Words, topics and sentences
Encode, decode information and meaning.
1h20 per week, for 3 weeks
The n-gram model
The n-gram model partly circuments the limits of bag of characters / words in which terms are independent. By capturing dependencies between characters and words, for example between pair of words or triplet of characters, it allows for a finer representation, useful for instance to read DNA sequences (ACGT -> ACT -> Peptide) or understand and translate ingredients with Pyfood.
Note: When using n-gram models, the value of n is important (min, max) and depends on your domain of application.
Application:
- Find latin roots from French, Italian, Spanish words. Embed words with n-grams/tfidf + pca.
- Study language evolution. Word context. W2vec, Glove (300d, oov) → fastvec (next chapter)
A challenge with n-gram models is related to the curse of dimensionality.
Corpus and query preprocessing steps
| Preprocessing step | Description | Beware |
|---|---|---|
| Tokenization | Split text in sequences of tokens | Abbreviations (U.N.), Hyphenation (New-York), Apostrophes, Accents, Dates, Numéros/nombres |
| Stop-words | Frequent words that do not carry semantic information (the, a, an, and, or, to) are removed, reducing dimension and distances | “Let it be”, “To be or not to be” |
| Case-insensitive | Lower case words, reducing dimension and distances | - |
| Lemmatization | Reduce inflected forms of a word: am, were, being, been -> be (requires knowledge of grammar, part of speech) | - |
| Stemming | Reduces tokens to a “root” form, for example Porter Algorithm in English | Resulting terms are not always readable |
Reference
Laure Delisle et al. A large-scale crowdsourced analysis of abuse against women journalists and politicians on Twitter. 2019.
Michel Deudon. On food, bias and seasons: A recipe for sustainability. HAL-02532348. 2020.
Local Seasonal. Pyfood: A Python package to process food. Pypi.