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Discussion 7 | CS 61A Spring 2024

Discussion 7: OOP​

Pick someone in your group to join Discord. It's fine if multiple people join, but one is enough.

Now switch to Pensieve:

  • Everyone: Go to discuss.pensieve.co and log in with your @berkeley.edu email, then enter your group number. (Your group number is the number of your Discord channel.)

Once you're on Pensieve, you don't need to return to this page; Pensieve has all the same content (but more features). If for some reason Penseive doesn't work, return to this page and continue with the discussion.

Post in the #help channel on Discord if you have trouble.

Getting Started​

Say your name, another class you're taking besides CS 61A, and something you've practiced for a while, such as playing an instrument, juggling, or martial arts. Did you discover any common interests among your group members?

Q1: Draw​

The draw function takes a list hand and a list of unique non-negative integers positions that are all less than the length of hand. It removes hand[p] for each p in positions and returns a list of those elements in the order they appeared in hand (not the order they appeared in positions).

Fill in each blank with one of these names: list, map, filter, reverse, reversed, sort, sorted, append, insert, index, remove, pop, zip, or sum. See the built-in functions and list methods documentation for descriptions of what these do.

Discussion Time: Before writing anything, talk as a group about what process you'll implement in order to make sure the right cards are removed and returned. Try not to guess-and-check! The purpose of discussion is for you to try to solve problems without the help of an interpreter checking your work.

Run in 61A Code

For a list s and integer i, s.pop(i) returns and removes the ith element, which changes the position (index) of all the later elements but does not affect the position of prior elements.

Calling reversed(s) on a list s returns an iterator. Calling list(reversed(s)) returns a list of the elements in s in reversed order.

Aced it? Give yourselves a hand!

Object-Oriented Programming​

A productive approach to defining new classes is to determine what instance attributes each object should have and what class attributes each class should have. First, describe the type of each attribute and how it will be used, then try to implement the class's methods in terms of those attributes.

Q2: Keyboard​

Overview: A keyboard has a button for every letter of the alphabet. When a button is pressed, it outputs its letter by calling an output function (such as print). Whether that letter is uppercase or lowercase depends on how many times the caps lock key has been pressed.

First, implement the Button class, which takes a lowercase letter (a string) and a one-argument output function, such as Button('c', print).

The press method of a Button calls its output attribute (a function) on its letter attribute: either uppercase if caps_lock has been pressed an odd number of times or lowercase otherwise. The press method also increments pressed and returns the key that was pressed. Hint: 'hi'.upper() evaluates to 'HI'.

Second, implement the Keyboard class. A Keyboard has a dictionary called keys containing a Button (with its letter as its key) for each letter in LOWERCASE_LETTERS. It also has a list of the letters typed, which may be a mix of uppercase and lowercase letters.

The type method takes a string word containing only lowercase letters. It invokes the press method of the Button in keys for each letter in word, which adds a letter (either lowercase or uppercase depending on caps_lock) to the Keyboard's typed list. Important: Do not use upper or letter in your implementation of type; just call press instead.

Read the doctests and talk about:

  • Why it's possible to press a button repeatedly with .press().press().press().
  • Why pressing a button repeatedly sometimes prints on only one line and sometimes prints multiple lines.
  • Why bored.typed has 10 elements at the end.

Discussion Time: Before anyone types anything, have a conversation describing the type of each attribute and how it will be used. Start with Button: how will letter and output be used? Then discuss Keyboard: how will typed and keys be used? How will new letters be added to the list called typed each time a Button in keys is pressed? Call the staff if you're not sure! Once everyone understands the answers to these questions, you can try writing the code together.

Run in 61A Code

Please don't look at the hints until you've discussed as a group and agreed that you need a hint.

Since self.letter is always lowercase, use self.letter.upper() to produce the uppercase version.

The number of times caps_lock has been pressed is either self.caps_lock.pressed or Button.caps_lock.pressed.

The output attribute is a function that can be called: self.output(self.letter) or self.output(self.letter.upper()). You do not need to return the result. Instead return self afterward in order to return the button that was pressed.

The keys can be created using a dictionary comprehension: self.keys = {c: Button(c, ...) for c in LETTERS}. The call to Button should take c and an output function that appends to self.typed, so that every time one of these buttons is pressed, it appends a letter to self.typed.

Call the press method of self.key[w] for each w in word. It should be the case that when you call press, the Button is already set up (in the Keyboard.__init__ method) to output to the typed list of this Keyboard.

Presentation Time: Describe how new letters are added to typed each time a Button in keys is pressed. Instead of just reading your code, say what it does (e.g., "When the button of a keyboard is pressed ..."). One short sentence is enough to describe how new letters are added to typed. When you're ready, send a message to the #discuss-queue channel with the @discuss tag, your discussion group number, and the message "Put it on our tab!" and a member of the course staff will join your voice channel to hear your description and give feedback.

Q3: Bear​

Implement the SleepyBear, and WinkingBear classes so that calling their print method matches the doctests. Use as little code as possible and try not to repeat any logic from Eye or Bear. Each blank can be filled with just two short lines.

Discussion Time: Before writing code, talk about what is different about a SleepyBear and a Bear. When using inheritance, you only need to implement the differences between the base class and subclass. Then, talk about what is different about a WinkingBear and a Bear. Can you think of a way to make the bear wink without a new implementation of print?

Run in 61A Code

Implement a next_eye method that returns an Eye instance that is closed.

One way to make the bear wink is to track how may times the next_eye method is invoked using a new instance attribute and return a closed eye if next_eye has been called an even number of times.

Document the Occasion​

Please all fill out the attendance form (one submission per person per week).