构建词汇表
将每个词语映射到固定索引
构建词汇表 是 CoddyKit 上的免费 NLP Academy 课时。 这是第 2 节课，共 4 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 NLP Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 NLP Academy 课程共包含 4 节课。
本课时的部分内容尚未翻译，以英文显示。
What Is a Vocabulary?
A vocabulary is the full set of unique words your model knows. Each word gets one fixed slot in every document vector. 📖
Collect Every Unique Word
To build it, gather all words across your documents and keep only the distinct ones, dropping repeats.
words = set("the cat sat the mat".split())Give Each Word an Index
Sort the unique words and assign each a number. This word to index map is your vocabulary lookup.
vocab = {w: i for i, w in enumerate(sorted(words))}The Index Defines the Slot
The index tells a word which position it occupies in every vector, so cat always lands in the same column.
Vocabulary Sets Vector Length
If your vocabulary has 5000 words, every document becomes a vector of 5000 numbers, one slot per word.
Order Must Stay Fixed
Once set, the mapping must never change. A stable order guarantees that column 3 means the same word for every document.
Out-of-Vocabulary Words
A word not in your vocabulary is called out of vocabulary. The simplest choice is to ignore it during counting.
Pruning Rare Words
Words that appear once add noise and size. Dropping very rare terms keeps the vocabulary smaller and cleaner.
Dropping Very Common Words
You can also cut words that appear in nearly every document. These stopwords rarely help tell documents apart.
Look Up an Index
With the map built, finding a word position is instant. Just index the dictionary by the word you want.
print(vocab["cat"]) # the column index for catVocabulary Powers Counting
This map is the backbone of vectorization. Next you fill each slot with how often that word appears in a document.
Quick Check
What does a word index represent?
Recap: Your Vocabulary
You built a vocabulary of unique words, mapped each to a fixed index, and saw how pruning keeps it lean and useful. 🎉
常见问题解答
「构建词汇表」课时是免费的吗？
是的 — 「构建词汇表」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 NLP Academy 课程的其余内容，请升级到 CoddyKit PRO。 NLP Academy 课程共包含 4 节课。
「构建词汇表」这节课中我会学到什么？
将每个词语映射到固定索引 你通过在浏览器中直接运行的动手代码来练习 NLP Academy，全天候 AI 导师会在你学习这节课的过程中回答你的问题。
学习 NLP Academy 需要有经验吗？
无需任何先前经验。CoddyKit 上的 NLP Academy 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 2 节课，共 4 节。
「构建词汇表」课时需要多长时间？
大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。
我能在这节 NLP Academy 课中编写并运行代码吗？
能。每节 NLP Academy 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。