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Relational Expressions
• Decision-making: comparison of two numerical values
• Relational expression
– Also known as a condition
– Evaluates to 1 (true) or 0 (false)
– Simple type: two operands and relational operator
• Six relational operators: <, >, <=, >=, = =, !=
• Char type coerced to int for comparison
• Strings compared at character level
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Figure 5-1
Anatomy of a Simple Relational Expression
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Table 5-1
Relational Operators for Primitive Data Types
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Logical Operators
• Complex expressions
– Comprised of simple relational expressions
– Logical connectors required
• AND ( && ), OR ( | | ), NOT (!)
• Precedence
– AND and OR lower than relational operators
– NOT (unary) higher than relational operators
• Associativity
– AND and OR: left to right
– NOT: right to left
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Table 5-2
Operator Precedence
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Table 5-3
Equivalent Expressions
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A Numerical Accuracy Problem
• Caveat: equality comparison of floating-point types
– Avoid use of equality operator
– Computer representation slightly inaccurate
• Work around problem
– If possible, replace floating-point data with integers
– If not, use alternative syntax
• abs (operandOne - operandTwo) < EPSILON
• EPSILON is very small value such as .0000001
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The if-else Statement
• if-else statement: fundamental selection structure
• Purpose: alter instruction sequence
• Syntax:
if (expression)
statement1;
else
statement2;
• Expression
– Relational expression
– May consist of single variable, such as type bool
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Figure 5-2
The if-else Flowchart
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Compound Statements
• Compound statement
– Sequence of statements enclosed by braces
– Supports construction of complex selection structures
• Syntax:
if (expression){
//sequence of statements
}
else{
//sequence of statements
}
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Block Scope
• Code block
– Set of compound statements
– May be nested
• Variable scope
– Variable meaningful between closing braces
– Name conflict resolved by location
• Inner block takes precedence over outer
• Compiler seeks declaration moving inside out
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One-Way Selection
• One-way selection: excludes else portion
• Syntax:
if (expression)
statement; // code block might follow
• Non-zero expression triggers statement execution
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Figure 5-3
Flowchart for the One-Way if Statement
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Problems Associated with the
if-else Statement
• Semantic problems
– Logical form
– Correct by reviewing original design
• Syntax problems
– Misuse of assignment operator (=) in expression
• Assigns value to operand
• Non-zero assignments always evaluate to true
– Use equality operator (= =) for comparisons
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Nested if Statements
• Selection structures may be nested
– if or if-else statements nest in either (or both) parts
of larger if-else statement
– Nesting may be deeper than one level
• Syntax caveat
– Use braces to define logical unit
– Misused (or missing) braces may cause fatal logical
error
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Figure 5-4a
The if-else Nested Within the if Part
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Figure 5-4b
The if-else Nested Within the else Part
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The if-else chain
• if-else chain: useful, readable form of nesting
• Syntax:
if (expression1)
statement1;
// may be code block
else if (expression2)
statement2; // may be code block
else
statement3;
// may be code block
• Additional else-if components may be added
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The Switch Statement
• Switch statement
– Variation on chained if-else statement
– Control “switches” to case based on condition
– Caveat: condition evaluates to an integer
• Cases may include complex structures
• Break statement follows each case
• Default statement is optional
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Figure 5-5
The Expression Determines an Entry Point
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Program Design and Development:
Introduction to UML
• Think and plan before coding
– Primary concern: classes and objects needed
• Uniform Modeling Language (UML)
– Supports object-oriented design
– Set of rules and diagrams
• Focus on four UML diagrams
– Class, object, state, and sequence
– Analogy to specialized blueprints
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Figure 5-7
Basic UML Symbols and Notation
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Class and Object Diagrams
• Commonality of class and object diagrams
– Both employ rectangular containers
– Names, attributes, behaviors found in both
• Chief differences
–
–
–
–
Class diagram: describes classes and relationships
Object diagram: describes objects and relationships
Class at higher level of abstraction
One class can generate many particular objects
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Figure 5-6
A Class and Object Representation
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Figure 5-8
Including Attributes in UML Class and Object Diagrams
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Class and Object Diagrams
(continued)
• Two aspects to class attributes
– Data type: may be primitive or class
– Visibility: where variable may be seen (or used)
• Plus (+) sign indicates public
• Minus (-) sign indicates private
• No sign for protected status
• Operations
– Become methods that transform attributes
– Follow attribute sign convention for visibility
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Figure 5-9
A Class with Attributes
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Figure 5-10
Including Operations in Class Diagrams
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Relationships
• Three basic relationships
– Association, aggregation, generalization
• Association
– Signified by phrases such as “works for”, “has a”
– Indicated by straight line connecting classes/
objects
– Multiplicity
• Numerical relationship between objects/classes
• Quantities: 0, 1, many, unlimited
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Figure 5-11
An Association
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Table 5-4
UML Association Notation
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Relationships (continued)
• Aggregation
– One class/object consists of other classes/objects
– Visualize as relation of whole to parts
– Symbolized by diamond
• Generalization
– Relationship between class and its refinement
– Ford Taurus is a type of automobile
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Figure 5-12
Single-Level Aggregation
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Figure 5-13
Another Single-Level Aggregation
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Figure 5-14
Multi-Level Aggregation
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Figure 5-15
A Generalization Relationship
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Application: A Date Class
• Stage one: identify and name objects
• Stage two
– Define attributes
• Month, day, and year
• Integer data types
• Stage three
– Create object diagram
– Object diagram shows assignment of values
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Figure 5-16
Initial Date Class Diagram
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Figure 5-17
First Refinement-Date Class Diagram
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Figure 5-18
A Date Object Diagram
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Application: A Date Class
(continued)
• Stage four
– Identify operations that become methods
– Basic operations: constructor, mutator, accessor
– Additional operations: queries with comparisons
• Stage five
– Construct second refinement of class diagram
– Name, attributes, operations detailed
– Visibility denoted
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Table 5-5
Required Operations for the Date Class
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Figure 5-19
Second Refinement-Date Class Diagram
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Explanation of the Basic Date
Class
• Default constructor
– Initializes month, day, year
– cout object echo prints default data
• Overloaded constructor
– Initializes Date object with parameterized interface
– cout object echo prints default data
• setDate( ): almost identical to overloaded
constructor
• showDate( ): accessor manipulates output stream
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Using the Basic Date Class
• Constructors instantiate two Date objects
• Syntax of object declaration
– If default constructor used, follow primitive form
– If overloaded constructor used, supply arguments
• Accessors retrieve data in attributes
• Output modified according to current values
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Simplifying the Code
• Opportunities for optimization exist
• Call internal methods when possible
– Eliminate redundant code
– Do not reinvent your own wheel
– Example: call setDate( ) in constructors
• Dealing with other redundancies
– Target common or repeated actions
– Fold action into method
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Adding Additional Class
Methods
• Virtue of OO programming: scalability
• Add isLeapYear ( )
– Based on leap year algorithm
– Returns a Boolean value
• Add dayOfWeek ( )
– Based on Zeller’s algorithm
– Returns an integer value
• Include appropriate declarations, definition, visibility
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A Closer Look at Program
Testing
• Added complexity increases likelihood of errors
• Selection structures introduce new control paths
–
–
–
–
Ideally, programmer tests each path
Not physically possible
Growth of test paths: 2n
n corresponds to number of if-else statements
• Choose critical elements to test
– Legal and limiting input values
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Summary
• Relational expression (condition) evaluates to 1
(true) or 0 (false)
• Relational operators: <, >, <=, >=, = =, !=
• Logical connectors: AND (&&), OR (| |), NOT (!)
• Basic selection structure: if-else statement
• Compound statement: set enclosed by braces
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Summary (continued)
• Selection variations: nesting, chained if-else, switch
• Uniform Modeling Language (UML): OO design
rules/templates
• Basic UML diagrams: class, object, state, sequence
• Use UML tools to construct/implement Date class
• Testing: selectivity avoids combinatorial explosion
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