Continuous bag of words (CBOW) in NLP

In order to make the computer understand a written text, we can represent the words as numerical vectors. One way to do so is by Using Word embeddings, they are a way of representing words as numerical vectors. These vectors capture the meaning of the words and their relationships to other words in the language. Word embeddings can be generated using unsupervised learning algorithms such as Word2vec, GloVe, or FastText. 

Word2vec is a neural network-based method for generating word embeddings, which are dense vector representations of words that capture their semantic meaning and relationships. There are two main approaches to implementing Word2vec:

• Continuous bag-of-words (CBOW) 
• Skip-gram

What is a Continuous Bag of Words (CBOW)?

Continuous Bag of Words (CBOW) is a popular natural language processing technique used to generate word embeddings. Word embeddings are important for many NLP tasks because they capture semantic and syntactic relationships between words in a language. CBOW is a neural network-based algorithm that predicts a target word given its surrounding context words. It is a type of “unsupervised” learning, meaning that it can learn from unlabeled data, and it is often used to pre-train word embeddings that can be used for various NLP tasks such as sentiment analysis, text classification, and machine translation. 

Example of a CBOW Model

Is there any difference between  Bag-of-Words (BoW) model and the Continuous Bag-of-Words (CBOW)?

• The Bag-of-Words model and the Continuous Bag-of-Words model are both techniques used in natural language processing to represent text in a computer-readable format, but they differ in how they capture context.
• The BoW model represents text as a collection of words and their frequency in a given document or corpus. It does not consider the order or context in which the words appear, and therefore, it may not capture the full meaning of the text. The BoW model is simple and easy to implement, but it has limitations in capturing the meaning of language.
• In contrast, the CBOW model is a neural network-based approach that captures the context of words. It learns to predict the target word based on the words that appear before and after it in a given context window. By considering the surrounding words, the CBOW model can better capture the meaning of a word in a given context.

Architecture of the CBOW model

The CBOW model uses the target word around the context word in order to predict it. Consider the above example “She is a great dancer.” The CBOW model converts this phrase into pairs of context words and target words. The word pairings would appear like this ([she, a], is), ([is, great], a) ([a, dancer], great) having window size=2. 

CBOW Architecture

The model considers the context words and tries to predict the target term. The four 1∗W input vectors will be passed to the input layer if have four words as context words are used to predict one target word. The hidden layer will receive the input vectors and then multiply them by a W∗N matrix. The 1∗N output from the hidden layer finally enters the sum layer, where the vectors are element-wise summed before a final activation is carried out and the output is obtained from the output layer.

Code Implementation of CBOW

Let’s implement a word embedding to show the similarity of words using the CBOW model. In this article I have defined my own corpus of words, you use any dataset. First, we will import all the necessary libraries and load the dataset. Next, we will tokenize each word and convert it into a vector of integers.

# Re-import necessary modules
import tensorflow as tf
from tensorflow.keras.models import Sequential
from tensorflow.keras.layers import Dense, Embedding, Lambda
from tensorflow.keras.preprocessing.text import Tokenizer
from tensorflow.keras.utils import to_categorical
import numpy as np
import matplotlib.pyplot as plt
from sklearn.decomposition import PCA

# Define the corpus
corpus = [
    'The cat sat on the mat',
    'The dog ran in the park',
    'The bird sang in the tree'
]

# Convert the corpus to a sequence of integers
tokenizer = Tokenizer()
tokenizer.fit_on_texts(corpus)
sequences = tokenizer.texts_to_sequences(corpus)
print("After converting our words in the corpus into vector of integers:")
print(sequences)

Output:

After converting our words in the corpus into vector of integers:
[[1, 3, 4, 5, 1, 6], [1, 7, 8, 2, 1, 9], [1, 10, 11, 2, 1, 12]]

Now, we will build the CBOW model having window size = 2.

# Define the parameters
vocab_size = len(tokenizer.word_index) + 1
embedding_size = 10
window_size = 2

# Generate the context-target pairs
contexts = []
targets = []
for sequence in sequences:
    for i in range(window_size, len(sequence) - window_size):
        context = sequence[i - window_size:i] + sequence[i + 1:i + window_size + 1]
        target = sequence[i]
        contexts.append(context)
        targets.append(target)

# Convert the contexts and targets to numpy arrays
X = np.array(contexts)
y = to_categorical(targets, num_classes=vocab_size)

# Define the CBOW model
model = Sequential()
model.add(Embedding(input_dim=vocab_size, output_dim=embedding_size, input_length=2 * window_size))
model.add(Lambda(lambda x: tf.reduce_mean(x, axis=1)))
model.add(Dense(units=vocab_size, activation='softmax'))

# Compile the model
model.compile(optimizer='adam', loss='categorical_crossentropy', metrics=['accuracy'])

# Train the model
model.fit(X, y, epochs=100, verbose=0)

Next, we will use the model to visualize the embeddings.

# Extract the embeddings
embedding_layer = model.layers[0]
embeddings = embedding_layer.get_weights()[0]

# Perform PCA to reduce the dimensionality of the embeddings
pca = PCA(n_components=2)
reduced_embeddings = pca.fit_transform(embeddings)

# Visualize the embeddings
plt.figure(figsize=(5, 5))
for word, idx in tokenizer.word_index.items():
    x, y = reduced_embeddings[idx]
    plt.scatter(x, y)
    plt.annotate(word, xy=(x, y), xytext=(5, 2),
                 textcoords='offset points', ha='right', va='bottom')
plt.title("Word Embeddings Visualized")
plt.show()

Output:

Graphical Representation of the Word Vectors Created via COBW

This visualization allows us to observe the similarity of the words based on their embeddings. Words that are similar in meaning or context are expected to be close to each other in the plot.

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