How to use gensim for lda on news articles?

Question

I'm trying to retrieve list of topics from a large corpus of news articles, I'm planning to use gensim to extract a topic distribution for each document using LDA. I want to know the format of processed articles required by gensim implementation of lda and how to convert raw articles to that format. I saw this link about using lda on wikipedia dump but I found the corpus to be in a processed state whose format was not mentioned anywhere

Answer 1

There is an offline learning step and an online feature creation step.

Offline Learning

Assume you have a big corpus such as Wikipedia or downloaded a bunch of news articles.

For each article/document:

1. You get the raw text
2. You lemmatize it. Gensim has utils.lemmatize
3. You create a dictionary
4. You create a bag of word representation

Then you train the TF-IDF model and convert the whole corpus to the TF-IDF space. Finally, you train the LDA model on the "TF-IDF corpus".

Online

With an incoming news article you do almost the same:

1. Lemmatize it
2. Create a bag of word representaiton using the dictionary.
3. Convert it to TF-IDF space using the TF-IDF model
4. Convert it to LDA space.