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Discussion 1 | CS 61A Spring 2024

Discussion 1: Control, Environment Diagrams

What everyone will need:

  • A way to view this worksheet (on your phone is fine)
  • A way to take notes (paper or a device)

Pick someone in your group to join Discord, find your group's channel, and post "Here we go!" in your group's Discord text channel. It's fine if multiple people join, but one is enough.

Find your facilitator: Now figure out who will be your facilitator for the rest of the discussion.

  • If you know you're the facilitator, let your group know.
  • If no one is the facilitator, then whoever has the brightest phone case is the facilitator.

Now switch to Pensieve:

  • Everyone: Go to discuss.pensieve.co and log in with your @berkeley.edu email.
  • Facilitator: Click Create Room, pick this discussion, and share the room code with your group.
  • Others: Don't create a room; instead, click "Join Room" after typing in the room code that the facilitator shared with you.
  • Post in the #help channel on Discord if you have trouble.

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.

Expressions

Say your name and an expression that people say when they really like something, such as "that's awesome" or "nice one". Each person should try to come up with a different expression. Feel free to ask your group for help if you're stuck. You can even use other languages than English. Now you have things to say later in the discussion when the group solves a problem.

While and If

Learning to use if and while is an essential skill. During this discussion, focus on what we've studied in the first three lectures: if, while, assignment (=), comparison (<, >, ==, ...), and arithmetic. Please don't use features of Python that we haven't discussed in class yet, such as for, range, and lists. We'll have plenty of time for those later in the course, but now is the time to practice the use of if (textbook section 1.5.4) and while (textbook section 1.5.5).

Q1: Race

The race function below sometimes returns the wrong value and sometimes runs forever.

Run in 61A Code

Find positive integers x and y (with y larger than x but not larger than 2 * x) for which either:

  • race(x, y) returns the wrong value or
  • race(x, y) runs forever

You just need to find one pair of numbers that satisfies either of these conditions to finish the question, but if you want to think of more you can.

Notes:

  • x += 1 is the same as x = x + 1 when x is assigned to a number.
  • 0 is a false value and all other numbers are true values.

The value of race(x, y) is incorrect when it is not the first time the tortoise passes the hare. Try some small numbers (below 5) to see if you can find a case where tortoise has become larger than hare, but the expression tortoise - hare was not zero when it happened.

Put your (x, y) examples in the text chat of your group's voice channel. If you get stuck for more than 10 minutes, ask for help!

Q2: Fizzbuzz

Implement the classic Fizz Buzz sequence. The fizzbuzz function takes a positive integer n and prints out a single line for each integer from 1 to n. For each i:

  • If i is divisible by both 3 and 5, print fizzbuzz.
  • If i is divisible by 3 (but not 5), print fizz.
  • If i is divisible by 5 (but not 3), print buzz.
  • Otherwise, print the number i.

Try to make your implementation of fizzbuzz concise.

Run in 61A Code

Be careful about the order of your if and elif clauses: try first checking if the current number is divisible by both 3 and 5, then check for just divisibility by 3 and just divisibility by 5.

Problem Solving

A useful approach to implementing a function is to:

  1. Pick an example input and corresponding output.
  2. Describe a process (in English) that computes the output from the input using simple steps.
  3. Figure out what additional names you'll need to carry out this process.
  4. Implement the process in code using those additional names.
  5. Determine whether the implementation really works on your original example.
  6. Determine whether the implementation really works on other examples. (If not, you might need to revise step 2.)

Importantly, this approach doesn't go straight from reading a question to writing code.

For example, in the is_prime problem below, you could:

  1. Pick n is 9 as the input and False as the output.
  2. Here's a process: Check that 9 (n) is not a multiple of any integers between 1 and 9 (n).
  3. Introduce i to represent each number between 1 and 9 (n).
  4. Implement is_prime (you get to do this part with your group).
  5. Check that is_prime(9) will return False by thinking through the execution of the code.
  6. Check that is_prime(3) will return True and is_prime(1) will return False.

Try this approach together on the next two problems.

Important: It's highly recommended that you don't check your work using a computer right away. Instead, talk to your group and think to try to figure out if an answer is correct. On exams, you won't be able to guess and check because you won't have a Python interpreter. Now is a great time to practice checking your work by thinking through examples. You could even draw an environment diagram!

If you're not sure about how something works or get stuck, ask for help from the course staff.

Q3: Is Prime?

Write a function that returns True if a positive integer n is a prime number and False otherwise.

A prime number n is a number that is not divisible by any numbers other than 1 and n itself. For example, 13 is prime, since it is only divisible by 1 and 13, but 14 is not, since it is divisible by 1, 2, 7, and 14.

Use the % operator: x % y returns the remainder of x when divided by y.

Here's a while statement that goes through all numbers above 1 and below n :

i = 2
while i < n:
...
i = i + 1

You can use n % i == 0 to check whether i is a factor of n. If it is, return False.

Run in 61A Code

Presentation Time: Come up with a one sentence description of the process you implemented to solve is_prime that you think someone could understand without looking at your code. Pick someone to present, then send a message to the discuss-queue channel with the @discuss tag, your discussion group number, and the message "Prime Time!" and a member of the course staff will join your voice channel to hear your description and give feedback.

Q4: Unique Digits

Write a function that returns the number of unique digits in a positive integer.

Hints: You can use // and % to separate a positive integer into its one's digit and the rest of its digits.

You may find it helpful to first define a function has_digit(n, k), which determines whether a number n has digit k.

Run in 61A Code

One approach is to loop through every digit from 0 to 9 and check whether n has the digit. Count up the ones it has.

Environment Diagrams

An environment diagram keeps track of names and their values in frames, which are drawn as boxes.

Q5: Bottles

Answer the following questions with your group. Step through the diagram to check your answers.

1) What determines how many different frames appear in an environment diagram?

  • The number of functions defined in the code
  • The number of call expressions in the code
  • The number of return statements in the code
  • The number of times user-defined functions are called when running the code

2) What happens to the return value of pass(bottles)?

  • It is used as the new value of remaining in the global frame
  • It is used as the new value of bottles in the global frame
  • It is used as the new value of pass in the global frame
  • None of the above

3) What effect does the line bottles = 98 have on the global frame?

  • It temporarily changes the value bound to bottles in the global frame.
  • It permanently changes the value bound to bottles in the global frame.
  • It has no effect on the global frame.

Q6: Double Trouble

Draw the environment diagram on paper or a whiteboard (without having the computer draw it for you)! Then, check your work by stepping through the diagram.

Document the occasion

If you'd like, take another group selfie and add it to the text chat of your group's Discord channel. Then, please all fill out the attendance form (one submission per person per week).