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# Neural Networks Lecture

# Briefing

## What is a neural network

- The single Neuron
- Weighted Input
- Activation

- The network model
- Input/Output
- Weights
- Activation Function

- The Tensor Model

## Output and Loss Function

- Classification versus Regression
- Two-class classification (0 or 1)
- Regression \(y=f(x)\in \mathbb{R}\)
- Multiclass \(y=(y_1,y_2,\ldots,y_n)\)
- \(y_i=1\) means membership in class 1

- Soft-decision: \(y\) is a continuous variable
- higher values are more probably one

- Loss functiion
- Mean-squared error (common for regression) \[L = (x-y)^2\]
- Cross Entropy (common for classification) \[L = \log \frac{ \exp x_{y} } { \sum \exp x_i }\]
- There are others

## Training

- Optimisation problem
- tune the weights to minimise the loss function
- if the activation function is differentiable, the entire system is
- different optimisation algorithms; trust the API or do a more advanced module

## Activation Functions

- Threshold functions
- Approximations to the threshold function
- Logistic: \(f(x) = \frac1{1+e^{-\beta x}}\)
- ReLU: \(f(x)=\max(x,0)\)
- not differentiable

## Tools

Two main contenders.

- TensorFlow
- PyTorch
- A replacement for NumPy to use the power of GPUs and other accelerators.
- An automatic differentiation library that is useful to implement neural networks.

Note that PyTorch replaces NumPy; i.e. it is primarily a python tool, and operaes in the object oriented framework of python.

The reason for using PyTorch in these examples is primarily that I have lately been working off some code created by some final year students this Spring, and they happened to choose PyTorch. The choice of TensorFlow or PyTorch is otherwise arbitrary.

## Sample Program

### Training

```
model = Inception3(num_outputs=nparams)
criterion = nn.MSELoss()
optimizer = torch.optim.Adam(model.parameters(),
model.train()
for epoch in range(num_epochs):
tloss = 0.0
for i, (images, params) in enumerate(trainloader):
optimizer.zero_grad()
output = model(images)
loss = criterion(output, params)
loss.backward()
optimizer.step()
tloss += loss.item() * len(images)
print( f"Epoch {epoch+1}: Loss = {tloss}" )
```

### Testing

```
total_loss = 0
model.eval()
with torch.no_grad():
for images, params in testloader:
output = model(images)
loss = criterion(output, params)
total_loss += loss * len(images)
```

## Loss Functions and Evaluation

- Accuracy: ratio of correctly classified items
- What is the difference between a rate and a probability?
- Statistics
- Standard deviation
- Hypothesis Tests
- Confidence Interval

- Other heuristics

## Computational Power

- Neural Networks are Computationally Expensive
- GPU or CPU - what’s the difference?
- what resources do you have?

- Remedies
- Reduce image resolution
- Reduce number of images
- Reduce number of epochs

- In particular, it is necessary to sacrifice accuracy during development and testing.
- In the final stages you may need big datasets to achieve satisfactory results, and then you may need more computing power.