Text Processing Exercise

What you'll learn

• How to preprocess text data using NLTK
• Text tokenization and stemming techniques
• Feature extraction using bag-of-words
• Working with real-world text datasets
• Handling punctuation and special characters

Introduction

💻 Activity Type: Coding Exercise

⏱️ Total Time: 90 minutes

Access the exercise notebook here:

💡 Make sure you're signed in to your Google account to access the Colab notebook. Create a copy of the notebook before starting.

Question One: Processing List of Words

⏱️ Time: 15 minutes

Text Preprocessing

1. Import the necessary NLTK components
   • Import nltk and SnowballStemmer
2. Download the Punkt Tokenizer
   • Use nltk.download('punkt')
3. Initialize a Snowball stemmer for English
   • Create stemmer instance: stemmer = SnowballStemmer('english')
4. Stem each word in the provided list
   • Apply stemmer to each word using a loop or list comprehension
5. Store and print the stemmed words
   • Save results in a new list and display them tip The Snowball stemmer reduces words to their root form. For example, "running" becomes "run".

Feature Extraction

1. Import CountVectorizer from sklearn

   • from sklearn.feature_extraction.text import CountVectorizer

2. Initialize the vectorizer

   • vectorizer = CountVectorizer()

3. Fit the vectorizer with the word list

   • Use vectorizer.fit(word_list)

4. Extract and print the vocabulary

   • Access vocabulary using vectorizer.vocabulary_

5. Transform the words into a bag-of-words matrix

   • Use vectorizer.transform(word_list)

6. Print the resulting matrix

   • Display the sparse matrix representation

Question 2: Processing Sentence

⏱️ Time: 15 minutes

Text Preprocessing

1. Tokenize the provided sentence

   • Use nltk.word_tokenize(sentence)

2. Stem each token using the Snowball stemmer

   • Apply stemmer to each token in the list

3. Join the stemmed tokens back into a sentence

   • Use ' '.join(stemmed_tokens)

4. Print the processed sentence

   • Display both original and processed versions

Feature Extraction

1. Initialize CountVectorizer

   • Create new vectorizer instance

2. Fit with the processed sentence

   • Remember to wrap sentence in a list: [processed_sentence]

3. Extract and print vocabulary

   • Display the vocabulary dictionary

4. Transform sentence to bag-of-words

   • Transform the processed sentence

5. Print the resulting matrix

   • Show the sparse matrix representation

💡 CountVectorizer expects a list of documents, so even for a single sentence, wrap it in a list!

Question 3: Process Sentence with Punctuations

⏱️ Time: 20 minutes

Text Processing

1. Tokenize the sentence
   • Use NLTK's word tokenizer
2. Stem each token
   • Apply stemmer to all tokens
3. Remove non-alphanumeric characters
   • Filter tokens using token.isalnum()
   • This removes punctuation and special characters
4. Join tokens back into a sentence
   • Use space as separator
5. Print the cleaned sentence

   • Compare with original to see changes

⚠️ Warning Removing punctuation can change meaning! Consider if this is appropriate for your specific use case.

Feature Extraction

1. Initialize CountVectorizer
   • Create fresh vectorizer instance
2. Fit with cleaned sentence
   • Remember to use list format
3. Extract vocabulary
   • Print vocabulary items
4. Transform to bag-of-words
   • Apply transformation
5. Print the matrix
   • Display the result

Advanced Challenges

Challenge One: Processing List of Sentences

⏱️ Time: 20 minutes

1. Create empty list for processed sentences
   • processed_sentences = []
2. For each sentence:
   • Tokenize using word_tokenize()
   • Stem tokens with your stemmer
   • Remove non-alphanumeric characters using isalnum()
   • Join back into sentence with spaces
   • Append to results list
3. Print processed sentences
   • Display original vs. processed for comparison tip Use a helper function to avoid repeating the same preprocessing steps!

Feature Extraction

1. Initialize CountVectorizer

   • Consider setting parameters like max_features

2. Fit with all processed sentences

   • Use the entire list of processed sentences

3. Extract vocabulary

   • Check vocabulary size

4. Transform all sentences to bag-of-words

   • This creates a matrix with one row per sentence

5. Print the matrix

   • Use .toarray() to see the dense representation

Challenge 2: Processing Dataset

⏱️ Time: 20 minutes

1. Download and load the emotion dataset

   • Use pandas to read the CSV file
   • Check dataset structure with .head()

2. Create text preprocessing function that:

   python

   def preprocess_text(text):
       # Tokenize
       # Stem tokens
       # Remove non-alphanumeric characters
       # Join back into text
       return processed_text
   

3. Apply function to entire dataset

   • Use .apply() method on the text column

4. Print first 5 original and processed sentences

   • Compare side by side to verify processing

⚠️ Warning Large datasets may take time to process. Consider using a subset for testing first!

Feature Extraction

1. Initialize CountVectorizer

   • Consider limiting vocabulary size for large datasets

2. Fit with all processed text

   • This learns the vocabulary from the entire dataset

3. Extract vocabulary

   • Check total number of unique words

4. Transform dataset to bag-of-words

   • Creates a sparse matrix representation

5. Print first sample's sparse representation

   • Shows non-zero elements only

6. Convert and print first sample as array

   • Use .toarray()[0] to see full vector

Troubleshooting

Common Issues:

1. NLTK Download Errors

   • If nltk.download('punkt') fails, try:
   python

   import ssl
   ssl._create_default_https_context = ssl._create_unverified_context
   nltk.download('punkt')
   

2. Memory Issues with Large Datasets

   • Use max_features parameter in CountVectorizer
   • Process data in batches
   • Consider using TfidfVectorizer for better memory efficiency

3. Empty Vocabulary

   • Check if text preprocessing is removing all words
   • Ensure you're fitting the vectorizer before transforming

Resources

• NLTK Documentation
• CountVectorizer Documentation
• Emotions dataset for NLP on Kaggle

Exercise Submission

ℹ️ Info

📝 Submission Instructions

Once you've completed all questions and challenges:

1. Save your Colab notebook (File -> Save a copy in Drive)
2. Generate a shareable link (Share -> Copy link)
3. Submit your work through the form below
4. Include any observations or challenges you faced

Submission Deadline: Check with your instructor

Submit Your Exercise Here