> For the complete documentation index, see [llms.txt](https://koshizuow.gitbook.io/compilerbook/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://koshizuow.gitbook.io/compilerbook/calculator_level_language.md).

# 創造計算機等級的語言

本章作為C編譯器的第一步，會支援四則運算或其他的算術運算子以支援編譯如以下這樣的算式：

```
30 + (4 - 2) * -5
```

雖然看起來微不足道，但其實這是滿難的目標。算式有括號內的計算優先、乘除法優先等等規則，如果不想個辦法去正確解讀這些架構就沒辦法正確計算。但是，輸入的算式只是一些文字符號，這些資料並沒有包含這些架構。要正確判斷這些算式，就需要分析文字列，並好好抓出這些隱藏的架構。

這類語法分析的問題，如果沒有相關的背景知識，想要解這類的問題會相當困難。其實在以前，這類問題被認為是困難的問題，尤其在1950年代到1970年代花了非常多努力在研究，開發出了各式各樣的演算法。多虧了這些成果，在今天如果想要做語法分析，只要知道做法，是越來越簡單了。

本章將會介紹語法分析最常見演算法之一的遞迴下降分析法（recursive descendent parsing）。如 GCC 或 Clang 等大家日常所使用的 C/C++ 編譯器使用的也是遞迴下降分析法。

不僅限於編譯器，只要有解析某種結構的文本的需求而寫程式時，常常會出現這個方法。在這章所學到的方法，可以直接應用在這類問題上。本章所學的語法分析方法，說是一輩子通用的技術也不誇張吧。好好理解本章的演算法，在身為程式設計師的百寶箱中加入語法分析的招式吧。


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://koshizuow.gitbook.io/compilerbook/calculator_level_language.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.