Solutions: You can find the file with solutions for all questions here.

OOP terminology

Object-oriented programming (OOP) is a style of programming that allows you to think of code in terms of "objects." Here's an example of a Car class:

class Car(object):
    num_wheels = 4

    def __init__(self, color):
        self.wheels = Car.num_wheels
        self.color = color

    def drive(self):
        if self.wheels <= Car.num_wheels:
            return self.color + ' car cannot drive!'
        return self.color + ' car goes vroom!'

    def pop_tire(self):
        if self.wheels > 0:
            self.wheels -= 1

Here's some terminology:

• class: a blueprint for how to build a certain type of object. The Car class (shown above) describes the behavior and data that all Car objects have.

• instance: a particular occurrence of a class. In Python, we create instances of a class like this:

  >>> my_car = Car('red')

  my_car is an instance of the Car class.

• attribute or field: a variable that belongs to the class. Think of an attribute as a quality of the object: cars have wheels and color, so we have given our Car class self.wheels and self.color attributes. We can access attributes using dot notation:

  >>> my_car.color
  'red'
  >>> my_car.wheels
  4

• method: Methods are just like normal functions, except that they are tied to an instance or a class. Think of a method as a "verb" of the class: cars can drive and also pop their tires, so we have given our Car class the methods drive and pop_tire. We call methods using dot notation:

  >>> my_car = Car('red')
  >>> my_car.drive()
  'red car goes vroom!'

• constructor: As with data abstraction, constructors describe how to build an instance of the class. Most classes have a constructor. In Python, the constructor of the class defined as __init__. For example, here is the Car class's constructor:

  def __init__(self, color):
      self.wheels = Car.num_wheels
      self.color = color

  The constructor takes in one argument, color. As you can see, the constructor also creates the self.wheels and self.color attributes.

• self: in Python, self is the first parameter for many methods (in this class, we will only use methods whose first parameter is self). When a method is called, self is bound to an instance of the class. For example:

  >>> my_car = Car('red')
  >>> car.drive()

  Notice that the drive method takes in self as an argument, but it looks like we didn't pass one in! This is because the dot notation implicitly passes in car as self for us.

Inheritance

Question 1: Cat

Here's the pet class. Your task is to create a specific pet: Cat

CURRENT_YEAR = 2018

class Animal(object):
    def __init__(self):
        self.is_alive = True  # It's alive!!

class Pet(Animal):
    def __init__(self, name, year_of_birth, owner=None):
        Animal.__init__(self)   # call the parent's constructor
        self.name = name
        self.age = CURRENT_YEAR - year_of_birth
        self.owner = owner

    def eat(self, thing):
        self.talk()
        if thing == "poison":
            self.lose_life()
        print(self.name + " ate a " + str(thing) + "!")

    def talk(self):
        print("..")

Implement a Cat class that inherits from Pet. Use superclass methods wherever possible.

class Cat(Pet):
    """
    >>> my_cat = Cat("Furball", 2011, "Me", lives=2)
    >>> my_cat.talk()
    Meow!
    >>> my_cat.name
    'Furball'
    >>> my_cat.lose_life()
    >>> my_cat.is_alive
    True
    >>> my_cat.eat("poison")
    Meow!
    Furball ate a poison!
    >>> my_cat.is_alive
    False
    >>> my_cat.lose_life()
    'Cat is dead x_x'
    """
    def __init__(self, name, year_of_birth, owner, lives=9):
        assert type(lives) == int and  lives > 0
        "*** YOUR CODE HERE ***"
        Pet.__init__(self, name, year_of_birth, owner)
        self.lives = lives
    def talk(self):
        """A cat says 'Meow!' when asked to talk."""
        "*** YOUR CODE HERE ***"
        print("Meow!")
    def lose_life(self):
        """A cat can only lose a life if it has at least one
        life. When there are zero lives left, the 'is_alive'
        variable becomes False.
        """
        "*** YOUR CODE HERE ***"
        if not self.is_alive:
            return "Cat is dead x_x"
        self.lives -= 1
        if self.lives == 0:
            self.is_alive = False

Toggle Solution

Use OK to test your code:

python3 ok -q Cat --local

Table

In this lab, you will implement a version of the Table object from Data 8, that we are calling T88ble, appropriately pronounced T-88-bel. We aren't going to make you implement every single function, but we have table.select()ed the functions table.where(difficulty, are.below(insane)) and table.where(concepts, are.equal_to(relevant)).

Notice that we are representing tables here as a list of rows.

Question 2: Basic

Complete the num_rows, num_cols, and labels functions according to the docstrings.

    def num_rows(self):
        """
        Compute the number of rows in a table.

        >>> simple_table = T88ble(simple_table_rows, simple_table_labels)
        >>> simple_table.num_rows()
        2
        >>> longer_table = T88ble(longer_table_rows, longer_table_labels)
        >>> longer_table.num_rows()
        3
        """
        "*** YOUR CODE HERE ***"
        return ______
        return len(self.rows)
    def num_cols(self):
        """
        Compute the number of cols in a table.

        >>> simple_table = T88ble(simple_table_rows, simple_table_labels)
        >>> simple_table.num_cols()
        2
        >>> longer_table = T88ble(longer_table_rows, longer_table_labels)
        >>> longer_table.num_cols()
        5
        """
        "*** YOUR CODE HERE ***"
        return ______
        return len(self.rows[0])
    def labels(self):
        """
        Lists the column labels in a table.

        >>> simple_table = T88ble(simple_table_rows, simple_table_labels)
        >>> simple_table.labels()
        ['Flavor', 'Price']
        >>> longer_table = T88ble(longer_table_rows, longer_table_labels)
        >>> longer_table.labels()
        ['Year', 'Bread price', 'Eggs price', 'Average tank of gas', 'Rent per day']
        """
        "*** YOUR CODE HERE ***"
        return ______
        return self.column_labels

Toggle Solution

Use OK to test your code:

python3 ok -q T88ble.num_rows --local

Use OK to test your code:

python3 ok -q T88ble.num_cols --local

Use OK to test your code:

python3 ok -q T88ble.labels --local

Question 3: Column Methods

Complete the column, select, and with_column functions according to the docstrings.

You might find the list.index(value) function to be particularly useful for this problem. Check out the python docs here

    def column(self, label):
        """
        Returns the values of the column represented by label.

        >>> simple_table = T88ble(simple_table_rows, simple_table_labels)
        >>> simple_table.column("Flavor")
        ['chocolate', 'vanilla']
        >>> longer_table = T88ble(longer_table_rows, longer_table_labels)
        >>> longer_table.column("Eggs price")
        [1.5, 2.5, 4]
        >>> longer_table
        ['Year', 'Bread price', 'Eggs price', 'Average tank of gas', 'Rent per day']
        [1990, 1, 1.5, 12, 7]
        [2000, 2, 2.5, 25, 10]
        [2010, 5, 4, 70, 36]
        """
        "*** YOUR CODE HERE ***"
        return ______
        label_index = self.labels().index(label)
        return [row[label_index] for row in self.rows]
    def with_column(self, label, values):
        """
        Returns a new table with an additional or replaced column.
        label is a string for the name of a column, values is an list

        >>> longer_table = T88ble(longer_table_rows, longer_table_labels)
        >>> longer_table.with_column('Inflation rate', [i for i in range(longer_table.num_rows())])
        ['Year', 'Bread price', 'Eggs price', 'Average tank of gas', 'Rent per day', 'Inflation rate']
        [1990, 1, 1.5, 12, 7, 0]
        [2000, 2, 2.5, 25, 10, 1]
        [2010, 5, 4, 70, 36, 2]
        >>> longer_table
        ['Year', 'Bread price', 'Eggs price', 'Average tank of gas', 'Rent per day']
        [1990, 1, 1.5, 12, 7]
        [2000, 2, 2.5, 25, 10]
        [2010, 5, 4, 70, 36]
        """
        "*** YOUR CODE HERE ***"
        return ______
        new_rows = [self.rows[i][:] + [values[i]] for i in range(self.num_rows())]
        new_labels = self.labels()[:] + [label]
        return T88ble(new_rows, new_labels)
    def select(self, labels):
        """
        Create a copy of a table with only some of the columns,
        reffered to by the list of labels.

        >>> simple_table = T88ble(simple_table_rows, simple_table_labels)
        >>> simple_table.select(["Flavor"])
        ['Flavor']
        ['chocolate']
        ['vanilla']
        >>> simple_table
        ['Flavor', 'Price']
        ['chocolate', 2]
        ['vanilla', 1]

        >>> longer_table = T88ble(longer_table_rows, longer_table_labels)
        >>> longer_table.select(['Year', 'Average tank of gas'])
        ['Year', 'Average tank of gas']
        [1990, 12]
        [2000, 25]
        [2010, 70]
        >>> longer_table
        ['Year', 'Bread price', 'Eggs price', 'Average tank of gas', 'Rent per day']
        [1990, 1, 1.5, 12, 7]
        [2000, 2, 2.5, 25, 10]
        [2010, 5, 4, 70, 36]
        """
        "*** YOUR CODE HERE ***"
        return ______
        label_indices = [self.labels().index(label) for label in labels]
        new_rows = [[row[i] for i in label_indices] for row in self.rows]
        return T88ble(new_rows, labels)

Toggle Solution

Use OK to test your code:

python3 ok -q T88ble.column --local

Use OK to test your code:

python3 ok -q T88ble.with_column --local

Use OK to test your code:

python3 ok -q T88ble.select --local

Question 4: Sort

Complete the sort function according to the docstring.

You might find the sorted method to be very useful for this question. sorted can take in a key = lambda... just like the min function from the project. Check out the python docs here

    def sort(self, label, descending=True):
        """
        Create a copy of a table sorted by the values in a column.
        Defaults to ascending order unless descending = True is included.

        >>> longer_table = T88ble(longer_table_rows, longer_table_labels)
        >>> longer_table.sort("Year")
        ['Year', 'Bread price', 'Eggs price', 'Average tank of gas', 'Rent per day']
        [2010, 5, 4, 70, 36]
        [2000, 2, 2.5, 25, 10]
        [1990, 1, 1.5, 12, 7]
        >>> longer_table
        ['Year', 'Bread price', 'Eggs price', 'Average tank of gas', 'Rent per day']
        [1990, 1, 1.5, 12, 7]
        [2000, 2, 2.5, 25, 10]
        [2010, 5, 4, 70, 36]

        >>> simple_table = T88ble(simple_table_rows, simple_table_labels)
        >>> simple_table.sort("Price", descending=False)
        ['Flavor', 'Price']
        ['vanilla', 1]
        ['chocolate', 2]
        >>> simple_table
        ['Flavor', 'Price']
        ['chocolate', 2]
        ['vanilla', 1]

        """
        "*** YOUR CODE HERE ***"
        return ______
        label_index = self.labels().index(label)
        new_rows = sorted(self.rows[:], key=lambda x: x[label_index], reverse = descending)
        return T88ble(new_rows, self.labels())

Toggle Solution

Use OK to test your code:

python3 ok -q T88ble.sort --local

Question 5: Where

Complete the where function according to the docstring.

    def where(self, label, filter_fn):
        """
        Create a copy of a table with only the rows that match a filter function.

        >>> def above(x):
        ...     return lambda y: y > x
        ...
        >>> longer_table = T88ble(longer_table_rows, longer_table_labels)
        >>> longer_table.where('Eggs price', above(2))
        ['Year', 'Bread price', 'Eggs price', 'Average tank of gas', 'Rent per day']
        [2000, 2, 2.5, 25, 10]
        [2010, 5, 4, 70, 36]
        >>> longer_table
        ['Year', 'Bread price', 'Eggs price', 'Average tank of gas', 'Rent per day']
        [1990, 1, 1.5, 12, 7]
        [2000, 2, 2.5, 25, 10]
        [2010, 5, 4, 70, 36]
        """
        "*** YOUR CODE HERE ***"
        return ______
        label_index = self.labels().index(label)
        new_rows = [row[:] for row in self.rows if filter_fn(row[label_index])]
        return T88ble(new_rows, self.labels())

Toggle Solution

Use OK to test your code:

python3 ok -q T88ble.where --local