bag of words

If we want to use text in Machine Learning algorithms, we’ll have to convert then to a numerical representation. It should be no surprise that computers are very well at handling numbers.

We convert text to a numerical representation called a feature vector. A feature vector can be as simple as a list of numbers.

The bag-of-words model is one of the feature extraction algorithms for text.

Related course: Data Science and Machine Learning with Python – Hands On!

Feature extraction from text

The bag of words model ignores grammar and order of words.
We start with two documents (the corpus):

‘All my cats in a row’,
‘When my cat sits down, she looks like a Furby toy!’,

A list in then created based on the two strings above:

{‘all’: 0, ‘cat’: 1, ‘cats’: 2, ‘down’: 3, ‘furby’: 4, ‘in’: 5, ‘like’: 6, ‘looks’: 7, ‘my’: 8, ‘row’: 9, ‘she’: 10, ‘sits’: 11, ‘toy’: 12, ‘when’: 13 }

The list contains 14 unique words: the vocabulary. That’s why every document is represented by a feature vector of 14 elements. The number of elements is called the dimension.

Then we can express the texts as numeric vectors:

[[1 0 1 0 0 1 0 0 1 1 0 0 0 0]
[0 1 0 1 1 0 1 1 1 0 1 1 1 1]]

Lets take a closer look:

‘All my cats in a row’ = [1 0 1 0 0 1 0 0 1 1 0 0 0 0]

If we follow the order of the vocabulary:

bag of words, feature extraction

we’ll get a vector, the bag of words representation.

Bag of words code

We’ll define a collection of strings called a corpus. Then we’ll use the CountVectorizer to create vectors from the corpus.

  # Feature extraction from text
# Method: bag of words

from sklearn.feature_extraction.text import CountVectorizer

corpus = [
'All my cats in a row',
'When my cat sits down, she looks like a Furby toy!',
'The cat from outer space',
'Sunshine loves to sit like this for some reason.'

vectorizer = CountVectorizer()
print( vectorizer.fit_transform(corpus).todense() )
print( vectorizer.vocabulary_ )

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