Why And How To Use LSTM In NLP Tasks

by | Jan 11, 2023 | artificial intelligence, Machine Learning, Natural Language Processing

With a text classification example using Keras

Long Short-Term Memory (LSTM) is a powerful natural language processing (NLP) technique. This powerful algorithm can learn and understand sequential data, making it ideal for analyzing text and speech. In this article, we will explore the concept of LSTMs and how they can be applied to NLP tasks such as language translation, text generation, and sentiment analysis. We will discuss the advantages and disadvantages of using LSTMs. As well as provide a how-to guide and code on how to get started with text classification.

What is an LSTM, and how does it work in NLP?

Natural language processing (NLP) tasks frequently employ the Recurrent Neural Network (RNN) variant known as Long Short-Term Memory (LSTM). RNNs are neural networks that process sequential data, such as time series data or text written in a natural language. A particular kind of RNN called LSTMs can solve the issue of vanishing gradients, which arises when traditional RNNs are trained on lengthy data sequences.

A collection of “memory cells” that can store information and transmit it from one time step to the next makeup LSTMs. A system of “gates” that regulate data flow into and out of the cells connects these cells. The input gate, forget gate, and output gate are the three different types of gates that make up an LSTM.

The input gate governs the flow of new information into the cell, the forget gate regulates the flow of information out of the cell, and the output gate manages the data flow into the LSTM’s output. By controlling the flow of information in this way, LSTMs can forget information that isn’t important while remembering other information for longer.

LSTM has been used in many Natural Language Processing (NLP) tasks, such as:

In NLP, LSTMs are typically trained to classify the overall meaning or sentiment of the text or to take in a sequence of words as input and predict the next word in the series. These NLP tasks are a good fit for LSTMs because they can handle sequential data well and keep track of previous inputs in “memory.”

LSTM in NLP can retain information for a long time while forgetting irrelevant information.

LSTMs can retain information for a long time while forgetting irrelevant information.

Bidirectional LSTM (BiLSTM) are another LSTM variant that helps maintain the context of the past and future when making predictions.

Why use an LSTM in NLP tasks?

When used for natural language processing (NLP) tasks, Long Short-Term Memory (LSTM) networks have several advantages.

  1. Handling sequential data: Since LSTMs are built to handle sequential data, they are ideal for NLP tasks like language modelling, machine translation, and text generation. In their concealed states, they can store information from the past and use it to forecast the future.
  2. Handling long-term dependencies: When dealing with long-term dependencies in sequential data, LSTMs excel. They can better comprehend context and meaning in text because they can keep information hidden for extended periods.
  3. Handling missing data: LSTMs are robust to errors and missing data because they can take missing data in the input sequence. This can be helpful when performing tasks like speech recognition, where the input may be noisy or lacking.
  4. Handling variable-length inputs: In tasks like text classification, where the length of the input text may vary, LSTMs’ ability to handle variable-length input sequences can be helpful.
  5. Handling a large amount of data: The training process can be sped up by using parallel computing methods like GPUs and TPUs in combination with LSTMs, which can handle large amounts of data.
  6. Attention Mechanism: When LSTM networks are combined with an attention mechanism, they can focus on specific parts of the input sequence. This helps with tasks like machine translation and summarising text.
  7. Combining LSTMs with other models: Like the encoder-decoder model used for machine translation and the attention-based model used for text summarization, LSTMs can be combined with other models to make more powerful architectures.

What are the disadvantages of using LSTM in NLP?

When used for natural language processing (NLP) tasks, Long Short-Term Memory (LSTM) networks have many drawbacks. Among the most significant disadvantages are the following:

  1. Computational complexity: Training LSTMs can be computationally expensive, mainly when using large datasets and lengthy text sequences. Because of this, we are using them in real-time applications or on devices with limited resources may be challenging.
  2. Overfitting: Overfitting is a problem with LSTMs, especially when using small datasets. This might result in subpar performance on hidden data.
  3. Limited context: LSTMs have a limited ability to handle context, despite being built to handle sequential data and having the ability to store information from the past in their hidden states. In some NLP tasks, the context needed to understand a sentence or passage can be spread over several sentences or paragraphs. This makes LSTMs less valuable.
  4. Difficult to interpret: A particular kind of neural network called LSTMs is regarded as a “black box model,” much like many other neural networks. It can take time to figure out how they make decisions and which features are most important for a particular job.
  5. Long-term dependencies: Long-term dependencies in sequential data are handled by LSTMs. But they continue to struggle with dependencies that last for many weeks, months, or even years.
  6. Data Preprocessing: To function appropriately, LSTMs typically need substantial data preprocessing. Before being fed into the model, the text data needs to be tokenized, cleaned and vectorized.

Transformers and their variations, such as BERT and GPT-3, are new alternatives to LSTM that have made NLP better but also have problems.

How to implement an LSTM in NLP for text classification

Long Short-Term Memory (LSTM) can be effectively used for text classification tasks. In text classification, the goal is to assign one or more predefined categories or labels to a piece of text. LSTMs can be trained by treating each word in the text as a time step and training the LSTM to predict the label of the text.

First, the text needs to be transformed into a numerical representation, which can be accomplished by employing tokenization and word embedding strategies. Tokenization involves separating the text into its words, and word embedding, which requires mapping words to high-dimensional vectors that accurately capture their meaning, are two methods for doing this.

The LSTM would then be fed these numerical representations of the text. Each word in the sequence will be processed by the LSTM one at a time, producing a hidden state for each word. The label of the text can be predicted using these hidden states, which capture the meaning of the text up to that point.

To generate the class scores, the output of the LSTM is fed into a fully connected layer and a softmax activation function. The class scores will represent the probability distribution of each possible class. The final predicted class is the one with the highest probability.

Summary

In summary, text classification using LSTMs typically involves:

  • Tokenization of the text to produce a sequence of words.
  • Word embedding of the series of words to make a sequence of vectors.
  • Feeding the sequence of vectors into the LSTM to create a sequence of hidden states.
  • Using the last hidden state to predict the label of the text.

Additionally, when dealing with lengthy documents, adding a method known as the Attention Mechanism on top of the LSTM can be helpful because it selectively considers various inputs while making predictions.

Text classification example of an LSTM in NLP using Python’s Keras

Here is an example of how you might use the Keras library in Python to train an LSTM model for text classification.

import keras.preprocessing.text Tokenizer
from keras.preprocessing.sequence import pad_sequences
from keras.layers import Embedding, LSTM, Dense
from keras.models import Sequential

# The input text, example could be list of sentences
texts = [...]

# The labels corresponding to the input text
labels = [...]

# Hyperparameters 
max_words = 10000 # max number of words to use in the vocabulary
max_len = 100 # max length of each text (in terms of number of words)
embedding_dim = 100 # dimension of word embeddings
lstm_units = 64 # number of units in the LSTM layer
num_classes = len(set(labels)) # number of classes

# Tokenize the texts and create a vocabulary
tokenizer = Tokenizer(num_words=max_words)
tokenizer.fit_on_texts(texts)
sequences = tokenizer.texts_to_sequences(texts)

# Pad the sequences so they all have the same length
x = pad_sequences(sequences, maxlen=max_len)

# Create one-hot encoded labels
y = keras.utils.to_categorical(labels, num_classes)

# Build the model
model = Sequential()
model.add(Embedding(max_words, embedding_dim, input_length=max_len))
model.add(LSTM(lstm_units))
model.add(Dense(num_classes, activation='softmax'))

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

# Train the model
model.fit(x, y, batch_size=32, epochs=10)

In the above example, the input texts is a list of sentences/documents, and the corresponding label is given in the labels list. The model starts by tokenizing the text and then converting them into a numerical representation. The sequences are then padded to have an equal length of max_len. Then the one-hot encoded labels are created, and the model is built on top of this.

The model consists of three layers: an embedding layer, an LSTM layer, and a dense layer with a softmax activation function. The embedding layer maps the words to high-dimensional vectors, and the LSTM layer processes the sequence of vectors, one word at a time. Finally, the dense layer with the softmax activation function produces the class scores.

The model is then compiled with categorical_crossentropy as the loss function, Adam as the optimizer and accuracy as the metric. Finally, the model is trained using the fit method by passing the input data and labels.

Note that the above example is simple, and the model’s architecture may need to be changed based on the size and complexity of the dataset. Also, consider using other architectures like 1D-CNNs with different pooling methods or attention mechanisms on top of LSTMs, depending on the problem and the dataset.

Conclusion

The ability of Long Short-Term Memory (LSTM) networks to manage sequential data, long-term dependencies, and variable-length inputs make them an effective tool for natural language processing (NLP) tasks. As a result, they have been extensively used in NLP tasks such as speech recognition, text generation, machine translation, and language modelling.

However, there are several drawbacks to LSTMs as well, including overfitting, computational complexity, and interpretability issues. Despite these difficulties, LSTMs are still popular for NLP tasks because they can consistently deliver state-of-the-art performance.

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