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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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