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# Assignment: Object-Oriented Programming - Class Variables and Methods
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In this assignment, you'll practice:
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* `if/else` statements
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* Lists and iteration
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* Defining classes and instantiating objects/instances
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* Defining and using instance variables
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* Defining and calling instance methods
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* Defining and using class variables
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* Defining and calling class methods
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This assignment can be challenging! Feel free to collaborate with other students on this assignment, but you should still hand in your own assignment.
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## Deliverables and Submitting
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You know what you're doing by now! :grin:
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---
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# Exercise 1: Bank Account
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Some starter code is already in the file named `exercise1.py`.
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Complete the rest of the program!
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*Do not modify any of the code in the `main()` function. Only modify the parts of the program indicated by `???`.*
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1. Fill in the rest of the `BankAccount` class.
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1. Add a **class variable** called `interest_rate` that is a **float** representing the interest rate for all the accounts in the bank
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* `interest_rate` is a class variable because it is used across all of the accounts in the bank!
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1. Add another **class variable** called `accounts` that starts as an empty list
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* This will eventually store the collection of all the accounts in the bank
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1. Add an `__init__` **instance method** that sets the bank account's `balance` to zero
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* The `balance` is stored in an instance variable because the value needs to be different from account to account
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* `__init__` should also add the account that is being initialized to the `accounts` class variable so that the bank can keep track of it
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1. Add an **instance method** called `deposit` that accepts a number as an argument and adds that amount to that account's balance
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* `deposit` needs to be an instance method because it pertains to a *single, specific* account.
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1. Add an **instance method** called `withdraw` that accepts a number as an argument and subtracts that amount from the account's balance
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* Why is `withdraw` an instance method, not a class method?
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* What should happen if you try to withdraw more money than you have?
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1. Add a **class method** called `total_funds` that returns the sum of all balances across all accounts in the `accounts` class variable
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* `total_funds` needs to be a class method because it *does not* pertain to any *single, specific* account
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1. Add a **class method** called `add_interest` that iterates through all accounts, and increases their balances according to the `interest_rate` in effect for all accounts
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* `add_interest` needs to be a class method because it operates on _all_ bank accounts, not a _single, specific_ account.
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### Example output
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When you run the `exercise1.py` program after completing the `BankAccount` class,assuming that you have set the `interest_rate` to `0.01`, you should get the following output:
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```
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0
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0
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200
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1000
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1200
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202.0
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1010.0
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1212.0
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152.0
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1162.0
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```
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---
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# Exercise 2: Vampire Infestation
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There's a vampire infestation! But that doesn't mean we don't have time to practice using class variables and methods.
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Some starter code is already in the file named `exercise2.py`.
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Complete the rest of the program!
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*Do not modify any of the code in the `main()` function. Only modify the parts of the program indicated by `???`.*
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Now that you've had some experience using class variables and methods it's time to test your knowledge of _when_ to use them.
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Your task is to write a `Vampire` class that stores a list of vampires (a `coven`, if you will).
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Every vampire has a `name`, `age`, an `in_coffin` boolean, and a `drank_blood_today` boolean.
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Every day at sunset the vampires leave their coffins in search of blood. If they don't drink blood and get back to their coffins before sunrise, they die.
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Your `Vampire` class should have the following methods:
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* `__init__`, which initializes a new vampire, assigns values for each of its attributes, and adds it to the coven
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* `drink_blood`, which sets a vampire's `drank_blood_today` boolean to true
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* `sunset`, which sets `drank_blood_today` and `in_coffin` to false for the entire coven, as they go out in search of blood
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* `go_home`, which sets a vampire's `in_coffin` boolean to true
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* `sunrise`, which removes from the coven any vampires who are out of their coffins or who haven't drank any blood in the last day
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It's up to you to determine whether each method should be an instance method or a class method.
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You'll also have to decide what instance and class variables you need.
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If you're not sure whether a method should be an instance method or a class method, starting to write the body of the method may help you figure it out based on what data you need access to.
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If you're still uncertain, don't be afraid to ask an instructor for help during office hours.
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Good luck!
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**Big Hint**: For the `sunrise` method, you'll have to iterate through a list. Be extremely careful: [Thou Shalt Not Modify A List During Iteration](https://unspecified.wordpress.com/2009/02/12/thou-shalt-not-modify-a-list-during-iteration/)!
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Instead, as the author of the article suggests, "You could construct a new list during iteration rather than mutating the existing one. (For example, rather than removing all the elements which satisfy a condition, insert into a new list all the elements which don’t)."
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### Example output
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When you run the `exercise2.py` program after completing the `Vampire` class, you should get the following output:
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```
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Coven at the beginning:
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Riley
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Alice
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Jasper
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Renesmee
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Marcus
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Zafrina
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Demetri
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Coven at the end:
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Riley
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Renesmee
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```
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---
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# Parting Thought: Bank Accounts and Vampires
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You're done! But here's a parting thought:
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Isn't it strange how, in most vampire fiction, vampires are lavishly wealthy, often living in expensive homes and wearing fine clothing?
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