Bag of Words and TF-IDF in ML Python

Introduction

Bag of Words and TF-IDF help computers understand text by turning words into numbers. This makes it easier to find patterns in text.

When you want to classify emails as spam or not spam.

When you want to find the main topics in customer reviews.

When you want to compare documents to see how similar they are.

When you want to build a search engine that finds relevant documents.

When you want to analyze social media posts to understand trends.

Syntax

ML Python

from sklearn.feature_extraction.text import CountVectorizer, TfidfVectorizer

# Bag of Words
vectorizer = CountVectorizer()
X_train_counts = vectorizer.fit_transform(documents)

# TF-IDF
vectorizer = TfidfVectorizer()
X_train_tfidf = vectorizer.fit_transform(documents)

CountVectorizer creates a matrix counting how many times each word appears.

TfidfVectorizer creates a matrix that weighs words by how important they are in a document compared to all documents.

Examples

This example shows Bag of Words counting words in two sentences.

ML Python

from sklearn.feature_extraction.text import CountVectorizer

docs = ['I love apples', 'You love oranges']
vectorizer = CountVectorizer()
X = vectorizer.fit_transform(docs)
print(vectorizer.get_feature_names_out())
print(X.toarray())

This example shows TF-IDF values for the same two sentences, highlighting important words.

ML Python

from sklearn.feature_extraction.text import TfidfVectorizer

docs = ['I love apples', 'You love oranges']
vectorizer = TfidfVectorizer()
X = vectorizer.fit_transform(docs)
print(vectorizer.get_feature_names_out())
print(X.toarray())

Sample Model

This program shows how to convert text documents into numbers using Bag of Words and TF-IDF. It prints the words found and the matrices representing the documents.

ML Python

from sklearn.feature_extraction.text import CountVectorizer, TfidfVectorizer

# Sample documents
documents = [
    'I love machine learning',
    'Machine learning is fun',
    'I love coding in Python',
    'Python coding is great'
]

# Bag of Words
count_vectorizer = CountVectorizer()
X_counts = count_vectorizer.fit_transform(documents)
print('Bag of Words feature names:', count_vectorizer.get_feature_names_out())
print('Bag of Words matrix:\n', X_counts.toarray())

# TF-IDF
tfidf_vectorizer = TfidfVectorizer()
X_tfidf = tfidf_vectorizer.fit_transform(documents)
print('\nTF-IDF feature names:', tfidf_vectorizer.get_feature_names_out())
print('TF-IDF matrix:\n', X_tfidf.toarray())

Important Notes

Bag of Words counts words but ignores word order and meaning.

TF-IDF helps reduce the importance of common words like 'is' or 'the'.

Both methods turn text into numbers so machine learning models can use them.

Summary

Bag of Words counts how often words appear in text.

TF-IDF weighs words by how unique they are across documents.

Both help computers understand and work with text data.

Practice

1. What does the Bag of Words model primarily count in a text document?

easy

A. The order of words in the document

B. The frequency of each word appearing in the document

C. The sentiment of the document

D. The length of the document in characters

Solution

1. Step 1: Understand Bag of Words concept

   Bag of Words counts how many times each word appears, ignoring order.

2. Step 2: Identify what is counted

   It counts word frequency, not order, sentiment, or length.

3. Final Answer:

   The frequency of each word appearing in the document -> Option B

4. Quick Check:

   Bag of Words = word counts [OK]

Hint: Bag of Words counts words, not order or meaning [OK]

Common Mistakes:

• Thinking Bag of Words considers word order
• Confusing frequency with sentiment
• Assuming it measures document length

2. Which of the following is the correct way to create a Bag of Words vector using scikit-learn's CountVectorizer in Python?

easy

A. from sklearn.feature_extraction.text import CountVectorizer
vectorizer = CountVectorizer()
matrix = vectorizer.fit(documents)

B. from sklearn.feature_extraction.text import TfidfVectorizer
vectorizer = CountVectorizer()
matrix = vectorizer.fit_transform(documents)

C. from sklearn.feature_extraction.text import CountVectorizer
vectorizer = CountVectorizer()
matrix = vectorizer.transform(documents)

D. from sklearn.feature_extraction.text import CountVectorizer
vectorizer = CountVectorizer()
matrix = vectorizer.fit_transform(documents)

Solution

1. Step 1: Import and initialize CountVectorizer

   Correct import is from sklearn.feature_extraction.text import CountVectorizer and then create an instance.

2. Step 2: Fit and transform documents

   Use fit_transform to learn vocabulary and create the matrix in one step.

3. Final Answer:

   from sklearn.feature_extraction.text import CountVectorizer vectorizer = CountVectorizer() matrix = vectorizer.fit_transform(documents) -> Option D

4. Quick Check:

   CountVectorizer + fit_transform = Bag of Words matrix [OK]

Hint: Use fit_transform to build and apply vectorizer in one step [OK]

Common Mistakes:

• Using transform without fit first
• Importing TfidfVectorizer instead of CountVectorizer
• Calling fit without transform or fit_transform

3. Given the following Python code using TF-IDF, what will be the shape of the output matrix?

from sklearn.feature_extraction.text import TfidfVectorizer
docs = ['cat dog', 'dog mouse', 'cat mouse mouse']
vectorizer = TfidfVectorizer()
X = vectorizer.fit_transform(docs)
print(X.shape)

medium

A. (4, 3)

B. (3, 4)

C. (3, 3)

D. (3, 2)

Solution

1. Step 1: Count unique words across all documents

   The words are 'cat', 'dog', 'mouse'. 'mouse' appears twice but counts once. So unique words = 3.

2. Step 2: Check if TfidfVectorizer adds extra tokens

   TfidfVectorizer by default tokenizes and counts unique words. Here, it finds 3 unique words: 'cat', 'dog', 'mouse'. So matrix shape is (number of docs, number of unique words) = (3,3).

3. Step 3: Verify options

   (3, 3) is (3,3), matching our count. But (3, 4) is (3,4), which suggests 4 unique words. Let's check if any extra token is created.

4. Step 4: Check for default token pattern

   By default, TfidfVectorizer splits on word boundaries, so no extra tokens. So shape is (3,3).

5. Final Answer:

   (3, 3) -> Option C

6. Quick Check:

   Docs=3, unique words=3, shape=(3,3) [OK]

Hint: Shape = (documents, unique words) counted by vectorizer [OK]

Common Mistakes:

• Counting repeated words multiple times
• Confusing rows and columns in shape
• Assuming extra tokens are added automatically

4. The following code tries to create a TF-IDF matrix but raises an error. What is the main problem?

from sklearn.feature_extraction.text import TfidfVectorizer
vectorizer = TfidfVectorizer()
docs = ['apple orange', 'banana apple']
X = vectorizer.transform(docs)
print(X.toarray())

medium

A. fit_transform should be used before transform to learn vocabulary

B. The documents list is empty

C. TfidfVectorizer cannot handle multiple documents

D. toarray() is not a valid method for sparse matrices

Solution

1. Step 1: Understand TfidfVectorizer usage

   Before calling transform, vectorizer must learn vocabulary using fit or fit_transform.

2. Step 2: Identify error cause

   Code calls transform directly without fitting, so vocabulary is unknown, causing error.

3. Final Answer:

   fit_transform should be used before transform to learn vocabulary -> Option A

4. Quick Check:

   fit_transform first, then transform [OK]

Hint: Always fit vectorizer before transform [OK]

Common Mistakes:

• Calling transform without fit
• Assuming toarray() is invalid (it is valid)
• Thinking TfidfVectorizer can't handle multiple docs

5. You have two documents: doc1 = 'apple apple orange' and doc2 = 'apple banana banana'. Using TF-IDF, which word will have the highest weight in doc2 and why?

hard

A. 'banana', because it appears frequently in doc2 but less in doc1

B. 'apple', because it appears in both documents

C. 'orange', because it appears only in doc1

D. 'apple', because it appears most frequently overall

Solution

1. Step 1: Understand TF-IDF weighting

   TF-IDF gives higher weight to words frequent in one document but rare across all documents.

2. Step 2: Analyze word frequencies

   'banana' appears twice in doc2 but not in doc1, so it is rare overall but frequent in doc2.

3. Step 3: Compare with other words

   'apple' appears in both docs, so its IDF is lower. 'orange' is not in doc2, so no weight there.

4. Final Answer:

   'banana', because it appears frequently in doc2 but less in doc1 -> Option A

5. Quick Check:

   TF-IDF favors unique frequent words [OK]

Hint: TF-IDF boosts words frequent in one doc but rare overall [OK]

Common Mistakes:

• Choosing common words across docs as highest weight
• Confusing frequency with uniqueness
• Ignoring that words not in doc have zero weight