Introduction to Gated Recurrent Unit (GRU)

Objective

• In sequence modeling techniques, the Gated Recurrent Unit is the newest entrant after RNN and LSTM, hence it offers an improvement over the other two.
• Understand the working of GRU and how it is different from LSTM

Introduction

In the ever-evolving world of artificial intelligence, where algorithms mimic the human brain’s ability to learn from data, Recurrent Neural Networks (RNNs) have emerged as a powerful deep learning algorithm for processing sequential data. However, RNNs struggle with long-term dependencies within sequences. This is where Gated Recurrent Units (GRUs) come in. As a type of RNN equipped with a specific learning algorithm, GRUs address this limitation by utilizing gating mechanisms to control information flow, making them a valuable tool for various tasks in machine learning.

Note: If you are more interested in learning concepts in an Audio-Visual format, We have this entire article explained in the video below. If not, you may continue reading.

What is GRU?

GRU or Gated recurrent unit is an advancement of the standard RNN i.e recurrent neural network. It was introduced by Kyunghyun Cho et al in the year 2014.

GRUs are very similar to Long Short Term Memory(LSTM). Just like LSTM, GRU uses gates to control the flow of information. They are relatively new as compared to LSTM. This is the reason they offer some improvement over LSTM and have simpler architecture.

Another Interesting thing about  GRU network is that, unlike LSTM, it does not have a separate cell state (Ct). It only has a hidden state(Ht). Due to the simpler architecture, GRUs are faster to train.

In case you are unaware of the LSTM network, I will suggest you go through the following article-Introduction to Long Short term Memory(LSTM).

Limitations of Standard RNN

Here are the limitations of standard RNNs in bullet points:

• Vanishing Gradient problem : This is a major limitation that occurs when processing long sequences. As information propagates through the network over many time steps, the gradients used to update the network weights become very small (vanish). This makes it difficult for the network to learn long-term dependencies in the data.
• Exploding Gradients: The opposite of vanishing gradients, exploding gradients occur when the gradients become very large during backpropagation. This can lead to unstable training and prevent the network from converging to an optimal solution.
• Limited Memory: Standard RNNs rely solely on the hidden state to capture information from previous time steps. This hidden state has a limited capacity, making it difficult for the network to remember information over long sequences.
• Difficulty in Training: Due to vanishing/exploding gradients and limited memory, standard RNNs can be challenging to train, especially for complex tasks involving long sequences.

How GRU Solve the Limitations of Standard RNN?

There are various types of recurrent neural network to solve the issues with standard RNN, GRU is one of them. Here’s how GRUs address the limitations of standard RNNs:

• Gated Mechanisms: Unlike standard RNNs, GRUs use special gates (Update gate and Reset gate) to control the flow of information within the network. These gates act as filters, deciding what information from the past to keep, forget, or update.
• Mitigating Vanishing Gradients: By selectively allowing relevant information through the gates, GRUs prevent gradients from vanishing entirely. This allows the network to learn long-term dependencies even in long sequences.
• Improved Memory Management: The gating mechanism allows GRUs to effectively manage the flow of information. The Reset gate can discard irrelevant past information, and the Update gate controls the balance between keeping past information and incorporating new information. This improves the network’s ability to remember important details for longer periods.
• Faster Training: Due to the efficient gating mechanisms, GRUs can often be trained faster than standard RNNs on tasks involving long sequences. The gates help the network learn more effectively, reducing the number of training iterations required.

The Architecture of Gated Recurrent Unit

Now lets’ understand how GRU works. Here we have a GRU cell which more or less similar to an LSTM cell or RNN cell.

At each timestamp t, it takes an input Xt and the hidden state Ht-1 from the previous timestamp t-1. Later it outputs a new hidden state Ht which again passed to the next timestamp.

Now there are primarily two gates in a GRU as opposed to three gates in an LSTM cell. The first gate is the Reset gate and the other one is the update gate.

Reset Gate (Short term memory)

The Reset Gate is responsible for the short-term memory of the network i.e the hidden state (Ht). Here is the equation of the Reset gate.

If you remember from the LSTM gate equation it is very similar to that. The value of rt will range from 0 to 1 because of the sigmoid function. Here Ur and Wr are weight matrices for the reset gate.

Update Gate (Long Term memory)

Similarly, we have an Update gate for long-term memory and the equation of the gate is shown below.

The only difference is of weight metrics i.e Uu and Wu.

How GRU Works?

Prepare the Inputs:

• The GRU takes two inputs as vectors: the current input (X_t) and the previous hidden state (h_(t-1)).

Gate Calculations:

• There are three gates in a GRU: Reset Gate, Update Gate, and Forget Gate (sometimes combined with Reset Gate). We’ll calculate the values for each gate.
• To do this, we perform an element-wise multiplication (like a dot product for each element) between the current input and the previous hidden state vectors. This is done separately for each gate, essentially creating “parameterized” versions of the inputs specific to each gate.
• Finally, we apply an activation function (a function that transforms the values) element-wise to each element in these parameterized vectors. This activation function typically outputs values between 0 and 1, which will be used by the gates to control information flow.

Now let’s see the functioning of these gates in detail. To find the Hidden state Ht in GRU, it follows a two-step process. The first step is to generate what is known as the candidate hidden state. As shown below

Candidate Hidden State

It takes in the current input and the hidden state from the previous timestamp t-1 which is multiplied by the reset gate output rt. Later passed this entire information to the tanh function, the resultant value is the candidate’s hidden state.

The most important part of this equation is how we are using the value of the reset gate to control how much influence the previous hidden state can have on the candidate state.

If the value of rt is equal to 1 then it means the entire information from the previous hidden state Ht-1 is being considered. Likewise, if the value of rt is 0 then that means the information from the previous hidden state is completely ignored.

Hidden State

Once we have the candidate state, it is used to generate the current hidden state Ht. It is where the Update gate comes into the picture. Now, this is a very interesting equation, instead of using a separate gate like in LSTM in GRU we use a single update gate to control both the historical information which is Ht-1 as well as the new information which comes from the candidate state.

Now assume the value of ut is around 0 then the first term in the equation will vanish which means the new hidden state will not have much information from the previous hidden state. On the other hand, the second part becomes almost one that essentially means the hidden state at the current timestamp will consist of the information from the candidate state only.

Similarly, if the value of ut is on the second term will become entirely 0 and the current hidden state will entirely depend on the first term i.e the information from the hidden state at the previous timestamp t-1.

Hence we can conclude that the value of ut is very critical in this equation and it can range from 0 to 1.

In case, you are interested to know more about GRU I suggest you read this Paper.

Advantages and Disadvantages of GRU

Advantages of GRU

• Faster Training and Efficiency: Compared to LSTMs (Long Short-Term Memory networks), GRUs have a simpler architecture with fewer parameters. This makes them faster to train and computationally less expensive.
• Effective for Sequential Tasks: GRUs excel at handling long-term dependencies in sequential data like language or time series. Their gating mechanisms allow them to selectively remember or forget information, leading to better performance on tasks like machine translation or forecasting.
• Less Prone to Gradient Problems: The gating mechanisms in GRUs help mitigate the vanishing/exploding gradient problems that plague standard RNNs. This allows for more stable training and better learning in long sequences.

Disadvantages of GRU

• Less Powerful Gating Mechanism: While effective, GRUs have a simpler gating mechanism compared to LSTMs which utilize three gates. This can limit their ability to capture very complex relationships or long-term dependencies in certain scenarios.
• Potential for Overfitting: With a simpler architecture, GRUs might be more susceptible to overfitting, especially on smaller datasets. Careful hyperparameter tuning is crucial to avoid this issue.
• Limited Interpretability: Understanding how a GRU arrives at its predictions can be challenging due to the complexity of the gating mechanisms. This makes it difficult to analyze or explain the network’s decision-making process.

Applications of Gated Recurrent Unit

Here are some applications of GRUs where their ability to handle sequential data shines:

Natural Language Processing (NLP)

• Machine translation: GRUs can analyze the context of a sentence in one language and generate a grammatically correct and fluent translation in another language.
• Text summarization: By processing sequences of sentences, GRUs can identify key points and generate concise summaries of longer texts.
• Chatbots: GRUs can be used to build chatbots that can understand the context of a conversation and respond in a natural way.
• Sentiment Analysis: GRUs excel at analyzing the sequence of words in a sentence and understanding the overall sentiment (positive, negative, or neutral).

Speech Recognition

GRUs can analyze the sequence of audio signals in speech to transcribe it into text. They can be particularly effective in handling variations in speech patterns and accents.

Time Series Forecasting

GRUs can analyze historical data like sales figures, website traffic, or stock prices to predict future trends. Their ability to capture long-term dependencies makes them well-suited for forecasting tasks.

Anomaly Detection

GRUs can identify unusual patterns in sequences of data, which can be helpful for tasks like fraud detection or network intrusion detection.

Music Generation

GRUs can be used to generate musical pieces by analyzing sequences of notes and chords. They can learn the patterns and styles of different musical genres and create new music that sounds similar.

These are just a few examples, and the potential applications of GRUs continue to grow as researchers explore their capabilities in various fields.

Conclusion

So just to summarize, Let’s see how different Gated recurrent unit is from LSTM.

LSTM has three gates on the other hand GRU has only two gates. In LSTM they are the Input gate, Forget gate, and Output gate. Whereas in GRU we have a Reset gate and Update gate.

In LSTM we have two states Cell state or Long term memory and Hidden state also known as Short term memory. In the case of GRU, there is only one state i.e Hidden state (Ht).

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This is all about GRU in this article. If you have any queries, let me know in the comments section!

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