Lists

Q1: Factors List

Write factors_list, which takes a number n and returns a list of its factors in ascending order.

We can know such a basic mathematical fact: the factor of a number can only be up to half of it (when the number is even). So we use for i in range(1, n // 2 + 1)

def factors_list(n):
    """Return a list containing all the numbers that divide `n` evenly, except
    for the number itself. Make sure the list is in ascending order.

    >>> factors_list(6)
    [1, 2, 3]
    >>> factors_list(8)
    [1, 2, 4]
    >>> factors_list(28)
    [1, 2, 4, 7, 14]
    """
    all_factors = []
    # the biggest number which can divide `n` evenly will be `n // 2`
    for i in range(1, n // 2 + 1):
        if n % i == 0:
            all_factors.append(i)
    return all_factors

Q2: Flatten

Write a function flatten that takes a list and “flattens” it. The list could be a deep list, meaning that there could be a multiple layers of nesting within the list.

Intro

I find it really interesting to solve recursive problems. I like this way of solving problems, the code is concise and intuitive, which is why I wrote this blog.

📒 How to solve a recursive problem ?

1. What is the base case ?
2. How to break down the current problem into simpler ones ?

Later, I will follow this idea to solve the recursion problem in this lab.

Recursion

Q2: Summation

Write a recursive implementation of summation, which takes a positive integer n and a function term. It applies term to every number from 1 to n including n and returns the sum.

hw03. Recursion, Tree Recursion

Q1: Num eights

Write a recursive function num_eights that takes a positive integer pos and returns the number of times the digit 8 appears in pos.

Important: Use recursion; the tests will fail if you use any assignment statements. (You can however use function definitions if you so wish.)

It is easy to think of the answer to this question, for we have seen a similar one in lecture. We can split the pos into all_bust_last and last. When we arrive at the base case, we just need to check if it is equals to 8. Because we can not use = in this problem, we need to use function instead.

Intro

I found that the windows management built in macOS is difficult to use. As a result, I always using my mouse to move and resize my window, which is less efficient. We should keep our hands on the keyboard as much as possible. After finishing the MIT-Missing-Semester, I came across the hammerspoon tool. I really like this one :)

What is hammerspoon ?

According to the official site’s introduction, hammerspoon is a tool for powerful automation of OS X, which is just a bridge between the operating system and a Lua scripting engine. The windows management is kind of automation.

hw02. Higher Order Functions

Q1: Product

The summation(n, term) function from the higher-order functions lecture adds up term(1) + ... + term(n). Write a similar function called product that returns term(1) * ... * term(n).

This problem is quite easy, we just need to use * instead of +. The logic is similar to summation(n, term) function in the lecture.

Remember to set ans = 1 at first, after all, 0 * any numbers is always equal to 0 🤗

hw01. Control

Q1. Welcome Forms

Skip :)

Q2. A Plus Abs B

Fill in the blanks in the following function for adding a to the absolute value of b, without calling abs. You may not modify any of the provided code other than the two blanks.

This problem is easy if we know that we can bind names to functions. In this problem:

• When b < 0, a + abs(b) = a - b, so we should use sub
• When b > 0, a + abs(b) = a + b, so we should use add

def a_plus_abs_b(a, b):
    """Return a+abs(b), but without calling abs.

    >>> a_plus_abs_b(2, 3)
    5
    >>> a_plus_abs_b(2, -3)
    5
    """
    if b < 0:
        f = sub         # b < 0 -> a+abs(b) = a - b, using sub
    else:
        f = add         # b > 0 -> a+abs(b) = a + b, using add
    return f(a, b)

Q3. Two of Three

Write a function that takes three positive numbers as arguments and returns the sum of the squares of the two smallest numbers. Use only a single line for the body of the function.

Lecture 09. Security and Cryptography

Entropy

Suppose a password is chosen as a concatenation of four lower-case dictionary words, where each word is selected uniformly at random from a dictionary of size 100,000. An example of such a password is correcthorsebatterystaple. How many bits of entropy does this have?

In order to calculate the bits of entropy, we need to know how many possibilites there.

We can easily know the combinations count are $100000^4$ and the bits of entropy are $log_2 (100000^4)\approx 66\ bit$ 🤗

Lecture 08. Metaprogramming

Most makefiles provide a target called clean. This isn’t intended to produce a file called clean, but instead to clean up any files that can be re-built by make. Think of it as a way to “undo” all of the build steps. Implement a clean target for the paper.pdf Makefile above. You will have to make the target phony. You may find the git ls-filessubcommand useful. A number of other very common make targets are listed here.

Intro

Recently I was fine-tuning my deep learning model, and I habitually started to use print to print some key information on the terminal. So my workflow is like:

1. I type some hyperparameters to train my model.
2. I manually opened an Excel to record the hyperparameters used and the model evaluation results. And I will go back to step 1. If I am not satisfied with results.

Soon, I became bored with this workflow (In fact, I kept this for quite a long time.). However, I suddenly forgot to record key information manually. As a result, I had to navigate in the history output of the the terminal slowly.

debugging

Use journalctl on Linux or log show on macOS to get the super user accesses and commands in the last day. If there aren’t any you can execute some harmless commands such as sudo ls and check again.

When I run log show --last 1d, it keeps running for along time. I don’t know how long it make take, so I will just execute harmless sudo ls to check the log.

Lecture05. Command-line Environment

Job control

From what we have seen, we can use some ps aux | grep commands to get our jobs’ pids and then kill them, but there are better ways to do it. Start a sleep 10000 job in a terminal, background it with Ctrl-Z and continue its execution with bg. Now use pgrep to find its pid and pkill to kill it without ever typing the pid itself. (Hint: use the -af flags).

Lecture 03. Editors (Vim)

Complete vimtutor. Note: it looks best in a 80x24 (80 columns by 24 lines) terminal window.

It is a tutorial for beginners of vim. I will just put some notes which are not mentioned in course here.

• U command: When we press u in normal mode, we can undo the last command. What U does is fixing a whole line.

• Ctrl + G: show your location in the file and the file status. Type the linenumber you want to go, then press G, then you are there.