Programming Logic and
Design
Fourth Edition, Comprehensive
Chapter 5
Making Decisions
Objectives
• Evaluate Boolean expressions to make comparisons
• Use the relational comparison operators
• Understand AND logic
• Understand OR logic
• Use selections within ranges
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Objectives (continued)
• Understand precedence when combining AND and
OR selections
• Understand the case structure
• Use decision tables
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Evaluating Boolean Expressions to Make
Comparisons
• Dual-alternative (or binary) selection structure:
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Evaluating Boolean Expressions to Make
Comparisons (continued)
• Single-alternative (or unary) selection structure
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Evaluating Boolean Expressions to
Make Comparisons (continued)
• Dual-alternative (or binary) selection structure:
– Also called an if-then-else structure
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Evaluating Boolean Expressions to
Make Comparisons (continued)
• Single-alternative (or unary) selection structure
– Also called an if-then structure
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Evaluating Boolean Expressions to
Make Comparisons (continued)
• Boolean expression
– Represents only one of two states
– Expression evaluates to either true or false
• Expressions with relational operators produce
Boolean results: ( true or false, 0 or 1 )
hours worked > 40
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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
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Using the Relational Comparison
Operators (continued)
•
Relational comparison operators:
– To express Boolean tests when comparing values
•
Different languages use different symbols
– Equals: = or ==
– Less than: <
– Greater than: <
– Less than or equal: <=
– Greater than or equal: >=
– Not Equal to != or <>
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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
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Using the Relational Comparison
Operators (continued)
• Rule of thumb: ask the question most likely to have a
positive outcome first
• Avoid “not equal” when it results in a double negative
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Using the Relational Comparison
Operators (continued)
• Rephrase in the positive
• Some comparisons are clearer when negative is used
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Using the Relational Comparison
Operators (continued)
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Understanding AND Logic
• AND decision
– Requires that both of two tests evaluate to True
– Requires a nested decision (nested if)
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Understanding AND Logic (continued)
• Developing the application
– The input data
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Understanding AND Logic (continued)
• Developing the application
– The intended output:
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Understanding AND Logic (continued)
• Developing the application
– The mainline logic:
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Understanding AND Logic (continued)
• housekeeping() module:
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Understanding AND Logic (continued)
• createReport() module:
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Understanding AND Logic (continued)
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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
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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
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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
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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
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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
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Combining Decisions in an AND
Selection (continued)
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Combining Decisions in an AND
Selection (continued)
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Avoiding Common Errors in an AND Selection
• Failure to nest 2nd decision entirely within 1st decision
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Avoiding Common Errors in an AND
Selection (continued)
• Correct questions to determine inclusion in a range
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Avoiding Common Errors in an AND
Selection (continued)
• Failure to use two complete Boolean expressions:
• Correct way:
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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
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Understanding OR Logic (continued)
• createReport() module:
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Avoiding Common Errors in an OR
Selection
• Unstructured OR selection:
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Avoiding Common Errors in an OR
Selection (continued)
• Incorrect interpretation of English:
– Casual use of AND when logic requires OR
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Avoiding Common Errors in an OR
Selection (continued)
• Correct logic:
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Avoiding Common Errors in an OR
Selection (continued)
• Incorrect interpretation of English
– Use of OR when AND logic is required
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Avoiding Common Errors in an OR
Selection (continued)
• Correct logic:
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Writing OR Decisions for Efficiency
• How many decisions?
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Writing OR Decisions for Efficiency
(continued)
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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
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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
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Combining Decisions in an OR
Selection (continued)
• Using an OR operator:
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Combining Decisions in an OR
Selection (continued)
• What the computer actually does:
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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 >
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Using Selections Within Ranges
(continued)
• Using high-end values in the range check:
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Using Selections Within Ranges
(continued)
• Using low-end values in the range check:
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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
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Common Errors Using Range Checks
(continued)
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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
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Common Errors Using Range Checks
(continued)
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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
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Understanding Precedence When
Combining AND and OR Selections
(continued)
• When only one of multiple conditions must be true,
use multiple ORs
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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
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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
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Understanding Precedence When
Combining AND and OR Selections
(continued)
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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
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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
• 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 ( rules )
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Using Decision Tables (continued)
• Developing the application:
– The data
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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
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Using Decision Tables (continued)
• Developing the application:
– The desired output
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Using Decision Tables (continued)
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Using Decision Tables (continued)
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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)
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Using Decision Tables (continued)
• To create a decision table (continued):
– Add rows to list possible outcome actions
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Using Decision Tables (continued)
• To create a decision table (continued):
– Choose one required outcome for each combination
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Using Decision Tables (continued)
• Create a flowchart from the decision table
– Draw a path for each column’s outcome
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Using Decision Tables (continued)
• Resulting flowchart created directly from table has
two identical outcomes: an unneeded question
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Using Decision Tables (continued)
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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
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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
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