# What is the formula and log base for idf?

To calculate tf-idf, we do:

tf*idf

tf=number of times word occurs in document

What is formula for idf and log base:

1. Log(number of documents/number of documents containing the word)

2. Log((1+number of documents)/(1+number of documents containing the word))

3. 1+Log(number of documents/number of documents containing the word)

4. 1+Log((1+number of documents)/(1+number of documents containing the word))

There a a number of variation how to calculate inverse document frequency. Have a look at the wiki page (Tf-Idf) or scikit-learn's TfidfVetorizer class.