# Defining a Neural Network in PyTorch¶

Deep learning uses artificial neural networks (models), which are computing systems that are composed of many layers of interconnected units. By passing data through these interconnected units, a neural network is able to learn how to approximate the computations required to transform inputs into outputs. In PyTorch, neural networks can be constructed using the torch.nn package.

## Introduction¶

PyTorch provides the elegantly designed modules and classes, including torch.nn, to help you create and train neural networks. An nn.Module contains layers, and a method forward(input) that returns the output.

In this recipe, we will use torch.nn to define a neural network intended for the MNIST dataset.

## Setup¶

Before we begin, we need to install torch if it isn’t already available.

pip install torch


## Steps¶

2. Define and intialize the neural network
3. Specify how data will pass through your model
4. [Optional] Pass data through your model to test

For this recipe, we will use torch and its subsidiaries torch.nn and torch.nn.functional.

import torch
import torch.nn as nn
import torch.nn.functional as F


### 2. Define and intialize the neural network¶

Our network will recognize images. We will use a process built into PyTorch called convolution. Convolution adds each element of an image to its local neighbors, weighted by a kernel, or a small matrix, that helps us extract certain features (like edge detection, sharpness, blurriness, etc.) from the input image.

There are two requirements for defining the Net class of your model. The first is writing an __init__ function that references nn.Module. This function is where you define the fully connected layers in your neural network.

Using convolution, we will define our model to take 1 input image channel, and output match our target of 10 labels representing numbers 0 through 9. This algorithm is yours to create, we will follow a standard MNIST algorithm.

class Net(nn.Module):
def __init__(self):
super(Net, self).__init__()

# First 2D convolutional layer, taking in 1 input channel (image),
# outputting 32 convolutional features, with a square kernel size of 3
self.conv1 = nn.Conv2d(1, 32, 3, 1)
# Second 2D convolutional layer, taking in the 32 input layers,
# outputting 64 convolutional features, with a square kernel size of 3
self.conv2 = nn.Conv2d(32, 64, 3, 1)

# Designed to ensure that adjacent pixels are either all 0s or all active
# with an input probability
self.dropout1 = nn.Dropout2d(0.25)
self.dropout2 = nn.Dropout2d(0.5)

# First fully connected layer
self.fc1 = nn.Linear(9216, 128)
# Second fully connected layer that outputs our 10 labels
self.fc2 = nn.Linear(128, 10)

my_nn = Net()
print(my_nn)


We have finished defining our neural network, now we have to define how our data will pass through it.

### 3. Specify how data will pass through your model¶

When you use PyTorch to build a model, you just have to define the forward function, that will pass the data into the computation graph (i.e. our neural network). This will represent our feed-forward algorithm.

You can use any of the Tensor operations in the forward function.

class Net(nn.Module):
def __init__(self):
super(Net, self).__init__()
self.conv1 = nn.Conv2d(1, 32, 3, 1)
self.conv2 = nn.Conv2d(32, 64, 3, 1)
self.dropout1 = nn.Dropout2d(0.25)
self.dropout2 = nn.Dropout2d(0.5)
self.fc1 = nn.Linear(9216, 128)
self.fc2 = nn.Linear(128, 10)

# x represents our data
def forward(self, x):
# Pass data through conv1
x = self.conv1(x)
# Use the rectified-linear activation function over x
x = F.relu(x)

x = self.conv2(x)
x = F.relu(x)

# Run max pooling over x
x = F.max_pool2d(x, 2)
# Pass data through dropout1
x = self.dropout1(x)
# Flatten x with start_dim=1
x = torch.flatten(x, 1)
# Pass data through fc1
x = self.fc1(x)
x = F.relu(x)
x = self.dropout2(x)
x = self.fc2(x)

# Apply softmax to x
output = F.log_softmax(x, dim=1)
return output


### 4. [Optional] Pass data through your model to test¶

To ensure we receive our desired output, let’s test our model by passing some random data through it.

# Equates to one random 28x28 image
random_data = torch.rand((1, 1, 28, 28))

my_nn = Net()
result = my_nn(random_data)
print (result)


Each number in this resulting tensor equates to the prediction of the label the random tensor is associated to.

Congratulations! You have successfully defined a neural network in PyTorch.