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Preface
What’s in This Book?
The book has many appendix chapters for topics that were relevant but did not fit directly into the main chapters.
Who Is “The Practitioner”?
Who Should Read This Book?
The Enterprise Machine Learning Practitioner
The Java engineer
The Enterprise Executive
The Academic
Conventions Used in This Book
Using Code Examples
Administrative Notes
O’Reilly Safari
How to Contact Us
Acknowledgments
Josh
I would like to note the support we received from our CEO at Skymind, Chris Nicholson. I would like to thank the people who contributed to the plugin chapters: Alex Black (Backprop, DataVec), Vyacheslav "Raver" Kokorin (GPU), Susan Eraly (GPU), and Ruben Fiszel (Reinforcement Learning).
Adam
The following are a few other people I would like to acknowledge who had an impact on my career leading up to this book: my parents (Lewis and Connie), Dr. Last, and especially not least, I would like to thank my wife Leslie and my sons Ethan, Griffin, and Dane for their patience while I worked late, often, and sometimes on vacation.
A Review of Machine Learning
The Learning Machines
Use the examples in the book to try out variations on practical deep networks. We hope you find the material practical and accessible. Let's start the book with a brief introduction to what machine learning is about and some of the core concepts you'll need to better understand the rest of the book.
How Can Machines Learn?
We will discuss our definition of deep learning in more detail in the next section.
Machine Learning Versus Data Mining
Let's take a quick look at another of the roots of deep learning: how neural networks are inspired by biology.
Biological Inspiration
First, the most basic unit of a neural network is an artificial neuron (or node for short). Artificial neurons are modeled after biological neurons in the brain and are stimulated by inputs just like biological neurons.
Biological Inspiration Across Computer Science
Biological neural networks are significantly more complex (several orders of magnitude) than artificial neural network versions. Second, just as neurons in the brain can be trained to forward only signals that are useful in achieving larger brain goals, we can train the neurons of a neural network to forward only useful signals.
What Is Deep Learning?
Ant colonies are able to perform intermediate tasks, defense, nest building, and foraging, while maintaining a near-optimal number of workers at each task based on relative need without the direct coordination of an individual ant.
Going Down the Rabbit Hole
Framing the Questions
The Math Behind Machine Learning: Linear Algebra
Scalars
Vectors
Matrices
Tensors
Hyperplanes
Relevant Mathematical Operations
Dot product
Element-wise product
Outer product
Converting Data Into Vectors
At some point we want our input data to be in the form of an array, but we can convert the data to intermediate representations (eg the "svmlight" file format shown in the code example that follows). The rest of the indexed numbers in the row are inserted into the corresponding slot in the matrix at runtime as "functions" to prepare for various linear algebra operations during the machine learning process.
Solving Systems of Equations
What Is a Feature?
An example of matrix decomposition in solving systems of linear equations is using upper-lower (LU) decomposition to solve the matrix A. Beyond matrix decomposition, let's take a look at general methods for solving sets of lin ‐.
Iterative methods
Iterative methods and linear algebra
The Math Behind Machine Learning: Statistics
Probability
Probability Versus Odds, Explained
Further Defining Probability: Bayesian Versus Frequentist
Bayesians, on the other hand, deal with "belief". in mathematical terms, "distributions") about the variable and update their beliefs about the variable as new information comes in.
Conditional Probabilities
Bayes’s Theorem
Posterior Probability
Distributions
The distribution of the training data in machine learning is important for understanding how to vectorize the data for modeling.
Central Limit Theorem
Long-tailed distributions deal with the real probability of events occurring that are five times the standard deviation. We need to be mindful of getting a good representation of the events in our training data to prevent overfitting the training data.
Samples Versus Population
Resampling Methods
Selection Bias
Likelihood
How Does Machine Learning Work?
Regression
Setting up the model
A simple example of a problem that linear regression solves would be to predict how much we will spend on gas per month based on the length of our commute. Visually, we can think of linear regression as finding a line that comes as close to as many points as possible in a scatter plot of data, as demonstrated in Figure 1-6.
Relating the linear regression model
Fitting is defining a function f(x) that produces y-values close to the measured or actual y-values.
Classification
We will discuss this further when we talk about multi-output neural networks versus single-output neural networks (binary classification). We will also discuss classification more in this chapter when we talk about logistic regression and then delve into the overall architecture of neural networks.
Clustering
In addition to two labels, we can have classification models with N labels for which we give each of the output labels a score, and then the label with the highest score is the output label. Recommendation is the process of suggesting items to users of a system based on similar other users or other items they have previously looked at.
Underfitting and Overfitting
Solving underfitting is the priority, but a lot of effort in machine learning goes into not overfitting the line to the data. The training set of data we are trying to draw a line through is just a sample of a larger unknown set, and the line we draw will need to fit the larger set equally well if it is to have any predictive power.
Optimization
If the line fits the data too well, we have the opposite problem, called "overfitting". You minimize the sum of the difference between the line at point x and the target point y to which it corresponds.
Convex Optimization
Now let's take a closer look at one subclass of optimization called convex optimization. tion. three-dimensional space) will produce something that looks more like a leaf that sticks out at each corner and sags convexly in the middle—a rather bowl-shaped feature. The slopes of these convex curves give our algorithm a hint as to which direction to take the next parameter step, as we will see in the gradient descent algorithm optimization.
Gradient Descent
In convex optimization, we look for a point where the derivative is equal to 0 for the function. This point is also known as the stationary point of the function or the minimum point.
What Is Gradient?
The cost function tells us how far we are from the global minimum of the loss function, and we use the derivative of the loss function as the update function to change the parameter. Taking the derivative of the loss function tells each parameter in x how much we need to adjust the parameter to get closer to the 0 point on the loss curves.
Stochastic Gradient Descent
The lowest point in the lowest valley is known as the global minimum, while the lows of all other valleys are known as local minima. In Chapter 8, we take an in-depth look at normalization methods in the context of vectorization and illustrate some ways to better address this problem.
Mini-batch training and SGD
Applying mini-batch to stochastic gradient descent has also been shown to lead to smoother convergence because the gradient com‐ . As the mini-log size increases, the computed gradient is closer to the "true" gradient of the entire training set.
Quasi-Newton Optimization Methods
If our dataset size is too small (eg, 1 training record), we're not using the hardware as effectively as we could, especially for situations like GPUs.
The Jacobian and the Hessian
Generative Versus Discriminative Models
Discriminative models focus on closely modeling the boundary between classes and can provide a more nuanced representation of this boundary than a generative model. Generative models are typically set up as probabilistic graphical models that capture the subtle relationships in the data.
Logistic Regression
The Logistic Function
Understanding Logistic Regression Output
If y is a function of x, and that function is sigmoidal or logistic, the more x increases, the closer we get to 1/1, because e is to the power of an infinitely large negative number. Because (1 + e–θx) is in the denominator, the larger it gets, the closer the quotient itself gets to zero.
Evaluating Models
If we are trying to estimate the probability that an e-mail is spam and f(x) happens to be equal to 0.6, we can rewrite it by saying that y has a 60 percent chance of being 1, or e- the email has a 60 percent chance of being spam, given the input.
The Confusion Matrix
If we define machine learning as a method of deriving unknown outputs from known inputs, the parameter vector x in a logistic regression model determines the strength and certainty of our deductions.
Sensitivity versus specificity
Accuracy
Precision
Recall
Context and interpreting scores
The goal is to accurately predict death in this scenario; this is where the model has the most value in the context of being clinically relevant in the real world. We delve deeper into the different facets of class imbalance and error distributions in the context of classification and regression.
Building an Understanding of Machine Learning
The goal of the challenge was to “predict hospital mortality with the highest accuracy. This was set up so that the contestants were focused not only on predicting that the patient would live most of the time and get a good F1 score, but to focus on predicting.
Foundations of Neural Networks and Deep Learning
Neural Networks
There has been a lot of hubris and a lot of talk spread across the internet and in the literature about the connection of neural networks to the human mind. We provide a brief overview of the biological neuron and then look at the early predecessor of modern neural networks: the perceptron.
The Biological Neuron
Building on our understanding of the perceptron, we will then look at how it evolved into the more generalized artificial neuron that underpins today's modern multi-layer perceptrons. Most synapses send signals from the axon of one neuron to the dendrite of another neuron.
Dendrites
Axons
Information flow across the biological neuron
From biological to artificial
The Perceptron
Both the TLU and the perceptron inspired the biological neuron, as we will explore.
History of the perceptron
Definition of the perceptron
While we don't have a complete model for how the brain works, we can see how the perceptron was modeled after the biological neuron. In this case, we can see how the perceptron receives input through connections with weights on them, in a similar way to how synapses transmit information to other biological neurons.
The perceptron learning algorithm
The input values do not affect the bias term, but the bias term is learned via per‐. As a basic linear classifier, we consider the single-layer perceptron to be the simplest form of the family of feed-forward neural networks.
Limitations of the early perceptron
Multilayer Feed-Forward Networks
Evolution of the artificial neuron
The artificial neuron (see Figure 2-6) takes inputs that, based on the weights on the connections, can be ignored (by a 0.0 weight on an input connection) or passed to the activation function. For the input layer, we just take the attribute at that particular index, and the activation function is linear (it passes the attribute value).
Comparing the biological neuron and the artificial neuron
Activation functions and their use will be an ongoing theme in almost every chapter for the rest of this book.
Feed-forward neural network architecture
This is controlled by the type of activation function we use in the neurons in the output layer. We will discuss the difference between these two types of activation functions later in the chapter.
Training Neural Networks
Weights can be understood mathematically by treating them as a vector of parameters in the previous section on linear algebra, which describes the process of machine learning as optimizing a vector of parameters (e.g., “weights” here) to minimize error.
Backpropagation Learning
Algorithm intuition
In the next section, we'll look at the mathematical notation you'll see in most literature. Learning algorithms for multilayer neural networks are neither guaranteed to converge to a global optimum, nor are they absolute.
A closer look at backpropagation
This is a direct effect of how learning a general function from training examples is considered an intractable problem in the worst case. The loss function (described later in this chapter) is not explicitly called in the pseudocode in Example 2-2.
Understanding backpropagation pseudocode Example 2-2 has the following inputs
This rule also contains an expression for the derivative of the activation function to obtain the gradient of the activation function. We saw in the previous section that g'(z) gave us the derivative of the activation function.
Backpropagation and Fractional Error Responsibility
This produces the partial error value for the node in the previous layer, which is used when we update the input links in that layer. We progressively perform this algorithm layer by layer until we have updated every layer in the network.
Backpropagation and Mini-Batch Stochastic Gradient Descent
This equation takes a look at each node i in the current layer and takes the current error value Δi and multiplies it by the weight of the incoming connection times the derivative of the activation function. The learning rate is a parameter we define (as opposed to being a measurement of network performance).
Activation Functions
The length of these learning steps, or the amount of weights changed with each iteration, is known as the learning rate. We use activation functions for hidden neurons in a neural network to introduce nonlinearity into the network's modeling capabilities.
Linear
The family of sigmoid functions contains several variations, one of which is known as the Sigmoid function.
Sigmoid
A sigmoid function is a machine that converts independent variables of nearly infinite range into simple probabilities between 0 and 1, and most of the output will be very close to 0 or 1.
Tanh
Hard Tanh
Softmax
If we want to get multiple classifications per output (eg "person + car"), we don't want softmax as the output layer. In the case where we have a large set of labels (eg thousands of labels), we would use a version of the softmax activation function called hierarchical softmax activa‐.
Rectified Linear
Compared to the sigmoid and tanh activation functions, the ReLU activation function does not suffer from gradient vanishing problems. Research has shown deep networks using ReLU activation functions to train well without using pre-training techniques.
Leaky ReLU
Because the gradient of a ReLU is either zero or a constant, it is possible to rule in the vanishing exploding gradient problem.
Softplus
Loss Functions
Loss Function Notation
We will use the notation h Xi =Yi to denote the neural network that transforms the input Xi to give the output Yi. So our notation h X =Y must be conditioned by a set of weights and biases; Thus, we will change our notation to say hw,b X =Y.
Loss Functions for Regression
We will modify this notation a bit later to emphasize its dependence on weights and biases. When we refer to the jth output function, we will use it as a signature that binds our notation to a matrix where the rows are different data points and col‐.
Mean squared error loss
Given the data available, the loss function notation indicates that its value depends only on W and b, the weights and the biases of the neural network. In the universe of a given neural network with a fixed number of layers, configurations, etc. to be trained on a given set of data, the value of the loss function depends solely on the state of the network, as defined by the weights and prejudices.
Other loss functions for regression
Regression loss function discussion
Although MSLE and MAPE are approaches to deal with large ranges, common practice with neural networks is to normalize input to a suitable range and use the MSE or MAE to optimize for either the mean or the median.
Loss Functions for Classification
Hinge loss
Logistic loss
For the sake of mathematical convenience, when dealing with the product of probabilities, it is customary to transform them into the log of proba‐. Note that we want to minimize our loss function, which we do by maximizing the likelihood, which in turn we do by minimizing the negative log likelihood.
Loss Functions for Reconstruction
This is mathematically equivalent to what is called the cross-entropy between two probability distributions—in this case, what we predict and what we observe under the same criteria.
Hyperparameters
Learning Rate
Too much learning rate exceeds the low point, causing the algorithm to bounce back and forth on either side of the minimum without ever coming to a stop. If you can't wait another week for results, choose a moderate learning rate (e.g. 0.1) and experiment with several others in the same ballpark to get the best speed and accuracy.
Regularization
A large learning rate coefficient (eg, 1) will make your parameters fast, and small ones (eg, will make it slow. Large jumps will save time initially, but can be disastrous if they cause us to exceed our minimum.
Momentum
Conversely, small learning rates should eventually lead you to an error minimum (it may be a local minimum rather than a global one), but they can take too long and add to the burden of an already computationally intensive process.
Sparsity
Fundamentals of Deep Networks
Defining Deep Learning
1 Referring to the character of Lewis Carroll's Red Queen from the book Through the Looking Glass; she has to keep running to stay in the same place. As the demands of the industry are changing and reaching more and more, the capabilities of neural networks must grow forward.
Defining deep networks
More connections means our network has more parameters to optimize, and that required the explosion in computing power that has occurred over the last 20 years.
Deep Reinforcement Learning
Deep reinforcement learning is a variant of reinforcement learning where a neural network is used as the universal function approximator. Deep reinforcement learning has grown in popularity during the production of this book, and hopefully we will see it included in a future edition.
Evolutionary progress and resurgence
This eventually led to the creation of the MNIST handwriting benchmark (we cover this more later in the chapter) and a progressive number of records for accuracy achieved by deep learning. Term Memory [LSTM] Memory Cell and Memory Cell with Forget Gate) during the late 1990s.
HAL 9000
This application will be more of the latent intelligence variety (eg recommendations or voice recognition) coupled with pragmatic engineering to make it useful in everyday aspects of our lives. We probably won't realize all the big commercial applications until they are right in front of our faces.
Advances in network architecture
Based on these achievements, we can easily design deep learning that will influence many applications in the next decade.
From feature engineering to automated feature learning
With CNNs, we train the network to understand the edge of the nose and then the general shape of the nose from the low-level "nose edge" features. Early layers in the network can pick up these nose edge features and then pass them on to later layers in the network as larger feature maps.
Generative modeling
The CNN extracts the artist's style into the network's parameters, which can later be applied to arbitrary images to be rendered in the same style. Another interesting application of recurrent neural networks is the work of Lipton and Elkan in which the network models proper names such as "Coors Light" and other aspects of beer jargon.
The Tao of deep learning
Chapter 5, we take a look at an example of Recurrent Neural Networks in which we generate new lines of Shakespeare by modeling all of Shakespeare's other works. Crafted beer reviews can be prompt-driven (eg, "give me a 3-star review of a German lager") and are impressive.
Organization of This Chapter
Common Architectural Principles of Deep Networks
Parameters
In deep networks, we still have a parameter vector that represents the connection in the network model we are trying to optimize. One network's layers are composed of RBMs (subnetworks in their own right, which we'll review later in the chapter) that are used to extract features for the other network.
Layers
The biggest change in deep networks regarding parameters is how the layers in the different architectures are connected. In terms of working with the core linear algebra of deep networks, DL4J relies on the ND4J library to represent these linear algebra primitives.
Activation functions for general architecture
In this case, we would use a sigmoid output layer with a single neuron to give us a real value in the range 0.0 to 1.0 (excluding these values) for the single class. Instead, we would use the sigmoid output layer with n number of neurons, which gives us a probability distribution (0.0 to 1.0) for each class independently.
Reconstruction cross-entropy
An example of this would be to use the multiclass cross entropy as a loss function in a layer with a soft‐.
Optimization Algorithms
The Jacobian has one partial derivative per parameter (to calculate the partial derivatives, all other variables are currently treated as constants). Second-order algorithms compute the derivative of the Jacobian (ie, the derivative of a matrix of derivatives) by approximating the Hessian.
First-order methods
There are other variations of optimization algorithms (such as "meta heuristics") that we will not cover in this book. You can adjust the SGD by adjusting the learning rate (e.g., with methods such as Adagrad, discussed shortly) or by using second-order information (i.e., the Hessian), as we'll see later. after.
Second-order methods
BFGS in Practice
Here we define a hyperparameter as any configuration setting that is free to be selected by the user that can affect performance.
Layer size
However, the link weights, as we saw in Chapter 1, are the parameters we need to train. A large number of parameters can lead to long training times and models that struggle to converge.
Magnitude hyperparameters
Momentum is a factor between 0.0 and 1.0 that is applied to the rate of change of weights over time. You multiply half the sum of the squared weights by a coefficient called weight-cost.
Mini-batching
We can also relate dropout to the concept of averaging the output of multiple models. L2 improves generalization, smooths the model output as the input changes, and helps the network ignore weights it doesn't use.
Summary
Building Blocks of Deep Networks
Over time, better optimization methods, activation functions, and weight initialization methods have reduced the importance of pre-training based deep networks. This gives us the first layer of weights for our main neural network (eg, multi-layer multi-layer perceptron).
RBMs
Geoff Hinton
Network layout
The nodes of the visible and hidden layers are connected by links with associated weights. In an RBM, every single node of the input (visible) layer is connected by weights to every single node of the hidden layer, but no two nodes are of the same layer.
Training
On” means that they forward the signal on the network; "off" means no. Normally "on" means that the data passing through the node is valuable; it contains information that will help the network make a decision.
Reconstruction
If we use an RBM to learn the MNIST dataset, we can sample from the trainers. If we know the mean and variance, or sigma, of the normal data, we can reconstruct that curve.
Other uses of RBMs
Autoencoders
Similarities to multilayer perceptrons
Defining features of autoencoders
Training autoencoders
Common variants of autoencoders
Applications of autoencoders
In addition to the output layer, there are a few other differences that we outline in the next section. Autoencoders are commonly used in systems where we know what the normal data will look like, but it is difficult to describe what is abnormal.
Variational Autoencoders
The VAE model assumes that data x is generated in two steps: (a) a value zi∼p z is generated from a prior distribution, and (b) the data instance is generated according to some conditional distribution xi∼p x z. For example, if P z x is Gaussian, the forward activation of the encoder yields the Gaussian distribution parameters μ and σ2.
Major Architectures of Deep Networks
Unsupervised Pretrained Networks
Deep Belief Networks
Feature Extraction with RBM Layers
These features move forward through these layers of RBMs in one direction, creating more elaborate features on the top layer. These feature layers are then used as initial weights in a traditional feedforward neural network driven by backpropagation.
Fine-tuning a DBN with a feed-forward multilayer neural network
As this raw data is modeled in the generative modeling process with each layer of the RBM, the system is able to extract increasingly higher-level features from the raw input data produced by our baseline input vectorization process. These initialization values help the training algorithm guide the parameters of the traditional neural network to better regions of parameter search space.
Current state of DBNs
Generative Adversarial Networks
Training generative models, unsupervised learning, and GANs
The goal here is to make the images realistic enough to fool the discriminator network to the point where it cannot tell the difference between real and syn‐. The goal here is to update the parameters of the generating network to the point where the discriminant network is sufficiently "fooled" by the generating network because the output is so realistic compared to the real training data images.
Deconvolutional Networks
Building generative models and Deep Convolutional Generative Adversarial Networks
This network takes random numbers (from a uniform distribution) and generates an image from the network model as output. As the input random numbers change, we see that the DCGAN generates different types of images.
Conditional GANs
Comparing GANs and variational autoencoders
Sometimes with generated images we also get a different type of noise in the generated output image. The downside of images created with VAE is that the images are sometimes slightly blurry due to the way they are created.
Convolutional Neural Networks (CNNs)
The images generated by GANs tend to capture the style of the input data, but sometimes they don't compose the scene in a consistent way (eg, the image is of a dog, but the dog doesn't look quite right). The biological inspiration for CNNs is the visual cortex in animals.9 Cells in the visual cortex are sensitive to small subregions of input.
Intuition
Columns next to each other are thus materialized in the exported materialized view of the database. Cells are well suited to exploiting the strong spatio-local correlation found in the types of images our brains process, acting as local filters in the input space.
What Is the CIFAR-10 Dataset?
In many cases, we will want to have multiple hidden layers in our multilayer neural network, which will also multiply those weights. The structure of image data allows us to change the architecture of a neural network in a way that we can take advantage of this structure.
CNN Architecture Overview
The input layer accepts three-dimensional input generally in terms of spatial size (width × height) of the image and has a depth representing the color channels (generally three for RGB color channels). These layers find a series of features in the images and gradually construct higher order features.
Neuron spatial arrangements
We express the rectified linear unit (ReLU) activation function as a layer in the diagram here to be consistent with other literature. These layers are fully connected to all the neurons in the previous layer, as their name implies.
Evolution of the connections between layers
Input Layers
Convolutional Layers
Convolution
At each step, the kernel is multiplied by the input data values within its bounds, creating a single entry in the output feature map. Convolutional layers perform transformations on the input data volume that are a function of the activations in the input volume and the parameters (weights and biases of the neurons).
Filters
The figure illustrates how the kernel is shifted over the input data to produce the con‐. In practice, if the feature we are looking for is detected in the input, the output is large.
Activation maps
To calculate the activation map, we push the filter over the depth slice of the input volume. We calculate the dot product between the inputs to the filter and the input volume.
Controlling Local Connectivity with the Receptive Field
We control the local connectivity of this process with the hyperparameter called the receptive field, which determines how much of the width and height of the input volume our filter maps against.
Parameter sharing
We cannot take advantage of parameter sharing if the input images we train on have a specific centered structure.
Learned filters and renders
ReLU activation functions as layers
Convolutional layer hyperparameters
The last hyperparameter is zero-padding, which can be used to control the spatial size of the output volumes. We would want to do this in cases where we want to preserve the spatial size of the input volume in the output volume.
Batch normalization and layers
Stride configures how far our filter slider will move based on the use of the filter function. Each time we apply the filter function to an input column, we create a new depth column in the output volume.
Pooling Layers
Fully Connected Layers
AlexNet is an example of this; it has two fully bonded layers followed by a softmax layer at the end.
Other Applications of CNNs
CNNs of Note
We have seen how convolutional layers, pooled layers and regular fully connected layers work together to do image classification.
Recurrent Neural Networks
Modeling the Time Dimension
Lost in time
Temporal feedback and loops in connections
Sequences and time-series data
An important facet of Recurrent Neural Networks is how we can work with input and output in unique ways.
Understanding model input and output
Now that we've seen the variations of input and output data, let's take a look at how this input data is represented.
3D Volumetric Input
Minibatch size is the number of input records (collections of time series points for a single source entity) that we want to model per batch. In the terminology of the previous section, we would consider any time step count above 1 to be "many-to-" in terms of input and output architecture.
Uneven time-series and masking
The number of columns matches up to the traditional feature column count found in a normal input vector.
Why Not Markov Models?
Recurrent Neural Networks, Hidden Layers, and Number of States
General Recurrent Neural Network Architecture
Recurrent Neural Networks architecture and time-steps
Vanishing Gradient Problem
LSTM Networks
Properties of LSTM networks
The computational complexity of the forward and backward pass operations scales linearly with the number of time steps in the input sequence. In the following sections, we provide an overview of the LSTM architecture and components.
LSTM network architecture
With recurrent neural networks, we introduce the idea of a type of connection that connects the output of a hidden layer neuron to the input of the same hidden layer neuron. With this recurrent connection, we can take the input from the previous time step into the neuron as part of the incoming information.
LSTM units
The output port exposes (or not) the contents of the memory cell to the output of the LSTM unit. The output of the LSTM block is recurrently connected to the block input and all ports for the LSTM block.
Gated Recurrent Units (GRU)
These events can span up to 1,000 discrete time steps compared to older recurrent architectures, which can model events over about 10 time steps.
LSTM layers
BPTT and truncated BPTT
Recurrent Neural Networks and Backpropagation
In the figure, we can see that the shortened BPTT transition size is four time steps. The overall complexity of the standard BPTT and the truncated BPTT are similar and require the same number of time steps during training.
Domain-Specific Applications and Blended Networks
Image Labeling with Mixed CNN/Recurrent Neural Network38 This type of network is a combination of CNN and BRNN.
Recursive Neural Networks
Network Architecture
Think of the goal as the top of the tree, while the stakes are the bottom.
Varieties of Recursive Neural Networks
Applications of Recursive Neural Networks
Summary and Discussion
Will Deep Learning Make Other Algorithms Obsolete?
Different Problems Have Different Best Methods
However, random forests and ensemble methods tend to be winners when deep learning doesn't win. The size of the input dataset can be another factor in how well deep learning is suited to a given problem.
When Do I Need Deep Learning?
Empirical results over the past few years have shown that deep learning provides the best predictive power when the data set is large enough. Neural networks have greater representational capacity than linear models and are more capable of exploiting data.
When to use deep learning
It is common in machine learning to try several models and find the one that works best for a particular problem. If the data is clearly linear, as seen in visualizations, you would try to fit the data with a non-linear model (eg a multilayer perceptron).
When to stick with traditional machine learning
Building Deep Networks
Matching Deep Networks to the Right Problem
The applications in this chapter illustrate the concepts of deep networks that we have been building since Chapter 1. Even though a project like DL4J will evolve over time, we want to provide readers with a snapshot of the code that will always work with this version of the book.
Columnar Data and Multilayer Perceptrons
Historically, machine learning modeling has been relatively brittle when features are not in the correct "spots" in which the model expects them to be. A major strength of CNNs is their ability to take raw image data in which the objects in the scene are arbitrarily posed and skillfully recognize the features.
Evolution of CNN Applications
Columnar data, thanks to painstaking efforts, is displayed with all features aligned to a specific column. Objects in images rarely appear where we want or expect them, presenting non-trivial challenges in the feature extraction phase for classical machine learning techniques.
Time-series Sequences and Recurrent Neural Networks
However, in both time series and images, we are looking for specific artifacts that may occur in the data. These objects can be scaled in size and usually do not appear in the same place every time in the data, presenting us with a challenge.
Playing the Devil’s Advocate for Model Choice
Recurrent neural networks have evolved from multilayer perceptrons to better model the time domain of time series data. When we deal with variable-length time series with unknown but likely long dependencies, this is the case for which recurrent neural networks are particularly justified.
Using Hybrid Networks
If we were in the pure world of software engineering, we would consider this situation to be technical debt. If we're starting from a place where we don't know the right features, with the current state of the art in deep learning, it's easier to semiautomate hyperpara‐.
The DL4J Suite of Tools
The DL4J project also features robust vectorization capabilities with the introduction of DataVec as a first-class citizen in the suite. Let's take a quick look at each of these facets of the DL4J suite before we dive into setting up the tools on your laptop.
Vectorization and DataVec
Runtimes and ND4J
ND4J and the need for speed
JavaCPP
Benchmarking ND4J and DL4J
Basic Concepts of the DL4J API
Loading and Saving Models
Writing a trained model to disk
Reading a saved model from disk
Getting Input for the Model
Loading data during training
Setting Up Model Architecture
Building layer-oriented architectures
In the preceding code snippet, we can see hyperparameters such as a learning rate, an updater, and a moment value set for a given MultiLayerConfiguration object. We'll look at strategies for setting hyperparameters for different network architectures later in the book.
Training and Evaluation
We will commonly see this pattern for managing epoch loops in examples later in the chapter.
Making a prediction
Training, validation, and test data
Modeling CSV Data with Multilayer Perceptron Networks
