Understanding Neural Networks: A Comprehensive Guide
Tags: ai, machinelearning, datascience, tutorial
Published: 2023-11-28 07:03:37

## Introduction
Neural networks get their ideas from our brains. Like our brain uses many cells to think and learn, neural networks use many small parts to handle information. This makes computers smarter in some ways, like us!

In this article, we’ll explore neural networks in easy steps. First, we’ll learn the basics, then look at how they work, and finally see how they’re used in different things like phones, computers, and more.

## The Basics of Neural Networks

![neural-network](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/m0vu95ilvl5jl5tz3mfr.png)

Did you know that computers can learn like our brains? Our brain has neurons that talk to each other. When we learn, these connections grow stronger. Neural networks in computers work in a similar way. They have artificial neurons that get information, process it, and pass it on. The more they work with data, the better they get at tasks, like how we learn from doing things.

### Neural Network Layers

![A-typical-two-layer-neural-network-Input-layer-does-not-count-as-the-number-of-layers-of](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/xt3xei7zyryb7g1k95ep.png)

The neural network has 3 layers, each layer has a special job in handling information.

* **Input Layer**: It’s the starting point where you give the network information. For example, in a picture, each point of light in the picture goes into this layer.
* **Hidden Layers**: These are the main part of the network. They work on the information from the input layer quietly. They do the calculations and pass the results on.
* **Output Layer**: This is where the network gives us the answer. If the network is deciding if a picture is of a cat or a dog, the output layer tells us the answer.

### Nodes

![neuron-node-1024x466](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/q2ra2jau5240lcson20n.png)

Nodes are like small checkpoints in the network. Each node looks at a piece of data and decides what to do with it. They are like tiny helpers, each doing a small part of a big job.

### Activation functions

![activation-functions](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/tzgizyr8ffz82y0vmosg.png)

Activation functions are like rules for the nodes. They tell the nodes when to act or when to wait. This helps the network focus on the important information. There are a few common types:

* **Sigmoid**: This function is smooth and changes input values into a range from 0 to 1. It’s good for deciding things like yes or no.
* **Tanh (Hyperbolic Tangent)**: This is like sigmoid, but it works with values from -1 to 1. It’s useful when dealing with both positive and negative data.
* **ReLU (Rectified Linear Unit)**: This one is simple. It turns all negative values to zero and keeps positive values. It’s popular because it’s straightforward and works well.

### Weights
Weights are like scales. They decide how important each piece of information is to the network. They change how much each piece of information affects the final decision.

### Biases
Biases are like extra nudges that help the network make decisions. Even when the information seems not enough, biases can help the network to come up with an answer.

### Neural Network Example
Let’s go through an example of a simple neural network with layers, nodes, activation functions, weights, and biases to illustrate how these components work together.

Let’s consider a neural network with the following configuration:

**Layers:**
* 1 Input Layer
* 1 Hidden Layer
* 1 Output Layer

**Nodes:**
* Input Layer: 2 Nodes (Node1, Node2)
* Hidden Layer: 3 Nodes (Node3, Node4, Node5)
* Output Layer: 1 Node (Node6)

**Activation Functions:**
* Hidden Layer: ReLU (Rectified Linear Unit)
* Output Layer: Sigmoid

**Weights and Biases:**
Initial weights and biases are often randomly assigned. Let’s assume the following for simplicity:
Weights from Input to Hidden Layer:
* W1: 0.5 (from Node1 to Node3)
* W2: 0.3 (from Node1 to Node4)
* W3: 0.9 (from Node1 to Node5)
* W4: 0.8 (from Node2 to Node3)
* W5: 0.2 (from Node2 to Node4)
* W6: 0.1 (from Node2 to Node5)

Weights from Hidden to Output Layer:
* W7: 0.3 (from Node3 to Node6)
* W8: 0.6 (from Node4 to Node6)
* W9: 0.9 (from Node5 to Node6)

Biases:
* B1: 0.1 (for Node3)
* B2: 0.2 (for Node4)
* B3: 0.3 (for Node5)
* B4: 0.4 (for Node6)

![neural-network-example-1024x683](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/tg3a9jipo8m9lkttc07x.png)

Let’s say our input vector is [1, 0] (values for Node1 and Node2)

**Calculations in Hidden Layer:**
* Node3 = ReLU(Node1 input * W1 + Node2 input * W4 + B1) = ReLU(1 * 0.5 + 0 * 0.8 + 0.1) = ReLU(0.6) = 0.6
* Node4 = ReLU(Node1 input * W2 + Node2 input * W5 + B2) = ReLU(1 * 0.3 + 0 * 0.2 + 0.2) = ReLU(0.5) = 0.5
* Node5 = ReLU(Node1 input * W3 + Node2 input * W6 + B3) = ReLU(1 * 0.9 + 0 * 0.1 + 0.3) = ReLU(1.2) = 1.2

(Note: ReLU function is max(0, x), so all negative values become 0)

**Calculations in Output Layer:**

* Node6 = Sigmoid(Node3 output * W7 + Node4 output * W8 + Node5 output * W9 + B4) = Sigmoid(0.6 * 0.3 + 0.5 * 0.6 + 1.2 * 0.9 + 0.4) = Sigmoid(1.96)

(Note: Sigmoid function outputs a value between 0 and 1, useful for binary classification)

This is a simplified example. In a real-world scenario, the network would be more complex with more layers and nodes. Also, during training, these weights and biases would be adjusted iteratively to reduce prediction error.

## Training the Neural Network

Training a neural network helps it learn from data and get better at making predictions or decisions. The two key players in this process are **backpropagation** and **gradient descent**. They sound complex, but let’s break them down into simpler terms.

### Backpropagation: Learning from Mistakes

![backpropagation](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/gvh1y0weaboiwdlx5hn6.png)

Backpropagation is all about learning from errors. When a neural network makes a prediction, it might not always be right. Backpropagation measures how far off the prediction is from the correct answer. It then sends this error information back through the network. This is like the network looking back at its work and seeing where it went wrong.

As this error information travels back, the network adjusts its weights and biases. This adjustment is crucial as it helps the network learn from its mistakes. The next time it encounters similar data, it’s more likely to make a better prediction. It’s a continuous cycle of predicting, learning from errors, and improving.

### Gradient Descent: Finding the Best Path
Gradient descent is the method the network uses to change its weights and biases effectively. It finds the steepest path in terms of error reduction and takes small steps along that path.

In technical terms, gradient descent calculates the gradient (or slope) of the network’s error with respect to each weight and bias. It then adjusts these weights and biases to minimize the error.

By combining backpropagation and gradient descent, we get a powerful training process. It enables neural networks to learn from data, adjust their parameters, and improve their accuracy over time. This training is what makes neural networks so valuable in tasks like image recognition, language processing, and much more.

## Learning and Loss Functions
The learning process of a neural network involves adjusting its weights and biases based on the input data it receives and the output it’s supposed to produce. This adjustment happens during the network’s training phase. When a neural network is given data, it makes predictions or decisions based on its current state. It then compares its predictions to the actual, desired outcome and learns from any differences.

This process is a bit like a feedback loop. The network tries, learns from its mistakes, and tries again, but this time a bit smarter. Over time, through many rounds of this loop, the neural network becomes more and more accurate in its predictions or decisions.

A crucial part of this learning process is the use of loss functions. These functions measure how far off a neural network’s predictions are from the actual results. Think of them as scoring systems – they tell you how well the network is doing. Two common loss functions are:

* **Mean Squared Error (MSE)**: This is often used in regression problems, where the goal is to predict continuous values. MSE calculates the average of the squares of the differences between the predicted values and the actual values. It’s like measuring the average distance from the target in a game of darts. The smaller the distance (or error), the better the neural network is at hitting the target.
* **Cross-Entropy**: This one is popular in classification problems, where the goal is to choose between different categories (like identifying whether an image shows a cat or a dog). Cross-entropy measures the difference between two probability distributions – the actual distribution and the predicted distribution. It’s like measuring how well the network’s guess matches the real answer in terms of probability.

Both of these loss functions provide a way for the network to understand and quantify its mistakes. By minimizing these errors (as measured by the loss functions) through training, the neural network becomes more accurate and reliable in its predictions or classifications.

### Numerical Example
To provide a numerical example that illustrates training, backpropagation, and loss functions in a neural network, let’s consider a simple scenario. We’ll use a small neural network with one input layer, two hidden layers, and one output layer. We’ll focus on a regression problem using Mean Squared Error (MSE) as our loss function.

**Scenario**
* **Task**: Predict the output value based on the input.
* **Input**: A single value (for simplicity).
Output: A single predicted value.
* **True Output**: The actual value we want to predict.
* **Input Value (X)**: 2
* **True Output (Y)**: 6
* **Initial Weights**:
   * **Input to Hidden Layer**: W1 = 0.5, W2 = 0.5
   * **Hidden to Output Layer**: W3 = W4 = 0.5
* **Biases**:
   * **Hidden Layer**: B1 = 1, B2 = 1
   * **Output Layer**: B3 = 1

![numerical-example-that-illustrates-training-backpropagation-and-loss-functions-1024x683](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/lia22jsp2f04phws4f84.png)

**Forward Pass (Initial Prediction)**

1. **Hidden Layer Calculations** (using a simple linear activation for simplicity):
   * Node2 = (X * W1) + B1 = (2 * 0.5) + 1 = 2
   * Node3 = (X * W2) + B2 = (2 * 0.5) + 1 = 2
2. **Output Layer Calculation**:
Output = (Node2 * W3) + (Node3 * W4) + B3 = (2 * 0.5) + (2 * 0.5) + 1 = 3

**Loss Calculation (Mean Squared Error)**
* MSE: (True Output – Predicted Output)^2 = (6 – 3)^2 = 9

**Backpropagation and Weight Update (Simplified)**

Adjust weights to reduce loss. This is a complex calculation involving partial derivatives, but let’s simplify, suppose we adjust the weights slightly based on the error:

New W1 = 0.55, New W2 = 0.55, New W3 = 0.55

![numerical-example-that-illustrates-training-backpropagation-and-loss-functions2-1024x683](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/icvjxw7lj68p73t2njq1.png)

**New Prediction After Weight Update**
1. Repeat the forward pass with new weights:
   * New Node2 = old Node2 output * W3 + B1 = (2 * 0.55) + 1 = 2.1
   * New Node3 = old Node3 output * W4 + B2 = (2 * 0.55) + 1 = 2.1
   * New Output = New Node2 * W1 + New Node3 * W2 + B = (2.1 * 0.55) + (2.1 * 0.55) + 1 = 3.31
2. New MSE:
   * (True Output – New Predicted Output)^2 = (6 – 3.31)^2 = 7.2361

The loss (MSE) has decreased from 9 to 7.2361, showing improvement in the prediction after adjusting the weights.

## Practical Applications of Neural Networks

Neural networks, a fascinating aspect of artificial intelligence, have found their way into various domains, revolutionizing the way we approach problems and solutions. Let’s dive into the diverse applications of neural networks, highlighting their adaptability and potential future trends.

1. **Image Recognition**: One of the most striking uses of neural networks is in image recognition. They can identify patterns and objects in images with remarkable accuracy. From unlocking your phone with facial recognition to diagnosing diseases from medical images, neural networks are making significant contributions.
2. **Natural Language Processing (NLP)**: Neural networks are also at the forefront of understanding and processing human language. Whether it’s translating languages, powering voice assistants like Siri or Alexa, or enabling chatbots for customer service, NLP relies heavily on neural networks.
3. **Finance**: In the financial sector, neural networks are used for predictive analysis, such as forecasting stock prices or identifying fraudulent activities. Their ability to analyze vast amounts of data helps in making informed decisions.
4. **Healthcare**: The healthcare industry benefits immensely from neural networks. They assist in early disease detection, drug discovery, and personalized medicine, contributing to more efficient and effective patient care.

> One such application is the [**AI Background Remover**](https://www.codester.com/items/46073/clearcut-ai-background-remover-python?ref=DevEasyLearn), a handy tool for anyone looking to effortlessly remove backgrounds from images. This application, leveraging the power of AI, simplifies a task that traditionally requires complex editing skills.

## Conclusion
In conclusion, we’ve learned a lot about neural networks. These are smart systems in technology that work like the human brain. We looked at how they are made, how they learn, and how they make decisions. We also saw how important they are in different areas like recognizing pictures, understanding language, and helping in finance, and healthcare.

Neural networks are changing how we solve problems in many fields. They are becoming more important every day. In the future, they will do even more amazing things.

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