Hands-on Practical: Sentiment Analysis

Sentiment analysis, a common sequence modeling task, involves building and training a model. This model classifies text reviews as positive or negative using either an LSTM or a GRU layer. The development process demonstrates applying framework APIs, handling sequence data, and constructing a complete model.

We assume you have a basic understanding of text preprocessing steps like tokenization and padding, which are covered in detail in Chapter 8. Here, we'll focus on integrating these steps with LSTM/GRU model implementation.

Setting the Stage: The IMDB Dataset

We'll use the popular IMDB dataset, which contains 50,000 movie reviews labeled as either positive (1) or negative (0). This dataset is often included directly within deep learning frameworks, making it convenient to access.

# Example using TensorFlow/Keras
import tensorflow as tf
from tensorflow import keras

# Load the dataset, keeping only the top N most frequent words
VOCAB_SIZE = 10000
(train_data, train_labels), (test_data, test_labels) = keras.datasets.imdb.load_data(num_words=VOCAB_SIZE)

print(f"Training entries: {len(train_data)}, labels: {len(train_labels)}")
print(f"Sample review (integer encoded): {train_data[0][:20]}...")

The data is already integer-encoded, where each integer represents a specific word in the dataset's vocabulary.

Preparing the Data

Recurrent networks require inputs of uniform length. Since movie reviews vary in length, we need to pad or truncate them to a fixed size. We'll use post-padding, meaning we add zeros at the end of shorter sequences. Masking (often handled automatically by framework layers) will ensure these padded values are ignored during computation.

# Pad sequences to a maximum length
MAX_SEQUENCE_LENGTH = 256

train_data_padded = keras.preprocessing.sequence.pad_sequences(
    train_data,
    value=0, # Pad value
    padding='post', # Pad at the end
    maxlen=MAX_SEQUENCE_LENGTH
)

test_data_padded = keras.preprocessing.sequence.pad_sequences(
    test_data,
    value=0,
    padding='post',
    maxlen=MAX_SEQUENCE_LENGTH
)

print(f"Sample padded review length: {len(train_data_padded[0])}")
print(f"Sample padded review: {train_data_padded[0][:30]}...")

Building the Sentiment Analysis Model

Now, let's define our model architecture using the Keras Sequential API.

Embedding Layer: This layer takes the integer-encoded vocabulary indices and looks up a corresponding dense vector representation (embedding) for each word. It learns these embeddings during training. It expects input shaped (batch_size, sequence_length) and outputs (batch_size, sequence_length, embedding_dim).
LSTM or GRU Layer: This is the core recurrent layer. It processes the sequence of embeddings. We'll start with an LSTM layer.
Dense Output Layer: A single neuron with a sigmoid activation function outputs a value between 0 and 1, representing the probability of the review being positive.

EMBEDDING_DIM = 16
RNN_UNITS = 32 # Number of units in the LSTM/GRU layer

model = keras.Sequential([
    keras.layers.Embedding(input_dim=VOCAB_SIZE,
                           output_dim=EMBEDDING_DIM,
                           mask_zero=True, # Important: Enables masking for padded values
                           input_length=MAX_SEQUENCE_LENGTH),
    keras.layers.LSTM(RNN_UNITS), # You could replace LSTM with GRU here
    keras.layers.Dense(1, activation='sigmoid') # Output layer for binary classification
])

model.summary()

The mask_zero=True argument in the Embedding layer is significant. It tells downstream layers (like the LSTM) to ignore time steps where the input was 0 (our padding value).

Visualizing the Model Architecture

Basic model structure: Input -> Embedding -> LSTM -> Dense Output.

Compiling the Model

Before training, we need to configure the learning process using compile. We specify the optimizer, the loss function, and metrics to monitor.

Optimizer: adam is a generally good starting choice.
Loss Function: binary_crossentropy is appropriate for binary (0/1) classification problems with a sigmoid output.
Metrics: accuracy allows us to track the percentage of correctly classified reviews during training and evaluation.

model.compile(optimizer='adam',
              loss='binary_crossentropy',
              metrics=['accuracy'])

Training the Model

We can now train the model using the fit method, providing the padded training data and labels. We also set aside a portion of the training data for validation during training to monitor performance on unseen data and check for overfitting.

EPOCHS = 10
BATCH_SIZE = 512

# Create a validation set from the training data
validation_split = 0.2
num_validation_samples = int(validation_split * len(train_data_padded))

x_val = train_data_padded[:num_validation_samples]
partial_x_train = train_data_padded[num_validation_samples:]

y_val = train_labels[:num_validation_samples]
partial_y_train = train_labels[num_validation_samples:]

print("Training the model...")
history = model.fit(partial_x_train,
                    partial_y_train,
                    epochs=EPOCHS,
                    batch_size=BATCH_SIZE,
                    validation_data=(x_val, y_val),
                    verbose=1) # Set verbose=1 or 2 to see progress per epoch
print("Training complete.")

Evaluating Performance

After training, we evaluate the model's performance on the held-out test set. We can also visualize the training and validation accuracy and loss over epochs to understand the learning dynamics.

print("\nEvaluating on test data...")
results = model.evaluate(test_data_padded, test_labels, verbose=0)
print(f"Test Loss: {results[0]:.4f}")
print(f"Test Accuracy: {results[1]:.4f}")

# Plotting training history (requires Plotly)
import plotly.graph_objects as go
from plotly.subplots import make_subplots

history_dict = history.history
acc = history_dict['accuracy']
val_acc = history_dict['val_accuracy']
loss = history_dict['loss']
val_loss = history_dict['val_loss']
epochs_range = range(1, EPOCHS + 1)

fig = make_subplots(rows=1, cols=2, subplot_titles=("Training and Validation Loss", "Training and Validation Accuracy"))

fig.add_trace(go.Scatter(x=list(epochs_range), y=loss, name='Training Loss', mode='lines+markers', line=dict(color='#4263eb')), row=1, col=1)
fig.add_trace(go.Scatter(x=list(epochs_range), y=val_loss, name='Validation Loss', mode='lines+markers', line=dict(color='#f76707')), row=1, col=1)
fig.add_trace(go.Scatter(x=list(epochs_range), y=acc, name='Training Accuracy', mode='lines+markers', line=dict(color='#12b886')), row=1, col=2)
fig.add_trace(go.Scatter(x=list(epochs_range), y=val_acc, name='Validation Accuracy', mode='lines+markers', line=dict(color='#ae3ec9')), row=1, col=2)

fig.update_layout(height=400, width=800, title_text="Model Training History")
fig.update_xaxes(title_text="Epochs", row=1, col=1)
fig.update_xaxes(title_text="Epochs", row=1, col=2)
fig.update_yaxes(title_text="Loss", row=1, col=1)
fig.update_yaxes(title_text="Accuracy", row=1, col=2)

# The following line generates the Plotly JSON output
print(fig.to_json())

Example training history showing loss and accuracy curves for training and validation sets over epochs. (Note: Actual curve values are illustrative and depend on the specific training run).

The plot helps identify potential overfitting (where training accuracy keeps improving, but validation accuracy plateaus or decreases) and determine if more training epochs are needed.

Variations and Further Steps

GRU: Try replacing keras.layers.LSTM(RNN_UNITS) with keras.layers.GRU(RNN_UNITS) and retrain. Compare the performance and training time.

Stacked RNNs: To stack layers, ensure intermediate recurrent layers return the full sequence of outputs, not just the final output.

model_stacked = keras.Sequential([
    keras.layers.Embedding(VOCAB_SIZE, EMBEDDING_DIM, mask_zero=True, input_length=MAX_SEQUENCE_LENGTH),
    keras.layers.LSTM(RNN_UNITS, return_sequences=True), # Returns hidden state for each time step
    keras.layers.LSTM(RNN_UNITS), # This layer receives the sequence
    keras.layers.Dense(1, activation='sigmoid')
])

Bidirectional RNNs: Wrap a recurrent layer with keras.layers.Bidirectional to process the input sequence in both forward and backward directions, potentially capturing context more effectively.
```
model_bidirectional = keras.Sequential([
    keras.layers.Embedding(VOCAB_SIZE, EMBEDDING_DIM, mask_zero=True, input_length=MAX_SEQUENCE_LENGTH),
    keras.layers.Bidirectional(keras.layers.LSTM(RNN_UNITS)), # Wrap the LSTM layer
    keras.layers.Dense(1, activation='sigmoid')
])
```
Note that a Bidirectional layer typically doubles the output feature dimension (one set of features for forward, one for backward), unless configured otherwise.
Hyperparameter Tuning: Experiment with EMBEDDING_DIM, RNN_UNITS, optimizer choice, learning rate, and BATCH_SIZE. Add Dropout for regularization (covered later).

This practical example provides a concrete foundation for implementing LSTM and GRU models for sequence classification. You can adapt this structure for various other sequence-based tasks by modifying the input data preparation and the final output layer(s) of the model.

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