1. Chapter 3 Making Decisions
An Object-Oriented Approach to Programming Logic and Design Fourth Edition
Chapter 3
Making Decisions

2. Objectives In this chapter, you will learn about:
Evaluating Boolean expressions
Relational comparison operators
AND logic
OR logic
Making selections within ranges
Precedence when combining AND and OR selections
An Object-Oriented Approach to Programming Logic and Design

3. Evaluating Boolean Expressions
Selection structure (introduced in Chapter 2)
Used when a program’s logic can take one of two paths based on a decision
Also called an if-then-else structure
Example: driving directions
go North on Third Street
if Ninth Avenue is closed for repairs then
continue to Tenth Avenue and turn left
else
turn left on Ninth Avenue
endif
turn right on Sixth Street
An Object-Oriented Approach to Programming Logic and Design

4. Evaluating Boolean Expressions (cont’d)
Value can only be true or false
Every decision in a computer program involves evaluating a Boolean expression
True/false evaluation is “natural” for a computer
Computer circuitry consists of two-state on/off switches
Each state is represented by a 1 or a 0
Every computer decision yields true or false, yes or no
An Object-Oriented Approach to Programming Logic and Design

5. Pseudocode: Flowchart: if starts the selection Align else with if
Indent action taken when true
Align else with if
Indent action taken when false
Endif shows end of structure
Flowchart:
Logic takes one of two paths based on answer to question
No matter which path taken, logic continues
An Object-Oriented Approach to Programming Logic and Design

6. Dual-alternative selection Single-alternative selection
Logic flows to one of two alternatives
Mutually exclusive choices (never to both)
Single-alternative selection
Action is required for only one outcome
Also called if-then
An Object-Oriented Approach to Programming Logic and Design

7. Business program using if-then-else
Several variables and constants declared
After input data retrieved, makes decision about hoursWorked value
Figure 3-3
Figure 3-3
An Object-Oriented Approach to Programming Logic and Design

8. Flowchart and pseudocode for payroll program determining dental insurance
Program uses stacked structures, one “on top of” and completely separate from each other
Figure 3-4
Figure 3-4
An Object-Oriented Approach to Programming Logic and Design

9. Using Relational Comparison Operators
Three types of operations in a Boolean expression
Both values equal
First value greater than second value
First value less than second value
Relational operators
Used to create Boolean expressions
Require two operands
Relational Operators
Operator
Name ==
Equivalency operator
>
Greater-than operator
<
Less-than
>=
Greater-than or equal-to operator
<=
Less-than or equal-to operator
<> or !=
Not-equal-to operator
An Object-Oriented Approach to Programming Logic and Design

10. Using Relational Comparison Operators
Usually both operands are the same type
Expression evaluates to true or false
Can compare strings as well as numbers
Can compare variables and constants
An Object-Oriented Approach to Programming Logic and Design

11. Using the Relational Comparison Operators (cont’d)
Any relational situation can be expressed using three types of comparisons: equal, greater than, and less than
Example: code in the two boxes is equivalent
if a >= b is True, then a < b is False
if a >= b is False, then a < b is True
An Object-Oriented Approach to Programming Logic and Design

12. Using Relational Comparison Operators (cont’d)
Using “not equal to” in decisions
More prone to logical programming errors
Different in various languages
Figure 3-5
An Object-Oriented Approach to Programming Logic and Design

13. Using Relational Comparison Operators (cont’d)
Most programming languages support a NOT operator
Reverses the meaning of a Boolean expression
Unary operator: used in front of a single expression
Example:
age <> 21
NOT (age – 21)
An Object-Oriented Approach to Programming Logic and Design

14. Using Relational Comparison Operators (cont’d)
Eliminate double negative
Positive equivalent is more clear
Figure 3-6
An Object-Oriented Approach to Programming Logic and Design

15. Using the Wrong Relational Operator
Common error: choosing wrong operator
Using > when >= is what is really desired
All cases of = will be missed if this error is made
Best to double-check intended meaning with the person who requested the program
Phrases that can cause misunderstandings
No more than
At least
Not under
An Object-Oriented Approach to Programming Logic and Design

16. Understanding AND Logic
Compound condition
More than one selection structure to make decision
Multiple questions before determining outcome
AND decision
Requires both conditions to be true for action to occur
Can be constructed using nested decision
Also known as a nested selection or nested if
Nested decision contains a decision “inside of” another decision
An Object-Oriented Approach to Programming Logic and Design

17. Flowchart pseudocode for cell phone billing program
Second decision falls within one branch of the first decision
Outer selection: first selection of nested statements
Inner selection: second selection
Figure 3-7
Figure 3-7
An Object-Oriented Approach to Programming Logic and Design

18. Nesting AND Decisions for Efficiency
When nesting decisions, must decide which is first
With AND, result is the same no matter which is first
Choosing the right decision to put first can improve program performance
First ask the question that is less likely to be true
May eliminate or reduce second question evaluation
May result in faster program execution
An Object-Oriented Approach to Programming Logic and Design

19. Using the AND Operator Conditional AND operator (AND operator)
Evaluate two or more expressions in single statement
One or more AND operators combine two or more Boolean expressions
True evaluation requires each Boolean expression be true
An Object-Oriented Approach to Programming Logic and Design

20. Using the AND Operator Truth tables Short-circuit evaluation
Diagrams used in mathematics and logic
Describes truth of an entire expression
Truth based on the truth of each part
Short-circuit evaluation
Evaluates each expression using an AND operator
Evaluates only as far as necessary to determine whether entire expression is true or false
An Object-Oriented Approach to Programming Logic and Design

21. Computer executes both flowcharts in Figure 3-9 in the same manner
Computer makes decisions one at a time in the order they are given
In an AND expression, if first test is false, entire expression is false and second test is not performed
Figure 3-9
An Object-Oriented Approach to Programming Logic and Design

22. Avoiding Common Errors in an AND Selection
Common errors when making AND selections
When AND selections are nested incorrectly
Performing an action when only one condition is satisfied
Performing an action twice that should only occur once
Failing to include a complete Boolean expression on both sides of the operator
An Object-Oriented Approach to Programming Logic and Design

23. When satisfying two or more criteria to initiate an event, ensure that the second decision is nested entirely within the first
Figure 3-10
An Object-Oriented Approach to Programming Logic and Design

24. Avoiding Common Errors in an AND Selection
Must provide a complete Boolean expression on each side of the operator
Example of valid compound expression
callMinutes > 100 AND callMinutes < 200
Example of invalid compound expression
callMinutes > 100 AND < 200
An Object-Oriented Approach to Programming Logic and Design

25. Understanding OR Logic
Conditional OR operator (OR decision)
Take action if one of two conditions is true
Requires at least one decision evaluated to be true
If answers to both halves of question are false, entire expression value is false
Writing OR decisions
Can choose to ask either question first
First ask question that is more likely to be true
May result in faster program execution
An Object-Oriented Approach to Programming Logic and Design

26. Flowchart and pseudocode for cell phone billing program
Customer must meet one or both of two criteria to be billed a premium
Figure 3-11
An Object-Oriented Approach to Programming Logic and Design

27. Two ways to assign a premium to bills of customers who meet one of two criteria
Figure 3-12
Which program is more efficient depends on the relative likelihood of each condition
Figure 3-12
An Object-Oriented Approach to Programming Logic and Design

28. Using the OR Operator Conditional OR operator (OR operator)
Two or more questions in single comparison
One condition must be met for resulting action to occur
Short-circuiting
If first question is true, logic does not proceed to second question
Truth table for OR operator
Table 3-3
An Object-Oriented Approach to Programming Logic and Design

29. Computer executes both flowcharts in the same manner
Computer makes decisions one at a time in the order they are given
In an OR expression, If first test is true, entire expression is true and second test is not performed
Figure 3-13
An Object-Oriented Approach to Programming Logic and Design

30. Avoiding Common Errors in an OR Selection
Common errors when using OR operator
Creating unstructured logic
True and false decision paths must rejoin before proceeding to the next steps in the program
Using AND logic when OR logic is needed
Often, a casual request for A and B logically means a request for A or B
Need to clarify what is really being requested
Using OR logic when AND logic is needed
An Object-Oriented Approach to Programming Logic and Design

31. Avoiding Common Errors in an OR Selection (cont’d)
Unstructured flowchart for determining customer cell phone bill
Figure 3-14
An Object-Oriented Approach to Programming Logic and Design

32. Avoiding Common Errors in an OR Selection (cont’d)
Example of using AND logic when OR logic is needed
Figure 3-15
An Object-Oriented Approach to Programming Logic and Design

33. Avoiding Common Errors in an OR Selection (cont’d)
Correct logic that gives a discount for young and old movie patrons
Figure 3-16
An Object-Oriented Approach to Programming Logic and Design

34. Avoiding Common Errors in an OR Selection (cont’d)
Incorrect logic that attempts to charge full price for patrons whose age is over 12 and under 65
Figure 3-17
An Object-Oriented Approach to Programming Logic and Design

35. Avoiding Common Errors in an OR Selection (cont’d)
Correct logic that charges full price for patrons whose age is over 12 and under 65
Figure 3-18
An Object-Oriented Approach to Programming Logic and Design

36. Making Selections within Ranges
Range of values
Series of contiguous values that fall between specified limits
Range check
Compares variable to series of values between limits
Use range check to make comparisons using either lowest or highest value in each value range
Example: customer discount based on number of items ordered
Items
Ordered
Discount Rate (%)
0 to 10
11 to 24 10
25 to 50 15
51 or more 20
An Object-Oriented Approach to Programming Logic and Design

37. Flowchart and pseudocode of logic that selects correct discount based on items ordered
In pseudocode, notice how each if, else, and endif group aligns vertically
Figure 3-20
An Object-Oriented Approach to Programming Logic and Design

38. Avoiding Common Errors when Using Range Checks
Two common errors when using range checks
Using logic that contains an unreachable path
Asking questions when the answers are irrelevant
Program may work correctly but is inefficient and confusing
Examples of both types of errors follow
An Object-Oriented Approach to Programming Logic and Design

39. Inefficient range selection that includes an unreachable path
Programmer has asked one question too many
Figure 3-21
An Object-Oriented Approach to Programming Logic and Design

40. Inefficient range selection that includes two unnecessary questions
Wasting computer time asking a question that has already been answered
Figure 3-22
An Object-Oriented Approach to Programming Logic and Design

41. Understanding Precedence when Combining AND and OR Operators
Most languages allow multiple AND and OR operator combinations in an expression
When AND and OR are in the same statement, AND operator takes precedence
To control which operations are evaluated first:
Use parentheses to override default order of operations
Use parentheses for clarity
Use nested if statements instead of ANDs and ORs
Example of using nested if follows
An Object-Oriented Approach to Programming Logic and Design

42. Nested decisions that determine movie patron discount
Figure 3-23
An Object-Oriented Approach to Programming Logic and Design

43. Summary All decisions evaluate Boolean expressions
Can compare any two values of the same type using relational comparison operators
AND decision
requires both conditions to be true to return true result
first ask question less likely to be true
OR decision
requires at least one condition be true to return true result
first ask question more likely to be true
An Object-Oriented Approach to Programming Logic and Design

44. Summary (cont’d) Range checks
Make comparisons with either lowest or highest value in each range
Avoid unnecessary or previously answered questions
AND operators have higher precedence than OR operators
An Object-Oriented Approach to Programming Logic and Design