# Discussion 2: Environment Diagrams, Higher-Order Functions

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 [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 presses *Check 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 to check your work.

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

If you have questions, send a message to the `discuss-queue`

channel with the
@discuss tag and your discussion group number and talk it through with someone
from the course staff.

# Higher-Order Functions [60 minutes]

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

- Pick an example input and corresponding output.
*(This time it might be a function.)* - Describe a process (in English) that computes the output from the input using simple steps.
- Figure out what additional names you'll need to carry out this process.
- Implement the process in code using those additional names.
- Determine whether the implementation really works on your original example.
- 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.

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

`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!**. If you get stuck, ask the staff for help.

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 for practice. Then, send a message to the `discuss-queue`

channel with the @discuss tag, your discussion group number, and the message "Listen up!" and a member of the course staff will join your voice channel to hear your description.
If no one comes right away, feel free to work on the next problem until someone from the staff shows up.

### 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 `k`

th 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.

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

Run in 61A Code`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.

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

Run in 61A Code`k`

positions before it.
# Document the occasion

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