Fourth Edition

9. Using the Relational Comparison Operators
Six possible ways to compare two values:
Both are equal
The first is greater than the second
The first is less than the second
The first is greater than or equal to the second
The first is less than or equal to the second
The two values are not equal
Programming Logic and Design, Introductory, Fourth Edition

10. Using the Relational Comparison Operators (continued)
To express Boolean tests when comparing values
Different languages use different symbols
Equals: =
Less than: <
Greater than: <
Less than or equal: <=
Greater than or equal: >=
Programming Logic and Design, Introductory, Fourth Edition

11. Using the Relational Comparison Operators (continued)
Any logical situation can be expressed with only three types of comparisons: =, >, and <
>= and <= are not necessary, but make code more readable
Adjust the logic based on the comparison type
Programming Logic and Design, Introductory, Fourth Edition

12. Using the Relational Comparison Operators (continued)
Rule of thumb: ask the question most likely to have a positive outcome
Avoid “not equal” when it results in a double negative
Programming Logic and Design, Introductory, Fourth Edition

13. Using the Relational Comparison Operators (continued)
Rephrase in the positive
Some comparisons are clearer when negative is used
Programming Logic and Design, Introductory, Fourth Edition

14. Using the Relational Comparison Operators (continued)
Programming Logic and Design, Introductory, Fourth Edition

15. Understanding AND Logic
AND decision
Requires that both of two tests evaluate to True
Requires a nested decision (nested if)
Programming Logic and Design, Introductory, Fourth Edition

16. Understanding AND Logic (continued)
Developing the application
The input data
Programming Logic and Design, Introductory, Fourth Edition

17. Understanding AND Logic (continued)
Developing the application
The intended output:
Programming Logic and Design, Introductory, Fourth Edition

18. Understanding AND Logic (continued)
Developing the application
The mainline logic:
Programming Logic and Design, Introductory, Fourth Edition

19. Understanding AND Logic (continued)
housekeeping() module:
Programming Logic and Design, Introductory, Fourth Edition

20. Understanding AND Logic (continued)
createReport() module:
Programming Logic and Design, Introductory, Fourth Edition

21. Understanding AND Logic (continued)
Programming Logic and Design, Introductory, Fourth Edition

22. Writing Nested AND Decisions for Efficiency
For nested decisions, decide which to make first
An appropriate choice may improve performance
Consider expected outcomes to determine the most efficient way to nest the decisions
Programming Logic and Design, Introductory, Fourth Edition

23. Writing Nested AND Decisions for Efficiency (continued)
With 1000 employees, of which 90% (900) are in the medical plan, and 50% (500) are in the dental plan:
First question is asked 1000 times
Second question is asked 900 times
Programming Logic and Design, Introductory, Fourth Edition

24. Writing Nested AND Decisions for Efficiency (continued)
With 1000 employees, of which 90% (900) are in the medical plan, and 50% (500) are in the dental plan:
First question is asked 1000 times
Second question is asked 500 times
Programming Logic and Design, Introductory, Fourth Edition

25. Writing Nested AND Decisions for Efficiency (continued)
This analysis may not be possible:
You may not know decision outcome likelihoods
The decisions may not be mutually exclusive
Rule of Thumb: First ask the question that is less likely to be true
Reduces the number of times the second question will need to be asked
Programming Logic and Design, Introductory, Fourth Edition

26. Combining Decisions in an AND Selection
Logical AND operator:
Allows you to ask two or more questions (Boolean expressions) in a single comparison
Each Boolean expression in an AND selection must be true to produce a result of true
Question placed first will be asked first, so consider efficiency
Programming Logic and Design, Introductory, Fourth Edition

27. Combining Decisions in an AND Selection (continued)
Programming Logic and Design, Introductory, Fourth Edition

28. Combining Decisions in an AND Selection (continued)
Programming Logic and Design, Introductory, Fourth Edition

29. Avoiding Common Errors in an AND Selection
Failure to nest 2nd decision entirely within 1st decision
Programming Logic and Design, Introductory, Fourth Edition

30. Avoiding Common Errors in an AND Selection (continued)
Incorrect questions to determine inclusion in a range
Correct way:
Programming Logic and Design, Introductory, Fourth Edition

31. Avoiding Common Errors in an AND Selection (continued)
Failure to use two complete Boolean expressions:
Correct way:
Programming Logic and Design, Introductory, Fourth Edition

32. Understanding OR Logic
OR decision
At least one of two conditions must be true to produced a result of True
If first condition is true, no need to test the second condition
Programming Logic and Design, Introductory, Fourth Edition

33. Understanding OR Logic (continued)
createReport() module:
Programming Logic and Design, Introductory, Fourth Edition

34. Avoiding Common Errors in an OR Selection
Unstructured OR selection:
Programming Logic and Design, Introductory, Fourth Edition

35. Avoiding Common Errors in an OR Selection (continued)
Incorrect interpretation of English:
Casual use of AND when logic requires OR
Programming Logic and Design, Introductory, Fourth Edition

36. Avoiding Common Errors in an OR Selection (continued)
Correct logic:
Programming Logic and Design, Introductory, Fourth Edition

37. Avoiding Common Errors in an OR Selection (continued)
Incorrect interpretation of English
Use of OR when AND logic is required
Programming Logic and Design, Introductory, Fourth Edition

38. Avoiding Common Errors in an OR Selection (continued)
Correct logic:
Programming Logic and Design, Introductory, Fourth Edition

39. Writing OR Decisions for Efficiency
How many decisions?
Programming Logic and Design, Introductory, Fourth Edition

40. Writing OR Decisions for Efficiency (continued)
Programming Logic and Design, Introductory, Fourth Edition

41. Writing OR Decisions for Efficiency (continued)
Both produce the same output, but vary widely in number of questions asked
If first question is true, no need to ask second
Rule of thumb:
First ask the question that is more likely to be true
Programming Logic and Design, Introductory, Fourth Edition

42. Combining Decisions in an OR Selection
Logical OR operator:
Allows you to ask two or more questions (Boolean expressions) 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
Programming Logic and Design, Introductory, Fourth Edition

43. Combining Decisions in an OR Selection (continued)
Using an OR operator:
Programming Logic and Design, Introductory, Fourth Edition

44. Combining Decisions in an OR Selection (continued)
What the computer actually does:
Programming Logic and Design, Introductory, Fourth Edition

45. Using Selections Within Ranges
Range check: 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, Introductory, Fourth Edition

46. Using Selections Within Ranges (continued)
Using high-end values in the range check:
Programming Logic and Design, Introductory, Fourth Edition

47. Using Selections Within Ranges (continued)
Using low-end values in the range check:
Programming Logic and Design, Introductory, Fourth Edition

48. 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
Programming Logic and Design, Introductory, Fourth Edition

49. Common Errors Using Range Checks (continued)
Programming Logic and Design, Introductory, Fourth Edition

50. Common Errors Using Range Checks (continued)
Avoid asking an unneeded question
If there is only one possible outcome
If previous logic has already determined the answer
Programming Logic and Design, Introductory, Fourth Edition

51. Common Errors Using Range Checks (continued)
Programming Logic and Design, Introductory, Fourth Edition

52. Understanding Precedence When Combining AND and OR Selections
Can combine multiple AND and OR operators in an expression
When multiple conditions must all be true, use multiple ANDs
Programming Logic and Design, Introductory, Fourth Edition

53. Understanding Precedence When Combining AND and OR Selections (continued)
When only one of multiple conditions must be true, use multiple ORs
Programming Logic and Design, Introductory, Fourth Edition

54. 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
Use parentheses to correct logic and force evaluations to occur in the order desired
Programming Logic and Design, Introductory, Fourth Edition

55. 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, Introductory, Fourth Edition

56. Understanding Precedence When Combining AND and OR Selections (continued)
Programming Logic and Design, Introductory, Fourth Edition

57. Understanding the Case Structure
Used to provide a series of alternatives based on the value of a single variable
Replaces a series of chained if-else statements
May make the code easier to read
Programming Logic and Design, Introductory, Fourth Edition

58. Understanding the Case Structure (continued)
Programming Logic and Design, Introductory, Fourth Edition

59. Understanding the Case Structure (continued)
Programming Logic and Design, Introductory, Fourth Edition

60. 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, Introductory, Fourth Edition

61. Using Decision Tables (continued)
Developing the application:
The data
Programming Logic and Design, Introductory, Fourth Edition

62. 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, Introductory, Fourth Edition

63. Using Decision Tables (continued)
Developing the application:
The desired output
Programming Logic and Design, Introductory, Fourth Edition

64. Using Decision Tables (continued)
Programming Logic and Design, Introductory, Fourth Edition

65. Using Decision Tables (continued)
Programming Logic and Design, Introductory, Fourth Edition

66. 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)
Programming Logic and Design, Introductory, Fourth Edition

67. Using Decision Tables (continued)
To create a decision table (continued):
Add rows to list possible outcome actions
Programming Logic and Design, Introductory, Fourth Edition

68. Using Decision Tables (continued)
To create a decision table (continued):
Choose one required outcome for each combination
Programming Logic and Design, Introductory, Fourth Edition

69. Using Decision Tables (continued)
Create a flowchart from the decision table
Draw a path for each column’s outcome
Programming Logic and Design, Introductory, Fourth Edition

70. Using Decision Tables (continued)
Resulting flowchart created directly from table has two identical outcomes: an unneeded question
Programming Logic and Design, Introductory, Fourth Edition

71. Using Decision Tables (continued)
Programming Logic and Design, Introductory, Fourth Edition

72. 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, Introductory, Fourth Edition

73. 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, Introductory, Fourth Edition