Intro

Today I’m going to talk about a new feature introduced in Python 3.8: the Walrus operator（:=）, which is a much-debated feature, but it’s finally passed and released 🤔

In Python, an assignment statement (=) is not an expression but a statement. Walrus operator is expression though. The difference between statement and expression can be simply understood as: expression always returns a value, while statement does not return a value.

Intro

Today I want to talk about the new feature bring in Python 3.10 – Pattern matching 🎉

Those who have learned C language must be familiar with the following switch statement:

Intro

If you’ve been using dynamic languages like Python, Javascript, etc., you probably won’t notice the difference between stack and heap. Because these languages have garbage collectors (GCs) that will automatically manage memory for you, you just need to program at a high level without considering the details. The bad news is that GC is not a cost-free design. No matter how well designed a GC algorithm is, the performance of the code will be degraded to some extent. If you have been in programming for a long time, you may have heard something like “the recursion explodes the stack”. You may or may not click on the search engine to understand the difference between stack and heap. Chances are you just clicked into this article :)

Intro

In the process of learning Vim, the most enlightening sentence for me is :

Vim is a kind of programming language

I once tried to learn Vim a long time ago. However, the keystroke combinations are very difficult to memorize in my opinion. So I later gave up learning Vim and switched to a normal text editor. My view of Vim changed when I took this course: Missing semester. I started to regard Vim as a kind of programming language rather than just a text editor. Then I find that the combinations of various operations of Vim are the syntax of programming language.

What are the semantic actions

When the parser processes the input code, it not only determines whether the syntax is correct but also performs some useful actions. These actions are called semantic actions. In fact, it is a piece of code, which is generally embedded in the rules of the grammar file. Then when the parser applies this specific rule, the code you set will be executed. From another perspective, semantic actions are actually “triggers”, and the trigger condition is that the parser applies the corresponding rules.

Intro

Today I coded the Leetcode 169. Majority Element again. I vaguely remember what the optimal solution is called Boyer-Moore Majority Voting Algorithm. However, I have no idea what is except for its name. So I plan to systematically learn the principle of this algorithm and summarize it to write this blog. I once heard that:

If you want to master something, teach it :)

So, I’m here today to share this algorithm with you, and try to teach you this method in plain language, so let’s get started :)

Intro

I have finished the first two projects - Hog and Cats. The first two projects are relatively simple and uncomplicated. But today, the difficulty of the third project has indeed increased (you can see how complicated this is by looking at the rules of the game). It feels like Plants vs. Zombies

So I’m going to write a blog to sort out the ideas when writing code. 🤗

Trees

Q1: Prune Min

Write a function that prunes a Tree t mutatively. t and its branches always have zero or two branches. For the trees with two branches, reduce the number of branches from two to one by keeping the branch that has the smaller label value. Do nothing with trees with zero branches.

Prune the tree in a direction of your choosing (top down or bottom up). The result should be a linear tree.

BNF

Q1: Grouping and Pipes

In this question, you will add support for grouping and piping.

Recall that grouping allows for an entire regular expression to be treated as a single unit, and piping allows for a pattern to match an expression on either side. Combined, these will let us create patterns which match multiple strings!

Define the group and pipe expressions in your grammar.

Regular Expressions

Q1: Calculator Ops

Write a regular expression that parses strings written in the 61A Calculator language and returns any expressions which have two numeric operands, leaving out the parentheses around them.

We need to write a regular expression to match a pattern - (operand operator1 operator2). The operands consist of +, -, *, /. We can use [] here. Don’t forget to put a \ in front of - to escape it.

Q2: Roman Numerals

Write a regular expression that finds any string of letters that resemble a Roman numeral and aren’t part of another word. A Roman numeral is made up of the letters I, V, X, L, C, D, M and is at least one letter long.

For the purposes of this problem, don’t worry about whether or not a Roman numeral is valid. For example, “VIIIII” is not a Roman numeral, but it is fine if your regex matches it.