# Neural Network Implementation

This post is part of the series Neural Networks.

In the first post of this series, I introduced the basic concept of neural networks and wrote down the relation for a single neuron. Here, I provide more detail on the mathematics and implementation of a complete network.

## Nomenclature

As in the case of the single neuron, $$\{x_i\}$$ denotes inputs to a neuron, $$a$$ represents its activation (output), and $$\{w_i\}$$ and $$b$$ refer to weights and bias respectively. We also introduce here the “augmented input” $$z$$, corresponding to the input to the activation function $$g$$. In summary:

$z = \sum_i w_i x_i + b$ $a = g \left( z \right)$

As we will always use the sigmoid function $$\sigma(x)$$ as the activation function here, from now on we will refer to them directly, though it is possible to use other activation functions:

$g(t) = \sigma(t) = \frac{1}{1 + e^{-t}}$

In a network with multiple layers and multiple neurons per layer, we also need to introduce additional indices to refer to the relevant properties of individual neurons. Consider once again the example network shown in the first post of this series, now with the layers of the network numbered from 1 to 3:

The values referring to a particular neuron are now referred to using a superscript index $$l$$ to refer to the network layer, and subscript indices $$j$$ and $$k$$ to refer to the $$j$$th neuron of a particular layer and the $$k$$th input to that neuron.

In the above network, the highlighted neuron is number $$j=2$$ of layer $$l=2$$, so its activation, highlighted in blue, is given by $$a_2^{(2)}$$, and its input from neuron $$k=2$$ of layer $$l-1$$, highlighted in orange, is given by $$x_{22}^{(2)}$$. Similarly, the weight corresponding to input $$x_{22}^{(2)}$$ is denoted by $$w_{22}^{(2)}$$, and the bias of the highlighted neuron is $$b_2^{(2)}$$.

Introducing these notations to the above relations for a single neuron, we have:

$z_j^{(l)} = \sum_k w_{jk}^{(l)} x_{jk}^{(l)} + b_j^{(l)}$ $a_j^{(l)} = \sigma \left( z_j^{(l)} \right)$

## Vectorisation

Writing out the above indices can be time consuming, so it is usually convenient to write out the parameters and values mentioned above as vectors. All of the activations of a particular layer $$l$$, $$\{z_j^{(l)}\}$$, for example, can be written together as $$\mathbf{z}_j$$. The equations for a single neuron then become:

$z_j^{(l)} = \mathbf{w}_j^{(l)} \cdot \mathbf{x}_j^{(l)} + b_j^{(l)}$

or further:

$\mathbf{z}^{(l)} = \mathbf{w}^{(l)} \cdot \mathbf{x}^{(l)} + \mathbf{b}^{(l)}$

and

$\mathbf{a}^{(l)} = \sigma \left( \mathbf{z}^{(l)} \right)$

In addition, since $$x_{jk}^{(l)} = a_k^{(l-1)}$$ we can write the relation:

$\mathbf{z}^{(l)} = \mathbf{w}^{(l)} \cdot \mathbf{a}^{(l-1)} + \mathbf{b}^{(l)}$

Starting with input activations $$\mathbf{a}^{(0)}$$, we can propagate the activations through the network using the above relations for $$\mathbf{z}$$ and $$\mathbf{a}$$, and so arrive at the output activations.

## Making Predictions

When using a neural network, the output layer provides a set of activation values $$\{a_j^{(l)}\}$$. To use the network to perform a classification problem, each of these activations is assigned to a particular class, and the activations are considered as something analagous to the probability of the input being of that class. The class predicted by the network can then be determined by:

$\mathop{\mathrm{argmax}}\limits_j a_j^{(L)}$

where $$l=L$$ is the last layer of the network.

## Implementation

The above forward propagation and prediction methods are implemented in my neural network Python project, available on GitHub. The implementation there is divided into two main classes, NeuralNetwork and NeuralNetworkLayer, which store their parameters and perform processing using NumPy arrays.

These classes work together to perform the forward propagation of input activations to outputs. The NeuralNetworkLayer performs the propagation for a single layer:

The NeuralNetwork class passes activations through successive layers to process the whole network:

Predictions can be readily done using the predict method, which takes input activations and automatically maps the forward-propagated output activations to the appropriate category labels.