CS1371 Introduction to Computing for Engineers

1
CS1371Introduction to Computing for Engineers

• Classes and Objects (continued)

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Administrivia

• If have a re-grade issue, go to help desk.
  Deadline is two weeks after exam next Wed.
• Still reading
• Smith, Chapter 23,24
• It's Friday

3
Where we are so far

• OBJECT ORIENTED PROGRAMMING is like structs with
  behavior
• An OBJECT is an INSTANCE of a CLASS
• George is an INSTANCE of COLLIE
• Object Classes define
• Data (public and private)
• Methods functions (public and private)
• CONSTRUCTOR method makes the objects

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Behavioral Abstraction

• Behavioral Abstractions combine procedural and
  data abstractions.

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Do Simple BankAccount example

• ba BankAccount(100)
• getBalance, setBalance
• ba2 BankAccount(ba) copies
• when just type ba Matlab calls the display
  method, which calls the char method to get a
  string and the disp function to use it.
• For now made setBalance callable

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OO in Matlab (so far)

• To create a CLASS, all it's methods (including
  the constructor) must be in a directory called
  _at_NAME (where NAME is the the class NAME).
• Each class must have some system-useful methods
• char, display
• Also, are the methods that do useful work
• e.g. getBalance, Deposit
• And then there was this assignin nonsense

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We've told you a lie..

• We said
• In Matlab, all arguments are passed into
  functions by COPYING them ("call by VALUE")
• So if you change something in the argument
  variable, that variable in the CALLING
  environment doesn't change.
• This is true (show example)

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Matlab Struct Example

• Create a new function call, called AddPounds
• function addPounds(person,newfat)
• person.weight person.weight newfat
• disp(sprintf('s 's wieght is now f/n'
  person.name, person.weight)
• p1.name 'George Thomas'
• p1.weight 120
• addPounds(p1,10)
• p1.weight is still 120
• Look at setBalance (not private for now) return
  the new balance, in case account is frozen
• Comment out in assignin

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But

• But we also said
• Since you can't change the value of the passed in
  variable that will take effect outside the
  function, we must pass back the new value for
  assignment.
• Example again

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Modify setBalance

• Modify to return both the account and the new
  balance
• Look at getbalance should it return the account
  also???
• Gets ugly

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But

• But we also said
• Since you can't change the value of the passed in
  variable that will take effect outside the
  function, we must pass back the new value for
  assignment.
• That was a lie
• You can assign a value to a variable where that
  ASSIGNment is IN some particular environment.
• assignin(environment,name, value)
• Used whenever you change the object

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assignin

• add back into setBalance
• inputname(1) is the first argument

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Public Versus Private

• Data is NOT reachable from the outside unless you
  provide a method to get it.
• Can't do acct.balance
• So we defined getBalance
• Methods ARE reachable (callable) unless they are
  in the 'PRIVATE' subdirectory of the clas
  directory.
• Move setBalance

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Private data and methods

• move setBalance to the PRIVATE directory

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Having children

• The real power of OOP is the ability to have
  children classes.
• Children classes INHERIT all the abstractions
  (data and methods) of their parents, but can also
  have specialized ones. (Example in a minute.)
• Children have following properties
• All children can be treated as a instance of
  their own class, or any of their ancestor classes
  (parent, parent's parent, etc)
• But, will always get the "right" method for them.

Polymorhpism
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Children Classes Savings Account

• Consider Savings account A BankAccount with
  some more data and methods.
• New data
• interest rate
• Min Balance
• New methods
• calculate interest
• set, get the rate (??)

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Look at Methods for Savings code

• Look at system ones
• constructor adds interest rate and min_balance
• But more importantly look at
• acct class(acct, 'SavingsAccount', super)
• char gives new string - we'll look at in a
  minute
• Why is there no display method????
• Because all savings accounts are also
  BankAccounts, so they have the display method of
  BankAccount.

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Can Call Parent's Method Directly

• Sometimes, you think what your parent wants to do
  is completely wrong for you (later)
• But sometimes, they are almost right, and you
  just want to take what they do and change it a
  little.
• Eample the char method for SavingsAccount

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Char Method for Savings Account

• ba bankaccount(100)
• call char method ba.
• sa SavingsAccount(100)
• Then call char method on sa.
• Look at sa.BankAccount in the char method

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Adding a variable

• To initialize with many values, pass a struct
• Then need to have set and get functions

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Do for SSNUM

• Do it in BankAccount