Neural Networks is one of the most popular machine learning algorithms and also outperforms other algorithms in both accuracy and speed. Therefore it becomes critical to have an in-depth understanding of what a Neural Network is, how it is made up and what its reach and limitations are.
What Is a Neural Network?
A Neural Network consists of different layers connected to each other, working on the structure and function of a human brain. It learns from huge volumes of data and uses complex algorithms to train a neural net.
Here is an example of how neural networks can identify a dog’s breed based on their features.
- The image pixels of two different breeds of dogs are fed to the input layer of the neural network.
- The image pixels are then processed in the hidden layers for feature extraction.
- The output layer produces the result to identify if it’s a German Shepherd or a Labrador.
- Such networks do not require memorizing the past output.
Several neural networks can help solve different business problems. Let’s look at a few of them.
- Feed-Forward Neural Network: Used for general Regression and Classification problems.
- Convolutional Neural Network: Used for object detection and image classification.
- Deep Belief Network: Used in healthcare sectors for cancer detection.
- RNN: Used for speech recognition, voice recognition, time series prediction, and natural language processing.
What Is a Recurrent Neural Network (RNN)?
RNN works on the principle of saving the output of a particular layer and feeding this back to the input in order to predict the output of the layer.
Below is how you can convert a Feed-Forward Neural Network into a Recurrent Neural Network:
Fig: Simple Recurrent Neural Network
The nodes in different layers of the neural network are compressed to form a single layer of recurrent neural networks. A, B, and C are the parameters of the network.
Fig: Fully connected Recurrent Neural Network
Here, “x” is the input layer, “h” is the hidden layer, and “y” is the output layer. A, B, and C are the network parameters used to improve the output of the model. At any given time t, the current input is a combination of input at x(t) and x(t-1). The output at any given time is fetched back to the network to improve on the output.
Fig: Fully connected Recurrent Neural Network
Now that you understand what a recurrent neural network is let’s look at the different types of recurrent neural networks.
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Why Recurrent Neural Networks?
RNN were created because there were a few issues in the feed-forward neural network:
- Cannot handle sequential data
- Considers only the current input
- Cannot memorize previous inputs
The solution to these issues is the RNN. An RNN can handle sequential data, accepting the current input data, and previously received inputs. RNNs can memorize previous inputs due to their internal memory.
How Does Recurrent Neural Networks Work?
In Recurrent Neural networks, the information cycles through a loop to the middle hidden layer.
Fig: Working of Recurrent Neural Network
The input layer ‘x’ takes in the input to the neural network and processes it and passes it onto the middle layer.
The middle layer ‘h’ can consist of multiple hidden layers, each with its own activation functions and weights and biases. If you have a neural network where the various parameters of different hidden layers are not affected by the previous layer, ie: the neural network does not have memory, then you can use a recurrent neural network.
The Recurrent Neural Network will standardize the different activation functions and weights and biases so that each hidden layer has the same parameters. Then, instead of creating multiple hidden layers, it will create one and loop over it as many times as required.
Feed-Forward Neural Networks vs Recurrent Neural Networks
A feed-forward neural network allows information to flow only in the forward direction, from the input nodes, through the hidden layers, and to the output nodes. There are no cycles or loops in the network.
Below is how a simplified presentation of a feed-forward neural network looks like:
Fig: Feed-forward Neural Network
In a feed-forward neural network, the decisions are based on the current input. It doesn’t memorize the past data, and there’s no future scope. Feed-forward neural networks are used in general regression and classification problems.
Applications of Recurrent Neural Networks
RNNs are used to caption an image by analyzing the activities present.
Time Series Prediction
Any time series problem, like predicting the prices of stocks in a particular month, can be solved using an RNN.
Natural Language Processing
Text mining and Sentiment analysis can be carried out using an RNN for Natural Language Processing (NLP).
Given an input in one language, RNNs can be used to translate the input into different languages as output.
Types of Recurrent Neural Networks
There are four types of Recurrent Neural Networks:
- One to One
- One to Many
- Many to One
- Many to Many
One to One RNN
This type of neural network is known as the Vanilla Neural Network. It's used for general machine learning problems, which has a single input and a single output.
One to Many RNN
This type of neural network has a single input and multiple outputs. An example of this is the image caption.
Many to One RNN
This RNN takes a sequence of inputs and generates a single output. Sentiment analysis is a good example of this kind of network where a given sentence can be classified as expressing positive or negative sentiments.
Many to Many RNN
This RNN takes a sequence of inputs and generates a sequence of outputs. Machine translation is one of the examples.
Two Issues of Standard RNNs
1. Vanishing Gradient Problem
Recurrent Neural Networks enable you to model time-dependent and sequential data problems, such as stock market prediction, machine translation, and text generation. You will find, however, RNN is hard to train because of the gradient problem.
RNNs suffer from the problem of vanishing gradients. The gradients carry information used in the RNN, and when the gradient becomes too small, the parameter updates become insignificant. This makes the learning of long data sequences difficult.
2. Exploding Gradient Problem
While training a neural network, if the slope tends to grow exponentially instead of decaying, this is called an Exploding Gradient. This problem arises when large error gradients accumulate, resulting in very large updates to the neural network model weights during the training process.
Long training time, poor performance, and bad accuracy are the major issues in gradient problems.
Gradient Problem Solutions
Now, let’s discuss the most popular and efficient way to deal with gradient problems, i.e., Long Short-Term Memory Network (LSTMs).
First, let’s understand Long-Term Dependencies.
Suppose you want to predict the last word in the text: “The clouds are in the ______.”
The most obvious answer to this is the “sky.” We do not need any further context to predict the last word in the above sentence.
Consider this sentence: “I have been staying in Spain for the last 10 years…I can speak fluent ______.”
The word you predict will depend on the previous few words in context. Here, you need the context of Spain to predict the last word in the text, and the most suitable answer to this sentence is “Spanish.” The gap between the relevant information and the point where it's needed may have become very large. LSTMs help you solve this problem.
Backpropagation Through Time
Backpropagation through time is when we apply a Backpropagation algorithm to a Recurrent Neural network that has time series data as its input.
In a typical RNN, one input is fed into the network at a time, and a single output is obtained. But in backpropagation, you use the current as well as the previous inputs as input. This is called a timestep and one timestep will consist of many time series data points entering the RNN simultaneously.
Once the neural network has trained on a timeset and given you an output, that output is used to calculate and accumulate the errors. After this, the network is rolled back up and weights are recalculated and updated keeping the errors in mind.
Long Short-Term Memory Networks
LSTMs are a special kind of RNN — capable of learning long-term dependencies by remembering information for long periods is the default behavior.
All RNN are in the form of a chain of repeating modules of a neural network. In standard RNNs, this repeating module will have a very simple structure, such as a single tanh layer.
Fig: Long Short Term Memory Networks
LSTMs also have a chain-like structure, but the repeating module is a bit different structure. Instead of having a single neural network layer, four interacting layers are communicating extraordinarily.
Workings of LSTMs in RNN
LSTMs work in a 3-step process.
Step 1: Decide How Much Past Data It Should Remember
The first step in the LSTM is to decide which information should be omitted from the cell in that particular time step. The sigmoid function determines this. It looks at the previous state (ht-1) along with the current input xt and computes the function.
Consider the following two sentences:
Let the output of h(t-1) be “Alice is good in Physics. John, on the other hand, is good at Chemistry.”
Let the current input at x(t) be “John plays football well. He told me yesterday over the phone that he had served as the captain of his college football team.”
The forget gate realizes there might be a change in context after encountering the first full stop. It compares with the current input sentence at x(t). The next sentence talks about John, so the information on Alice is deleted. The position of the subject is vacated and assigned to John.
Step 2: Decide How Much This Unit Adds to the Current State
In the second layer, there are two parts. One is the sigmoid function, and the other is the tanh function. In the sigmoid function, it decides which values to let through (0 or 1). tanh function gives weightage to the values which are passed, deciding their level of importance (-1 to 1).
With the current input at x(t), the input gate analyzes the important information — John plays football, and the fact that he was the captain of his college team is important.
“He told me yesterday over the phone” is less important; hence it's forgotten. This process of adding some new information can be done via the input gate.
Step 3: Decide What Part of the Current Cell State Makes It to the Output
The third step is to decide what the output will be. First, we run a sigmoid layer, which decides what parts of the cell state make it to the output. Then, we put the cell state through tanh to push the values to be between -1 and 1 and multiply it by the output of the sigmoid gate.
Let’s consider this example to predict the next word in the sentence: “John played tremendously well against the opponent and won for his team. For his contributions, brave ____ was awarded player of the match.”
There could be many choices for the empty space. The current input brave is an adjective, and adjectives describe a noun. So, “John” could be the best output after brave.
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LSTM Use Case
Now that you understand how LSTMs work, let’s do a practical implementation to predict the prices of stocks using the “Google stock price” data.
Based on the stock price data between 2012 and 2016, we will predict the stock prices of 2017.
1. Import the required libraries
2. Import the training dataset
3. Perform feature scaling to transform the data
4. Create a data structure with 60-time steps and 1 output
5. Import Keras library and its packages
6. Initialize the RNN
7. Add the LSTM layers and some dropout regularization.
8. Add the output layer.
9. Compile the RNN
10. Fit the RNN to the training set
11. Load the stock price test data for 2017
12. Get the predicted stock price for 2017
13. Visualize the results of predicted and real stock price
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