Discussion 2: Environment Diagrams, Higher-Order Functions

Switch to Pensieve:

  • Everyone: Go to pensieve.co, log in with your @berkeley.edu email, and enter your group number as the room number (which was in the email that assigned you to this lab). As long as you all enter the same number (any number), you'll all be using a shared document.

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.

Attendance

Fill out this discussion attendance form with the unique number you receive from your TA. As soon as you get your number, fill out the form, selecting arrival (not departure -- that's later).

Getting Started [5 minutes]

Say your name and a city (or place) that you like, which is not Berkeley and is not where you have lived. Feel free to share why you like it.

VERY IMPORTANT: In this discussion, don't press Check Answer or run any Python code until your whole group is sure that the answer is right. Your goal should be to have all checks pass the first time! Figure things out and check your work by thinking about what your code will do. Not sure? Talk to your group! (You won't get to run Python during the midterm, so get used to solving problems without it now.)

Q1: Warm Up

What is the value of result after executing result = (lambda x: 2 * (lambda x: 3)(4) * x)(5)? Talk about it with your whole group and make sure you all agree before anybody checks the answer.

Call Expressions [15 minutes]

Draw an environment diagram for the code below. You can use paper or a tablet or the whiteboard. Talk to your group about how you are going to draw it, then go through each step together. Then, step through the diagram generated by Python Tutor to check your work.

Here's a blank diagram in case you're using a tablet:

template

If you have questions, ask them instead of just looking up the answer! First ask your group, and then your TA.

Higher-Order Functions [60 minutes]

Remember the problem-solving approach from last discussion; it works just as well for implementing higher-order functions.

  1. Pick an example input and corresponding output. (This time it might be a function.)
  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.)

Q2: Make Keeper

Implement make_keeper, which takes a positive integer n and returns a function f that takes as its argument another one-argument function cond. When f is called on cond, it prints out the integers from 1 to n (including n) for which cond returns a true value when called on each of those integers. Each integer is printed on a separate line.

Run in 61A Code

No peeking! First try to implement it without the hint.

To return a function f, include def f(cond): as the first line of the implementation and return f as the last. The f function should introduce i = 1 in order to loop through all integers, calling cond(i) to determine whether cond returns true for each integer.

Don't run Python to check your work. You can check it just by thinking!.

Once your group has converged on a solution, now it's time to practice your ability to describe your own code. A good description is like a good program: concise and accurate. Nominate someone to describe how your solution works and have them present to the group. If you want feedback, you can also present to your TA.

Q3: Digit Finder

Implement find_digit, which takes in a positive integer k and returns a function that takes in a positive integer x and returns the kth digit from the right of x. If x has fewer than k digits, it returns 0.

For example, in the number 4567, 7 is the 1st digit from the right, 6 is the 2nd digit from the right, and the 5th digit from the right is 0 (since there are only 4 digits).

Important: You may not use strings or indexing for this problem. Try to solve this problem using only one line.

Hint: Lambda expressions.

Hint: Use floor dividing by a power of 10 gets rid of the rightmost digits.

Run in 61A Code
First remove all of the digits after digit k, at which point digit k will be the last digit.

Q4: Match Maker

Implement match_k, which takes in an integer k and returns a function that takes in a variable x and returns True if all the digits in x that are k apart are the same.

For example, match_k(2) returns a one argument function that takes in x and checks if digits that are 2 away in x are the same.

match_k(2)(1010) has the value of x = 1010 and digits 1, 0, 1, 0 going from left to right. 1 == 1 and 0 == 0, so the match_k(2)(1010) results in True.

match_k(2)(2010) has the value of x = 2010 and digits 2, 0, 1, 0 going from left to right. 2 != 1 and 0 == 0, so the match_k(2)(2010) results in False.

Important: You may not use strings or indexing for this problem.

Hint: Floor dividing by powers of 10 gets rid of the rightmost digits.

Run in 61A Code
In each iteration, compare the last digit with the one that is k positions before it.

Document the occasion

Let your TA know you're done so that you can each get a departure number, and fill out the attendance form again (this time selecting departure instead of arrival). If your TA isn't in the room, go find them next door.