最近正在跟着《Crafting interpreter》这本书写解释器，原本书里面用 Java 实现了一个 Tree-walker 解释器 jlox，我正在用 Python 重写一遍，称为 pylox。看了这本书感觉对解释器的理解越来越深刻了，很推荐👍。此时的我突然想起来之前看完的 CS61A 的 Scheme 解释器还有几个小问题没有解决，导致它一直是未完成的状态，于是今天我打开了这个项目，打算从头到尾捋一遍，讲讲思路。

前两个项目还算简单, 比较不复杂. 但是今天这个第三个项目难度确实是上升了(看游戏规则就知道这个有多复杂了). 感觉像是植物大战僵尸

所以我打算为他写一篇博客来整理一下写代码时候的思路. 话不多说, 让我们进入正题吧 !

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.

]]> https://martinlwx.github.io/zh-cn/hw10-of-cs61a-of-ucb/ Wed, 02 Mar 2022 13:36:13 +0800 MartinLwx https://martinlwx.github.io/zh-cn/hw10-of-cs61a-of-ucb/ 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.

1. A group consists of any regex expression surrounded by parentheses (()).
2. A pipe operator consists of a regex expression, followed by a pipe (|) character, and lastly followed by another regex expression.

For example, r"apples" would match exactly the phrase “apples” in an input. If we wanted our pattern from before to match “oranges” as well, we could expand our rstring to do so using groupings and pipes: r"(apples)|(oranges)".

]]> https://martinlwx.github.io/zh-cn/lab12-ca61a-of-ucb/ Wed, 02 Mar 2022 11:15:08 +0800 MartinLwx https://martinlwx.github.io/zh-cn/lab12-ca61a-of-ucb/ 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.

写一个符合 (operand operator1 operator2) 格式的正则表达式. 这个是比较简单的, 因为这里的运算符只有 +, -, *, /. 我们用 [] 括起来就可以, 但是要注意**- 需要用 \ 转义**

]]> https://martinlwx.github.io/zh-cn/hw09-of-cs61a-of-ucb/ Tue, 01 Mar 2022 21:13:22 +0800 MartinLwx https://martinlwx.github.io/zh-cn/hw09-of-cs61a-of-ucb/ 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.

]]> https://martinlwx.github.io/zh-cn/lab11-cs61a-of-ucb/ Tue, 01 Mar 2022 00:41:21 +0800 MartinLwx https://martinlwx.github.io/zh-cn/lab11-cs61a-of-ucb/ Context

---

Problem 1

Important: Your code for this part should go in buffer.py.

Your job in this part is to implement the current and pop_first methods of the Buffer class.

current should return the current token of the current line we’re on in the Buffer instance without removing it. If there are no more tokens in the current line, then current should move onto the next valid line, and return the first token of this line. If there are no more tokens left to return from the entire source (we’ve reached the end of all input lines), then current should return None (this logic is already provided for you in the except StopIteration block).

]]> https://martinlwx.github.io/zh-cn/hw08-of-cs61a-of-ucb/ Mon, 28 Feb 2022 00:28:31 +0800 MartinLwx https://martinlwx.github.io/zh-cn/hw08-of-cs61a-of-ucb/ Q1: My Filter

Write a procedure my-filter, which takes a predicate func and a list lst, and returns a new list containing only elements of the list that satisfy the predicate. The output should contain the elements in the same order that they appeared in the original list.

Note: Make sure that you are not just calling the built-in filter function in Scheme - we are asking you to re-implement this!

]]> https://martinlwx.github.io/zh-cn/hw06-of-cs61a-of-ucb/ Sun, 27 Feb 2022 13:17:39 +0800 MartinLwx https://martinlwx.github.io/zh-cn/hw06-of-cs61a-of-ucb/ OOP

---

Q1: Vending Machine

In this question you’ll create a vending machine that only outputs a single product and provides change when needed.

Create a class called VendingMachine that represents a vending machine for some product. A VendingMachineobject returns strings describing its interactions. Remember to match exactly the strings in the doctests – including punctuation and spacing!

Fill in the VendingMachine class, adding attributes and methods as appropriate, such that its behavior matches the following doctests:

]]> https://martinlwx.github.io/zh-cn/lab10-cs61a-of-ucb/ Sun, 27 Feb 2022 01:04:02 +0800 MartinLwx https://martinlwx.github.io/zh-cn/lab10-cs61a-of-ucb/ Q2: Over or Under

Define a procedure over-or-under which takes in a number num1 and a number num2 and returns the following:

• -1 if num1 is less than num2
• 0 if num1 is equal to num2
• 1 if num1 is greater than num2

Challenge: Implement this in 2 different ways using if and cond!

text

]]>