# Backpropagation Made Easy With Examples And How To In Keras

Of course, we’ll want to do this multiple, or maybe thousands, of times. If you are still confused, I highly reccomend you check out this informative video which explains the structure of a neural network with the same example. In this case represents what we want our neural network to predict. In this case, we are predicting the test score of someone who studied for four hours and slept for eight hours based on their prior performance.

We just got a little lucky when I chose the random weights for this example. The role of an activation function is to introduce nonlinearity. An advantage of this is that the output is mapped from backpropagation tutorial a range of 0 and 1, making it easier to alter weights in the future. The gradients of the weights can thus be computed using a few matrix multiplications for each level; this is backpropagation.

• From there, we start looping over our number of epochs on Line 50.
• Repeatedly adjusts the weights of the connections in the network so as to minimize a measure of the difference between the actual output vector of the net and the desired output vector.
• Now, let’s train the network and see how the network will predict the output of the sample based on the current parameters.
• For details about how to build this script, please refer to this book.

Backpropagation had been derived repeatedly, as it is essentially an efficient application of the chain rule (derived by Gottfried Wilhelm Leibniz in 1673[1][26]) to neural networks. Backpropagation training is much smoother when the training data is of the highest quality, so clean your data before feeding it to your algorithm. This means normalizing the input values, which involves checking that the mean of the data is zero and the data set has a standard deviation of one.

## ML & Data Science

On Line 2, we import the only required package we’ll need for our implementation of back propagation — the NumPy numerical processing library. Construct an intuitive, easy to follow implementation of the backpropagation algorithm using the Python language. This can be as simple as MSE (mean squared error) or more complex like cross-entropy. You will see that z² can be expressed using (z_1)² and (z_2)² where (z_1)² and (z_2)² are the sums of the multiplication between every input x_i with the corresponding weight (W_ij)¹. The same operations can be applied to any layer in the network. If the derivative sign is negative, increasing the weight decreases the error.

• A high learning rate can cause the optimiser to overshoot the optimal weights and biases, leading to instability and slow convergence.
• It’s quite easy to implement the backpropagation algorithm for the example discussed in the previous section.
• In other words, we need to use the derivative of the loss function to understand how the weights affect the input.
• The error on weight w is calculated by differentiating total error with respect to w.

Now, how error function is used in Backpropagation and how Backpropagation works? Let start with an example and do it mathematically to understand how exactly updates the weight using Backpropagation. Sigmoid is the activation function used by the neurons, which maps the neuron’s input to a value between 0 and 1. This way we will try to reduce the error by changing the values of weights and biases. By knowing which way to alter our weights, our outputs can only get more accurate. Theoretically, with those weights, out neural network will calculate .85 as our test score!

## Backpropagation Process in Deep Neural Network

The optimization function, gradient descent in our example, will help us find the weights that will hopefully yield a smaller loss in the next iteration. Although backpropagation has its flaws, it’s still an effective model for testing and refining the performance of neural networks. Now that we understand the pros and cons of this algorithm, let’s take a deeper look at the ins and outs of backpropagation in neural networks. In this article, I would like to go over the mathematical process of training and optimizing a simple 4-layer neural network.

## Output Layer

On Line 47, we perform the bias trick by inserting a column of 1’s as the last entry in our feature matrix, X. From there, we start looping over our number of epochs on Line 50. For each epoch, we’ll loop over each individual data point in our training set, make a prediction on the data point, compute the backpropagation phase, and then update our weight matrix (Lines 53 and 54). Lines simply check to see if we should display a training update to our terminal. This image breaks down what our neural network actually does to produce an output. First, the products of the random generated weights (.2, .6, .1, .8, .3, .7) on each synapse and the corresponding inputs are summed to arrive as the first values of the hidden layer.

## Forward and backward passes in Neural Networks

Where
learning_rate
is a hyperparameter that controls the size of the weight updates. Consider W5, we will calculate the rate of change of error w.r.t change in weight W5. I am pretty sure, now you know, why we need Backpropagation or why and what is the meaning of training a model. The calculations we made, as complex as they seemed to be, all played a big role in our learning model. To run the network, all we have to do is to run the train function.

These sums are in a smaller font as they are not the final values for the hidden layer. There are only three things to consider in backpropagation for a fully connected network such as above. The passing gradient from the right, the local gradient calculated from the derivation of the activation function, and the passing gradient regarding the weights and the inputs to the left. One commonly used algorithm to find the set of weights that minimizes the error is gradient descent.

We’ll start by reviewing each of these phases at a high level. From there, we’ll implement the backpropagation algorithm using Python. Looking carefully, you can see that all of x, z², a², z³, a³, W¹, W², b¹ and b² are missing their subscripts presented in the 4-layer network illustration above. The reason is that we have combined all parameter values in matrices, grouped by layers. This is the standard way of working with neural networks and one should be comfortable with the calculations. However, I will go over the equations to clear out any confusion.

Here, we successfully updated the parameters without using the backpropagation algorithm. Now, let’s train the network and see how the network will predict the output of the sample based on the current parameters. Assume that the initial values for both weights and bias are like in the next table. GloVe, or Global Vectors for Word Representation, is an unsupervised learning algorithm that obtains vector word representations by analyzing…

Backpropagation is one of the important concepts of a neural network. For this, we have to update the weights of parameter and bias, but how can we do that in a deep neural network? In the linear regression model, we use gradient descent to optimize the parameter. Similarly here we also use gradient descent algorithm using Backpropagation.

For details about how to build this script, please refer to this book. The GitHub project also gives a simpler interface to build the network in the Ch09 directory. There’s an example that builds a network with 3 inputs and 1 output. At the end of the code, the function predict() is called to ask the network to predict the output of a new sample [0.2, 3.1, 1.7]. Let’s now update the weights according to the calculated derivatives.

On Key

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