OOP: exam-level questions

If you need help reviewing OOP, take a look at these resources:

• Albert's and Robert's slides (Pokemon examples!)
• Albert's and Robert's slides (inheritance)

Each question has a "Toggle Solution" button -- click it to reveal that question's solution.

What would Python print?

For the following questions, use the following class definition:

class Account:
    """A class computer account. Each account has a two-letter ID
    and the name of the student who is registered to the account.
    """
    num_of_accounts = 0
    def __init__(self, id):
        self.id = id
        Account.num_of_accounts += 1

    def register(self, student):
        self.student = student
        print('Registered ' + student)

    @property
    def type(self):
        return type(self)

Question 1

>>> acc_aa = Account("aa")
>>> acc_aa.register("Peter Perfect")
______Registered Peter Perfect
>>> Account.register(self, "Jom Magrotker")
______NameError: 'self' is not defined
>>> Account.register(acc_aa, "Jom Magrotker")
______Registered Jom Magrotker
>>> Account.register("Jom Magrotker")
______TypeError: requires 2 arguments, since it's not a bound method

Question 2

Use the acc_aa from the previous question.

>>> f1 = Account.register
>>> f1(acc_aa, "Peter Perfect")
______Registered Peter Perfect
>>> f2 = acc_aa.register
>>> f2(acc_aa, "Peter Perfect")
______TypeError: should only take one argument, since it is a bound method!
>>> f2("Peter Perfect")
______Registered Peter Perfect

Question 3

Use the acc_aa from the previous question.

>>> Account.register = lambda self: "WAT"
>>> Account.register(acc_aa, "Hello")
______TypeError: should only take one argument, self (but it is not a bound method!)
>>> Account.register("Hello")
______'WAT' # self is 'Hello'
>>> acc_aa.register("goodbye")
______TypeError: should only take one argument, given 2 (acc_aa, 'goodbye')
>>> acc_aa.register()
______'WAT'

Question 4

Assume you have started a new Python interactive session with the original definition of the Account class.

>>> acc_aa = Account("aa")
>>> acc_aa.register("Peter Perfect")
______Registered Peter Perfect
>>> acc_zz = Account("zz")
>>> acc_aa.register = acc_zz.register
>>> acc_aa.register("Bozo")
______Registered Bozo
>>> acc_aa.student
______'Peter Perfect'
>>> acc_zz.student
______'Bozo'

Code-Writing questions

Question 5

In computer science, a circular buffer a type of data structure that is used to store temporary, sequential data in a constant amount of space (this is commonly used to buffer data streams) in a first-in-first-out manner (the first element to be added is the first element to be removed). A circular buffer has the following properties:

• Each buffer has a fixed size of n elements (e.g. strings). This size is determined upon creation of the buffer. Note that the total number of elements that can be inserted into the buffer can exceed n, but the number of elements in buffer at any given time must be less than or equal to n.
• Each buffer has a start that keeps track of the earliest element that is currently in the buffer. Similarly, each buffer has an end that keeps track of the next available empty index in the buffer.
• The buffer has an append method, which adds a given element into the buffer. If the buffer is full, (i.e. the buffer already has n elements), do not add the element, and instead print "Buffer exceeded capacity".
• The buffer has a remove method, which removes the earliest element that is still in the buffer. If there are no elements in the buffer, print "Buffer is empty".

In order to implement the append and remove methods, you should have list of length n that stores the elements currently in the buffer. When you append the ith element, you should insert it into index (i mod n) of the list. Similarly, when you are removing the jth element, you should extract the element at index (j mod n) of the list. For more descriptions of the behavior, see the doctest.

class CircularBuffer:
    """Doctests:

    >>> buffer = CircularBuffer(3)
    >>> buffer.remove()
    Buffer is empty
    >>> buffer.append('a')
    >>> buffer.remove()
    'a'
    >>> buffer.remove()
    Buffer is empty
    >>> buffer.append('b')
    >>> buffer.append('c')
    >>> buffer.append('d')
    >>> buffer.append('e')
    Buffer capacity exceeded
    >>> buffer.remove()
    'b'
    >>> buffer.remove()
    'c'
    >>> buffer.remove()
    'd'
    >>> buffer.remove()
    Buffer is empty
    """
    def __init__(self, n):
        self.array = [None]*n   # list of length n
        self.n = n
        self.start = 0
        self.end = 0

    def append(self, elem):
        "*** YOUR CODE HERE ***"

    def remove(self):
        "*** YOUR CODE HERE ***"

class CircularBuffer:
    def __init__(self, n):
        self.array = [None]*n   # list of length n
        self.n = n
        self.start = 0
        self.end = 0

    def append(self, elem):
        if self.end - self.start == self.n:
            print('Buffer capacity exceeded')
        else:
            self.array[self.end % self.n] = elem
            self.end += 1

    def remove(self):
        if self.end == self.start:
            print('Buffer is empty')
        else:
            elem = self.array[self.start % self.n]
            self.start += 1
            return elem

Question 6

Write a Chef class with the following qualities:

• Each Chef is initialized with a list of required ingredients. Each item in the list is added to a storage that is shared by all the Chefs with an initial stock of 2. If the item is already in the storage, do NOT add it in again.
• Each Chef can fetch_ingredients from a storage that is shared by all the Chefs. Each Chef only needs 1 of each ingredient.
• Each Chef can serve, where they put their finished food in a shared list of finished foods.

For finer details of implementation, see the doctest.

class Chef:
    """Doctests:

    >>> albert = Chef('quiche', ['egg', 'cheese', 'cream', 'salt'])
    >>> ramsay = Chef('steak', ['meat', 'bbq sauce', 'salt'])
    >>> ramsay.cook()
    'Not enogh ingredients!'
    >>> ramsay.serve()
    'No food to serve!'
    >>> ramsay.fetch_ingredients()     # 1 salt remaining
    "Fetched: ['meat', 'bbq sauce', 'salt']"
    >>> ramsay.cook()
    'Cooked steak!'
    >>> ramsay.serve()
    >>> Chef.finished
    ['steak']
    >>> albert.fetch_ingredients()     # 0 salt remaining
    "Fetched: ['egg', 'cheese', 'cream', 'salt']"
    >>> albert.cook()
    'Cooked quiche!'
    >>> albert.serve()
    >>> Chef.finished
    ['steak', 'quiche']
    >>> ramsay.fetch_ingredients()
    'No more salt!'
    """
    "*** YOUR CODE HERE ***"

class Chef:
    storage = {}
    finished = []

    def __init__(self, food, ingredients):
        self.food, self.ingredients = food, ingredients
        self.fetched, self.cooked = False, False
        for elem in self.ingredients:
            Chef.storage[elem] = 2

    def fetch_ingredients(self):
        for elem in self.ingredients:
            if Chef.storage[elem] == 0:
                return 'No more ' + elem + '!'
            Chef.storage[elem] -= 1
        self.fetched = True
        return 'Fetched: ' + str(self.ingredients)

    def cook(self):
        if self.fetched:
            self.cooked, self.fetched = True, False
            return 'Cooked ' + self.food + '!'
        return 'Not enough ingredients!'

    def serve(self):
        if not self.cooked:
            return 'No food to serve!'
        Chef.finished.append(self.food)
        self.cooked = False