Joaquin Vila

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ACS 168Problem Solving Using the Computer
Chapter 6 Classes

• By
• Joaquin Vila
• Prepared
• bv
• Sally Scott

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Chapter 6Defining Classes and Abstract Data Types

• Structures
• Structures for Diverse Data
• Structures as Function Arguments
• Initializing Structures
• Classes
• Defining Classes and Member Functions
• Public and Private Members
• Summary of Properties of Classes
• Constructors for Initialization
• Abstract Data Types
• Classes to Produce ADTs

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Some terms

• Object
• a special variable with its own special functions
  as well as data
• example cin is an object of the istream class
• Class
• a data type whose variables are objects
• Abstract data type
• a programmer defined data type, such as a class,
  that meets certain standards.

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6.1 Structures

• A language feature for grouping of related data
• Provides a way to represent a collection of
  related data using a single unit.
• A record with fields
• A structure definition defines a type
• struct CDAccount structure
  tag
• 
  
• double balance Member names
• double interest_rate
• int term // months until maturity
• DONT FORGET THE SEMICOLON

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• Declaring structure variables
• CDAccount emp1 // declaration of Account
  //structure variable emp1
• Declaration with initialization
• CDAccount emp2 2550.00, .07, 24
• Assignment statements for member variables
• emp1.balance 3000.00
• emp1.interest_rate .10
• emp1.term 36
• emp2 emp1 // assigns emp2 values of emp1
• emp1.balance emp2.balance // assigns balance
  value

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• Specify member variables with the dot operator
• structure_name.member_variable_name
• emp1.balance
• Use member variables like any other variables
• Different structures may have same member
  variable names
• example
• CDAccount //structure for account with
  balance, interest_rate, and term member
  variables
• SavingAccount // structure for savings
  account with balance, interest_rate, and term
  member variables

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Example of program using a StudentRecord
structure include ltiostreamgt using
namespace std struct StudentRecord int
student_number char grade
int main( ) StudentRecord
your_record your_record.student_number
2001 your_record.grade A cout ltlt
The grade for student ltlt
your_record.student_number ltlt is
ltlt your_record.grade ltlt endl
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Practice
Given the structure and structure variable
declaration Struct CDAccount double
balance double interest_rate int term
char initial1 char initial2 CDAccount
account

• What is the type of each of the following?
• account.balance
• account.interest_rate
• CDAccount.term
• saving_account.initial1
• account.intital2
• account

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Practice
Lect. Notes, pgs. 98-99

• Define a structure type named PartRecord with the
  following fields
• an integer partNumber
• a double price
• an integer numOnHand
• an integer numOrdered
• Declare two variables, partA and PartB of type
  PartRecord
• Declare a PartRecord named partC, initializing it
  to be part number 151, with price 9.30, 10 on
  hand, and 0 ordered.

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• Write a function that gets information about a
  part from the user then returns a PartRecord.
  The function should have no parameters.

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Structures as Function Arguments

• A function can have
• call-by-value parameters of structure type
  and/or
• call-by-reference parameters of structure type
  and/or
• return type that is a structure type
• Example
• CDAccount shrink_wrap(double the_balance,
  double the_rate, int the_term)
• CDAccount temp
• temp.balance the_balance
• temp.interest_rate the_rate
• temp.term the_term
• return temp

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More on Initializing Structures

• A structure may be initialized at the time it is
  declared.
• struct Date
• int month
• int day
• int year
• Date due_date 12, 31, 2001
• The sequence of values is used to initialize the
  successive variables in the struct. The order is
  essential.
• It is an error to have more initializers than
  variables.
• If there are fewer initializers than variables,
  the initializers provided are used to initialize
  the data members. The remainder are initialized
  to 0 for primitive types.

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Use of Hierarchical Structures

• If a structure has a subset of its members that
  may be considered an entity, consider nested
  structures.
• Example A PersonInfo struct might
  include a birthday structure
• struct Date
• int month
• int day
• int year
• struct PersonInfo
• double height // inches
• int weight // pounds
• Date birthday // Date structure

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Use Hierarchical Structures, cont.

• Declare a variable of PersonInfo type as usual
• PersonInfo person1
• person1.birthday // This is a Date
  structure, with members accessible
  
• // as
  in any other structure variable.
• If the structure variable person1 has been set,
  the year a person was born can be output as
  follows
• cout ltlt person1.birthday.year
• structure person1 structure
  birthday structure
• variable member member

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6.2 Classes Defining Classes and Member Functions

• A class is also a user defined data type
• Class variables are called objects
• An object is a variable that
• Has member variables (data members) and can hold
  multiple values (like a structure)
• Also has member functions (methods)
• A class definition specifies the data members
  (much like a structure definition) and the
  function members

variables
functions
class
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A class is a blueprint from which objects are
created
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Defining a Class

• class DayOfYear
• public
• void output( ) member
  function prototype
• int month data members
• int day
• The scope of a class starts at the name of the
  class in the class definition and runs to the
  closing curly brace and semicolon.
• Member functions are declared inside of the class
  definition, but are usually defined outside of it
  (unless very short)

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• // Display 6.3 Class with a Member Function (1 of
  2)
• // Program to demonstrate a very simple example
  of a class.
• // A better version of the class DayOfYear will
  be given in Display 6.4.
• include ltiostreamgt
• using namespace std
• class DayOfYear
• public
• void output( )
  member function prototype
• int month
• int day
• int main( )
• DayOfYear today, birthday //
  declaration of two objects of DayOfYear class
• cout ltlt "Enter today's date\n"

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• // Display 6.3 Class with a Member Function (1 of
  2)
• cout ltlt "Today's date is "
• today. output( )
  calls to the member
  function output
• cout ltlt "Your birthday is "
  
• birthday.output( )
• 
  the calling
  object
• if (today.month birthday.month
  today.day birthday.day)
• cout ltlt "Happy Birthday!\n"
• else
• cout ltlt "Happy Unbirthday!\n"
• return 0
• scope resolution operator
• //Uses iostream
• void DayOfYear output( )
  member function definition
• cout ltlt "month " ltlt month
• ltlt ", day " ltlt day ltlt endl

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Classes

• Objects of class type are declared much the same
  as structure variables
• Class member variables are also specified using
  the dot operator
• Programmer defined member functions of a class
  are called exactly like structure member
  variables, using the dot operator.
• birthday.month birthday.output()
  
• class dot variable
  class dot
  member
• name operator name
  name operator function

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Defining Member Functions, cont.

• C provides the scope resolution operator
  to define a member function outside of the class
  definition
• similar to dot operator that is used with object
  members
• used with a class name
• When defined outside, the function heading must
  include the class name with the scope resolution
  operator. The class name is called the type
  qualifier
• return_type Class_name Function_name(Parameter_
  list)
• void DayOfYear output( )
• // body of function member function
  definition

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Defining Member Functions, cont.

• Anything in the block of the function definition
  is also considered to be in the scope of the
  class. You can use names of any members of that
  class (both data members and function members) in
  the definition of a member function without the
  dot operator.
• When a member function is called, as in
• today.output( )
• the object today is called the calling
  object.

void DayOfYearoutput( ) cout ltlt
month ltlt month ltlt , day ltlt day
ltlt endl
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Encapsulation and Information Hiding

• Combining several items such as variables, or
  variables and functions, into a single package,
  such as an object of some class, is called
  encapsulation
• Each object knows how to perform its own
  functions and how to interact (interface) with
  other objects.
• Information hiding the black box concept
• The goal is to allow the user to know how to use
  the functions, but does not know how the function
  is implemented

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Access Control with Access Specifiers

• use in class definition to label the member
  variables and functions
• public members public
• can be accessed from any function, including
  main( )
• private members private
• these members can only be accessed inside the
  class own member functions, not by outside
  functions.
• this is default
• best to make all member variables private
• order and number of access specifiers in class
  definition does not matter

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• Display 6.4 Class with Private Members (1 of 3)
• //Program to demonstrate the class DayOfYear.
• include ltiostreamgt
• using namespace std
• class DayOfYear
• public
• void input( )
• void output( )
• void set(int new_month, int new_day)
• //Precondition new_month and new_day form a
  possible date.
• //Postcondition The date is reset according
  to the arguments.
• int get_month( )
• //Returns the month, 1 for January, 2 for
  February, etc.

A better class definition
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• Display 6.4 Class with Private Members (2 of 3)
• int main( )
• DayOfYear today, bach_birthday
• cout ltlt "Enter today's date\n"
• today.input( )
• cout ltlt "Today's date is "
• today.output( )
• bach_birthday.set(3, 21)
• cout ltlt "J. S. Bach's birthday is "
• bach_birthday.output( )
• if ( today.get_month( ) bach_birthday.get_m
  onth( )
• today.get_day( )
  bach_birthday.get_day( ) )
• cout ltlt "Happy Birthday Johann
  Sebastian!\n"
• else

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• Display 6.4 Class with Private Members (3 of 3)
• //Uses iostream
• void DayOfYearinput( )
• cout ltlt "Enter the month as a number "
• cin gtgt month
• cout ltlt "Enter the day of the month "
• cin gtgt day
• void DayOfYearoutput( )
• cout ltlt "month " ltlt month
• ltlt ", day " ltlt day ltlt endl
• void DayOfYearset(int new_month, int new_day)
• month new_month
• day new_day

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Why Public and Private Members?

• With an ideal class definition, the class author
  should be able to change the details of the class
  implementation without necessitating changes in
  any program using the class .
• Use accessor functions to access the data members
  that are private and cannot be accessed directly.

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Public and Private Members

• There can be several public and private sections
  in a class.
• Members defined after public are accessible by
  all functions.
• Members defined after private are accessible
  only by all functions defined in the class.

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• Display 6.5 The Bank Account Class (1 of 3)
• //Program to demonstrate the class BankAccount.
• include ltiostreamgt
• using namespace std
• //Class for a bank account
• class BankAccount
• public
• void set(int dollars, int cents, double
  rate)
• //Postcondition The account balance has been
  set to dollars.cents
• //The interest rate has been set to rate
  percent.
• void set(int dollars, double rate)
• //Postcondition The account balance has been
  set to dollars.00.
• //The interest rate has been set to rate
  percent.
• void update( )
• //Postcondition One year of simple interest
  has been
• //added to the account balance.
• double get_balance( )

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• Display 6.5 The Bank Account Class (2 of 3)
• // class BankAccount
• private
• double balance
• double interest_rate
• double fraction(double percent)
• //Converts a percent to a fraction. For
  example, fraction(50.3) returns 0.503.
• int main( )
• BankAccount account1, account2
• cout ltlt "Start of Test\n"
• account1.set(123, 99, 3.0)
• cout ltlt "account1 initial statement\n"
• account1.output(cout)
• account1.set(100, 5.0)

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• Display 6.5 The Bank Account Class (3 of 3)
• void BankAccountset(int dollars, int cents,
  double rate)
• balance dollars 0.01cents
• interest_rate rate
• void BankAccountset(int dollars, double rate)
• balance dollars
• interest_rate rate
• void BankAccountupdate( )
• balance balance fraction(interest_rate)ba
  lance
• double BankAccountfraction(double percent)
• return (percent/100.0)
• double BankAccountget_balance( )
• return balance

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Programming Tips

• Make Data Members private
• make all member variables private. This means
  these variables can only be accessed or changed
  using member functions.
• Define Accessor Functions.
• consider providing a complete set of accessors to
  data in useful formats.
• Use the Assignment Operator with Objects The
  assignment operator applies to struct and
  class objects. You can use it to copy the same
  values from one object to another.

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• Summary of Properties of Classes
• 1. Classes have both member variables and member
  functions.
• 2. A member (either variable or function) may be
  public or private.
• 3. Normally, all variable members of a class are
  private.
• 4. A private member of a class cannot be used
  except in the definition of a function member of
  the same class.
• 5. The name of a member function for a class may
  be overloaded just as the name of an ordinary
  function.
• 6. A class may use another class as the type for
  a member variable.
• 7. A function may have formal parameters with
  class type.
• 8. A function may return an object that is, a
  class may be the type for the value returned by a
  function.
• (This works correctly with all the classes we
  have seen so far. Under circumstances we will
  encounter later, there are special members of
  the class that must be defined for this to work
  correctly. )

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Constructors for Initialization
C provides a special kind of member function
known as a constructor for automatic
initialization of class objects at definition.

• constructor is what builds the object.
• You can write your own constructors or C will
  provide a default constructor for you.
• A class constructor has the same name as the
  class.
• A constructor does not return a value, not even
  void. (This is the only type of function that
  does not have a return value.)

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Constructors for Initialization

• Each constructor should provide a value for every
  member variable.
• If the programmer does not write an explicit
  constructor, a default constructor is
  automatically provided. However, if any
  constructor is coded, this automatic default
  constructor is not provided.
• Constructors can be written to allow input of
  values for some or all variables.
• Class constructors may be overloaded as needed.
• A default constructor should ALWAYS be included
  in every class definition. If you write any
  constructor at all, you should also write a
  default constructor

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Constructors for Initialization

• class BankAccount
• public
• // Constructors
• BankAccount() // default constructor
• BankAccount( int dollars, int cents, double
  rate)
• . . .
• Private
• double balance
• double interest_rate
• BankAccountBankAccount(int dollars, int cents,
  double rate)
• balance dollars .01 cents
• interest_rate rate

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• Display 6.6 Class with Constructors (1 of 4)
• //Program to demonstrate the class BankAccount.
• include ltiostreamgt
• using namespace std
• //Class for a bank account
• class BankAccount
• public
• // Constructors
• BankAccount()
• //Initializes the account balance to 0.00
  and the interest rate to 0.0.
• BankAccount(int dollars, int cents, double
  rate)
• //Initializes the account balance to
  dollars.cents and
• //initializes the interest rate to rate
  percent.
• BankAccount(int dollars, double rate)
• //Initializes the account balance to
  dollars.00 and
• //initializes the interest rate to rate
  percent.
• // Accessor functions

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• Display 6.6 Class with Constructors (2 of 4)
• // other member functions
• void update()
• //Postcondition One year of simple interest
  has been added to the account balance.
• void output(ostream outs)
• //Precondition If outs is a file output
  stream, then outs has already been connected to a
  file.
• //Postcondition Account balance and interest
  rate have been written to the
• //stream outs.
• private
• double balance
• double interest_rate
• // private helper function
• double fraction(double percent)
• //Converts a percent to a decimal fraction.
  For example, fraction(50.3) returns 0.503.
• int main()
• BankAccount account1, // uses default
  constructor
• account2(100, 2.3) // uses third
  constructor

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• Display 6.6 Class with Constructors (3 of 4)
• account1 BankAccount(999, 99, 5.5) //
  calls second constructor to update account1
• cout ltlt "account1 reset to the following\n"
• account1.output(cout)
• return 0
• BankAccountBankAccount()
• balance 0
• interest_rate 0.0
• BankAccountBankAccount(int dollars, int cents,
  double rate)
• balance dollars 0.01cents
• interest_rate rate
• BankAccountBankAccount(int dollars, double
  rate)
• balance dollars

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• Display 6.6 Class with Constructors (4 of 4)
• void BankAccountupdate( )
• balance balance fraction(interest_rate)ba
  lance
• double BankAccountfraction(double percent)
• return (percent/100.0)
• double BankAccountget_balance( )
• return balance
• double BankAccountget_rate( )
• return interest_rate
• //Uses iostream
• void BankAccountoutput(ostream outs)

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• Calling a Constructor
• A constructor is called automatically when an
  object is declared. If you are not using the
  default constructor, you must give the arguments
  for the constructor when you declare the object.
• Syntax (for an object declaration when using a
  declared constructor other than the default
  constructor)
• ClassName ObjectName(Arguments_for_Const
  ructor)
• Example BankAccount account1(100, 2.3)
• Calling a constructor can be called after object
  declaration
• Syntax (for an explicit constructor call)
• ObjectName
  ConstructorName(Arguments_for_Constructor)
• Example account1 BankAccount(200, 3.5)
• A constructor must have the same name as the
  class of which it is a member.
• Hence Class_Name and Constructor_Name are
  the same identifier.

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Pitfall Constructors with no arguments

• The declaration
• BankAccount object_name(100, 2.3)
• invokes the BankAccount constructor that
  requires two parameters.
• The function call
• func()
• invokes a function func that takes no parameters
  
• Conversely,
• BankAccount objectname()
• does NOT invoke the no-parameter constructor.
• Rather, this line of code defines a function that
  returns an object of BankAccount type.

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• Constructors with No Arguments
• When you declare an object and want the
  constructor with zero arguments to be called, you
  do not include parentheses. For example, to
  declare an object and pass two arguments, you
  might do this
• BankAccount account(100, 2.3)
• However, to cause the constructor with NO
  arguments, to be called, you declare the object
• BankAccount account
• You do NOT declare the object
• BankAccount account()

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6.3 Abstract Data Types

• A data type is called an Abstract Data Type (ADT)
  if the programmers who use the type do not have
  access to the details of how the values and
  operations are implemented.
• Programmer defined types are not automatically
  ADTs. Care is required in construction of
  programmer defined types to prevent unintuitive
  and difficult-to-modify code.

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Classes to Produce ADTs How to make an ADT

• Member variables -
• make private.
• Basic operations that the programmer needs
• make a public member function of the class for
  each one, and fully specify how to use each such
  function (use clear comments).
• Helping functions
• make private member functions.
• The interface
• the public member functions along with commentary
  telling how to use the member functions. The
  interface of an ADT should tell all the
  programmer need to know to use the ADT.
• The implementation
• private members of the class and the definitions
  of all member functions. This is information the
  programmer should NOT NEED to use the class.

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Remember

• The client programmer does not need to know how
  data is stored, nor how the functions are
  implemented.
• Consequently alternative implementations may
  store different value types.

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• Display 6.7 Alternative BankAccount
  Implementation(1 of 6)
• //Demonstrates an alternative implementation of
  the class BankAccount.
• include ltiostreamgt
• include ltcmathgt
• using namespace std Notice
  that the public members of
• //Class for a bank account BankAccount
  look and behave
• class BankAccount exactly
  the same as in Display 6.6
• public
• BankAccount(int dollars, int cents, double
  rate)
• //Initializes the account balance to
  dollars.cents and
• //initializes the interest rate to rate
  percent.
• BankAccount(int dollars, double rate)
• //Initializes the account balance to
  dollars.00 and
• //initializes the interest rate to rate
  percent.
• BankAccount( )
• //Initializes the account balance to 0.00
  and the interest rate to 0.0.

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• Display 6.7 Alternative BankAccount
  Implementation(2 of 6)
• double get_balance( )
• //Returns the current account balance.
• double get_rate( )
• //Returns the current account interest rate
  as a percent.
• void output(ostream outs)
• //Precondition If outs is a file output
  stream, then
• //outs has already been connected to a file.
• //Postcondition Account balance and interest
  rate have been
• //written to the stream outs.
• private
• int dollars_part
• int cents_part
• double interest_rate//expressed as a
  fraction, e.g., 0.057 for 5.7
• double fraction(double percent)
  New
• //Converts a percent to a fraction. For
  example, fraction(50.3)
• //returns 0.503.
• double percent(double fraction_value)
• //Converts a fraction to a percent. For
  example, percent(0.503)

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• Display 6.7 Alternative BankAccount
  Implementation(3 of 6)
• int main( )
• BankAccount account1(100, 2.3), account2
• cout ltlt "account1 initialized as follows\n"
  The body of main is identical
• account1.output(cout)
  to that in Display 6.6, the
• cout ltlt "account2 initialized as follows\n"
  screen output is also identical
• account2.output(cout)
  Display 6.6.
• account1 BankAccount(999, 99, 5.5)
• cout ltlt "account1 reset to the following\n"
• account1.output(cout)
• return 0
• BankAccountBankAccount(int dollars, int cents,
  double rate)
• dollars_part dollars
• cents_part cents
• interest_rate fraction(rate)

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• Display 6.7 Alternative BankAccount
  Implementation(4 of 6)
• BankAccountBankAccount(int dollars, double
  rate)
• dollars_part dollars
• cents_part 0
• interest_rate fraction(rate)
• BankAccountBankAccount( )
• dollars_part 0
• cents_part 0
• interest_rate 0.0
• double BankAccountfraction(double percent)
• return (percent/100.0)

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• Display 6.7 Alternative BankAccount
  Implementation(5 of 6)
• //Uses cmath
• void BankAccountupdate( )
• double balance get_balance( )
• balance balance interest_ratebalance
• dollars_part floor(balance)
• cents_part floor((balance -
  dollars_part)100)
• double BankAccountget_balance( )
• return (dollars_part 0.01cents_part)
• double BankAccountpercent(double
  fraction_value)
• return (fraction_value100)
• double BankAccountget_rate( )

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• Display 6.7 Alternative BankAccount
  Implementation(6 of 6)
• //Uses iostream
• void BankAccountoutput(ostream outs)
• outs.setf(iosfixed)
  new definition
• outs.setf(iosshowpoint)
• outs.precision(2)
• outs ltlt "Account balance " ltlt get_balance( )
  ltlt endl
• outs ltlt "Interest rate " ltlt get_rate( ) ltlt
  "" ltlt endl
• The new definitions of get_balance and get_rate
• ensure that the output will still be in the
  correct units.

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• Information Hiding
• You should write the functions so that they can
  be used with no knowledge of how they were
  written as if they were black boxes. We know
  only the interface and specification.
• All the programmer needs to know about a function
  is its prototype and accompanying comment that
  explains how to use the function.
• The use of private member variables and private
  member functions in the definition of an abstract
  data type is another way to implement information
  hiding, where we now apply the principle to data
  values as well as to functions.

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The Class Diagram (lec. notes, p.97)
BankAccount
Class name member variables member functions
(with parameters) function return type
public members _ private members
-balance double -interest_rate
double BankAccount (dollars, cents,
rate) BankAccount(dollars, rate) BankAccount(
) update( ) void get_balance( )
double get_rate( ) double output(outs)
void -fraction(percent) double