An Object-Oriented Approach to
Programming Logic and Design
Chapter 5
Making Decisions
Objectives
• Evaluate Boolean expressions to make
comparisons
• Use the relational comparison operators
• Make decisions with objects
• Understand AND logic
• Understand OR logic
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Objectives (continued)
• Use selections within ranges
• Understand common errors using range checks
• Understand precedence when combining AND
and OR selections
• Understand the case structure
• Use a decision table
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Evaluating Boolean Expressions to
Make Comparisons
• People think computers are smart because
computers can make decisions
• Dual-alternative, or binary, selection structure:
logic flows to one of two alternatives; also
called the if-then-else structure
• If no else clause, it is called the singlealternative, or unary selection structure; also
called the if-then structure
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Evaluating Boolean Expressions to
Make Comparisons (continued)
Example:
if the answer to the question is yes, then
do something
else
do somethingElse
endif
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Evaluating Boolean Expressions to
Make Comparisons (continued)
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Evaluating Boolean Expressions to Make
Comparisons (continued)
Example:
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Evaluating Boolean Expressions to Make
Comparisons (continued)
• Boolean expression – one that represents only
one of two states, True or False
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Using the Relational Comparison
Operators
• Usually you compare values of the same type
• Three possible comparison decisions:
– Both values are equal
– First value is greater than second value
– First value is less than second value
• Values to be compared can be variables or
constants
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Using the Relational Comparison
Operators (continued)
• Each programming language has its own set of
relational comparison operators, or
comparison symbols
• Most languages allow the > and < signs for
comparison
• Comparison operators require a value on each
side of the operator
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Using the Relational Comparison
Operators (continued)
• Most languages support:
– >= First value is greater than or equal to the
second value
– <= First value is less than or equal to the
second value
– The two values are not equal
• While not required, these comparisons simplify
the code
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Using the Relational Comparison
Operators (continued)
• If a >= b is True, then a < b is False
• If a >= b is False, then a < b is True
• Rephrase the question and swap the action
taken to make the same decision
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Using the Relational Comparison
Operators (continued)
Example:
if customerAge >= 65 then
discount = 0.10
else
discount = 0
endif
if customerAge < 65 then
discount = 0
else
discount = 0.10
endif
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Using the Relational Comparison
Operators (continued)
• Negative comparisons can lead to double
negatives – use caution!
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Using the Relational Comparison
Operators (continued)
• Rephrase question to eliminate double negative:
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Using the Relational Comparison
Operators (continued)
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Making Decisions with Objects
• Object-oriented programs make decisions
about primitive types and objects
• Decision-making logic is the same for both
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Making Decisions
about Primitive Data Types
Example with primitive data:
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Making Decisions about Objects
• Decision making with objects may be more
complex because the data is usually hidden
• Object instance names are references, or
pointers, to the object
• Comparing pointers only determines if they
point to the same or different memory locations;
does NOT compare the value of the instance
data
• You must compare the object’s data values
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Making Decisions about Objects
(continued)
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Making Decisions about Objects
(continued)
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Understanding AND Logic
•
You may need more than one selection
structure to make a decision
• AND decision requires that both questions
evaluate as True
•
•
Also called a compound decision
Requires a nested decision – a nested if
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Understanding AND Logic (continued)
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Understanding AND Logic (continued)
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Understanding AND Logic (continued)
• Use care to match the else statements
correctly!
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Understanding AND Logic (continued)
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Nesting and Decisions for Efficiency
• In an AND decision, either decision can come
first (if a single action results)
• Efficiency may be improved by making a good
choice for which decision comes first
• The choice may be based on the data: What is
the outcome that is expected most often?
• Rule of thumb: first ask the question that is less
likely to be true – may eliminate the need to
evaluate the second question
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Nesting and Decisions for Efficiency
(continued)
Example: Both produce the same decision
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Combining Decisions in an AND
Selection
• Most languages allow multiple comparisons
using a logical AND operator
• Logical AND is equivalent to nested if
statements
• Consider the order of the questions for
efficiency
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Combining Decisions in an AND
Selection (continued)
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Avoiding Common Errors in an AND
Selection
Error: Decisions are not nested!
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Avoiding Common Errors in an AND
Selection (continued)
• Using AND to handle a range of values requires
two complete Boolean expressions
Correct example:
if itemsSold >= 5 AND itemsSold <= 10 then
bonus = 75
endif
Incorrect example:
if itemsSold >= 5 AND <= 10 then
bonus = 75
endif
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Understanding OR Logic
• OR decision requires that at least one of the
questions evaluates to True
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Writing Or Decisions for Efficiency
• In an OR decision, first ask the question that is
more likely to be True
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Combining Decisions
in an Or Selection
• Logical OR operator allows two or more
questions to be asked in a single statement
• Most languages require a complete Boolean
expression on each side of the OR operator
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Combining Decisions
in an Or Selection (continued)
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Avoiding Common Errors
in an Or Selection
Unstructured Example:
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Avoiding Common Errors
in an Or Selection (continued)
• Warning! Casual use of English may use
“and” and “or” incorrectly
• Be sure to state required decisions clearly and
unambiguously when there are multiple
decisions
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Avoiding Common Errors
in an Or Selection (continued)
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Avoiding Common Errors
in an Or Selection (continued)
• Most languages support a NOT operator
• Logical NOT operator reverses the meaning of a
Boolean expression
• NOT operator is unary – place it in front of a
single expression
Example:
if NOT (age < 21) then
print “OK”
endif
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Using Selection Within Ranges
• A range check compares a variable to a series
of values between limits
• Make comparisons using either the lowest or
highest value in each range of values
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Using Selection Within Ranges
(continued)
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Using Selection Within Ranges
(continued)
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Using Selection Within Ranges
(continued)
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Understanding Common Errors
Using Range Checks
• Avoid logical paths that are unreachable
• Avoid asking questions that have just one
possible answer or outcome
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Understanding Common Errors
Using Range Checks (continued)
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Understanding Common Errors
Using Range Checks (continued)
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Understanding Common Errors
Using Range Checks (continued)
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Understanding Common Errors
Using Range Checks (continued)
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Understanding Precedence when
Combining AND and Or Selections
• Most languages allow combination of multiple
AND and OR operators in an expression
• When AND and OR are in the same statement,
the And operator takes precedence
• Use parentheses to control which operators
are evaluated first, or use nested if
statements instead of ANDs and ORs
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Understanding Precedence when
Combining AND and Or Selections
(continued)
Examples:
if age<=12 OR age>=65 AND rating=“G” then
print “Discount applies”
if (age<=12 OR age>=65) AND rating=“G” then
print “Discount applies”
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Understanding Precedence when
Combining And and Or Selections
(continued)
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Understanding the Case Structure
• case structure allows a selection between
multiple alternatives
• Improves the readability of the code, because it
replaces “chained” if statements
• Tests a variable against a series of values, and
executes only the code for that value
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Understanding the Case Structure
(continued)
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Understanding the Case Structure
(continued)
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Using Decision Tables
• Decision table – a problem analysis tool
consisting of:
– Conditions
– Possible combinations of Boolean values for the
conditions
– Possible actions based on the outcomes
– A specific action corresponding to each Boolean
value of each condition
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Using Decision Tables (continued)
Example: College dorm assignment rules:
• Students under age 21 who request a quiet
dorm are assigned to Addams Hall
• Students under age 21 who do not request a
quiet dorm are assigned to Grant Hall
• Students 21 and over who request a quiet dorm
are assigned to Lincoln Hall
• Students 21 and over who do not request a
quiet dorm are assigned to Lincoln Hall (the
only dorm for over 21)
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Using Decision Tables (continued)
• List all possible conditions that affect the
outcome:
– Age is under 21, or not
– Requests quiet dorm, or not
• Determine all possible Boolean combinations
for the conditions:
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Using Decision Tables (continued)
• Add rows to list possible outcome actions:
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Using Decision Tables (continued)
• Choose one outcome for each possible
combination of conditions:
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Using Decision Tables (continued)
• Write pseudocode to describe the first and
second columns for which age is less than 21:
if age <21 then
if quietRequest = “Y” then
assignedHall = “Addams”
else
assignedHall = “Grant”
endif
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Using Decision Tables (continued)
• Now add pseudocode to describe the third and
fourth columns for which age is greater than or
equal to 21:
else
if quietRequest = “Y” then
assignedHall = “Lincoln”
else
assignedHall = “Lincoln”
endif
endif
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Using Decision Tables (continued)
• But the state of “quietRequest” does not matter
if the student is >= 21, so there is no need to
test the “quietRequest” variable:
else
assignedHall = “Lincoln”
endif
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Using Decision Tables (continued)
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Summary
• All decisions evaluate Boolean expressions
• Any two values can be compared using
relational comparison operators
• AND operator allows two or more conditions to
be tested in a single statement
• AND decision requires that both conditions are
true in order to return a True result
• In an AND decision, first ask the question that is
less likely to be true
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Summary (continued)
• OR decision requires that only one of the two
conditions is true to return a True result
• In an OR decision, first ask the question that is
more likely to be true
• For range checks, make comparisons with
either the lowest or highest value in each range
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Summary (continued)
• Avoid unnecessary or previously answered
questions
• case structure allows question with more than
two alternatives
• Decision table lists conditions and combinations
of outcomes
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