# Chapter 3: Under the Hood: Training a Digit Classifier

## Use Pytorch/SGD to fit a (quadratic) curve

• First, generate a dataset:

time = torch.arange(0, 20).float()
speed = (time - 10) ** 2 + (torch.randn(20) * 3)
plt.scatter(time, speed)

• We want to fit a quadratic equation of the form $$ax^2 + bx + c$$ to this data set (a, b, and c are the weights we want to tune). Define a generic quadratic function and a loss function:

def f(x, params):
a,b,c = params
return a*(x**2) + b*(x) + c

def loss_fn(actual, prediction):
return ((actual - prediction) ** 2).mean()

• Apply SGD:

lr = 1e-5

# 1. Initialize weights.
#    requires_grad_ indicates that we want to calculate the
#     gradient/derivative for these weights at some point.

for i in range(2000):
# 2. Use weights to obtain a result curve
predictions = f(time, params)

# 3. Use the loss function to compare the result curve to the actual curve
loss = loss_fn(speed, predictions)

# 4. Use backward to calculate derivatives for the weights against the loss function
#    (slope of the tangent; where x = one of the weights, and f(x) is the loss given that weight)
#    All intermediate computation between 1. and 4. are preserved behind the scenes
#    so pytorch can calculate this automatically, which is _really_ cool!
loss.backward()

# 5. Update the weights based on this derivative. Subtract here because:
#    - A negative gradient (downward slope) implies that we want to _increase_ the weight to reduce loss
#    - A positive gradient (upward slope) implies that we want to _decrease_ the weight to reduce loss

plt.scatter(time, to_np(predictions))
plt.scatter(time, speed)


## Train a classifier for the MNIST sample dataset

Train a classifier to pick threes and sevens apart from a dataset of handwritten digits.

url = URLs.MNIST_SAMPLE
path = untar_data(url)

threes = (path/'train/3').ls()
sevens = (path/'train/7').ls()

valid_threes = (path/'valid/3').ls()
valid_sevens = (path/'valid/7').ls()

t = tensor(Image.open(threes[1]))
df = pd.DataFrame(t)

• Convert each image into a pytorch-compatible “tensor”, using torch.stack to fold the enclosing list into the tensor too:

threes_tensor = [tensor(Image.open(i)) for i in threes]
sevens_tensor = [tensor(Image.open(i)) for i in sevens]

valid_threes_tensor = [tensor(Image.open(i)) for i in valid_threes]
valid_sevens_tensor = [tensor(Image.open(i)) for i in valid_sevens]

stacked_threes = torch.stack(threes_tensor)
stacked_sevens = torch.stack(sevens_tensor)

valid_stacked_threes = torch.stack(valid_threes_tensor)
valid_stacked_sevens = torch.stack(valid_sevens_tensor)

stacked_sevens.shape # torch.Size([6265, 28, 28])

• Flatten each image from a 28x28 2-d array into a flat vector, presumably to make matrix multiplication easier/possible:

flattened_threes = stacked_threes.view(-1, 28*28)
flattened_sevens = stacked_sevens.view(-1, 28*28)

valid_flattened_threes = valid_stacked_threes.view(-1, 28*28)
valid_flattened_sevens = valid_stacked_sevens.view(-1, 28*28)

flattened_threes.shape, flattened_sevens.shape # (torch.Size([6131, 784]), torch.Size([6265, 784]))

• Build pytorch Datasets; one each for the training set and the validation set, and build DataLoaders from it:

# Not sure why unsqueeze is required here, but the model doesn't work without it.
# Best guess: it's used so the target listing has the same matrix dimensions as the predictions it's being compared to later.

dset = list(zip(
torch.cat([flattened_threes, flattened_sevens], 0).float(),
torch.cat([tensor([0] * len(flattened_threes)), tensor([1] * len(flattened_sevens))], 0).unsqueeze(1)
))

valid_dset = list(zip(
torch.cat([valid_flattened_threes, valid_flattened_sevens], 0).float(),
torch.cat([tensor([0] * len(valid_flattened_threes)), tensor([1] * len(valid_flattened_sevens))], 0).unsqueeze(1)
))


• Next, define a model (this is [apparently] called a linear1 model), a loss function, and an accuracy function.

• Applying weights to the model results in one prediction per image; a matrix containing all images' pixels is multiplied by a matrix containing all weights.
• bias provides additional jitter that isn’t influenced by the pixel values.
• The loss and accuracy functions are distinct; the former is used for SGD, and the latter is used for human consumption.
• Here, both functions first normalize input predictions to $$[0,1]$$ via the sigmoid function. The target for a prediction is 0 for digit “3”, and 1 for digit “7”.
• The loss function takes the mean of every prediction’s distance from it’s target. The accuracy function is more binary; anything over 0.5 is considered a “7”, and anything below is considered a “3”; so a given prediction is either right or wrong.
• The accuracy function here is a poor fit for a loss function because it’s constant for the most part, except at the threshold (0.5), which makes SGD difficult - gradients are typically 0. So we need a loss function that’s much more responsive to small improvements.
def apply_weights(initials, weights, bias):
# initials has dimensions (256 x 28*28) {batch size x image dimensions}
# weights has dimensions (28*28 x 1) {one weight per pixel}
# the resultant prediction has dimensions {256 x 1} {one prediction per image}
return initials@weights + bias

def loss_fn(predictions, targets):
# each target is 0 or 1; 0 implies "3", 1 implies "7"
# each prediction is normalized to [0,1]
#    - 0 implies 100% confidence in a prediction of 3
#    - 1 implies 100% confidence in a prediction of 7
# smaller loss implies a better fit
predictions = predictions.sigmoid()

def accuracy_fn(predictions, targets):
predictions = predictions.sigmoid()
# A prediction over 0.5 is a "7", a prediction below 0.5 is a "3"
return ((predictions > 0.5).float() == targets).float().mean()

• Finally, execute the learning process:

epochs = 20
lr = 1.

def train_epoch():
# A DataLoader splits data into batches, so we iterate over batches here.
for initials, targets in dl:
predictions = apply_weights(initials, weights, bias)
loss = loss_fn(predictions, targets)
loss.backward()
for param in [weights, bias]:

def validate_epoch():
accuracies = [accuracy_fn(apply_weights(initials, weights, bias), targets) for initials, targets in valid_dl]

for _ in range(epochs):
train_epoch()
print("Accuracy: " + str(validate_epoch()))

# Accuracy: 0.8283473253250122
# Accuracy: 0.9078061580657959
# Accuracy: 0.8738964200019836
# Accuracy: 0.9475355744361877
# Accuracy: 0.948043704032898
# Accuracy: 0.9533552527427673
# Accuracy: 0.9523786902427673
# Accuracy: 0.9636488556861877
# Accuracy: 0.9628310799598694
# Accuracy: 0.9648040533065796
# Accuracy: 0.9336652159690857
# Accuracy: 0.9371824264526367
# Accuracy: 0.9515013694763184
# Accuracy: 0.961814820766449
# Accuracy: 0.9651533961296082
# Accuracy: 0.943160891532898
# Accuracy: 0.9676940441131592
# Accuracy: 0.9699568152427673
# Accuracy: 0.9549987316131592
# Accuracy: 0.9730651378631592

• The pytorch nn.Linear model does the same thing as apply_weights here, and incorporates both weights and biases.

• fastai provides an SGD function that performs parameter adjustment (after backward above).

• fastai also provides a Learner that can stand-in for almost the entire orchestration code above:

dls = DataLoaders(dl, valid_dl)
learn = Learner(dls, nn.Linear(28*28, 1), opt_func=SGD, loss_func=loss_fn, metrics=accuracy_fn)
learn.fit(20, lr=1.)

• This is a great way to build up an intuitive sense of the magic that fastai appears to be at first.

## Train a classifier for the full MNIST dataset

Train a classifier (as above) on the full MNIST dataset - handwritten digits from 0 - 9

• An initial attempt was not terrible. The loss function is missing something:

# 1. Build dataset
url = URLs.MNIST
path = untar_data(url)

train = [[int(os.path.basename(dir)), dir.ls()] for dir in (path/'training').ls()]
valid = [[int(os.path.basename(dir)), dir.ls()] for dir in (path/'testing').ls()]

from functools import reduce

def process_images(memo, val):
i, t = val
xb, yb = memo

xb.append(torch.stack([tensor(Image.open(img)).float() for img in t]).view(-1, 28*28))
yb.append(tensor([float(i)] * len(t)).unsqueeze(1))

return [xb, yb]

xb, yb = reduce(process_images, sorted(train, key=first), ([], []))
valid_xb, valid_yb = reduce(process_images, sorted(valid, key=first), ([], []))

train_dset = list(zip(torch.cat(xb, 0), torch.cat(yb, 0)))
valid_dset = list(zip(torch.cat(valid_xb, 0), torch.cat(valid_yb)))

# 2. Define loss/accuracy fns

# Normalize every prediction to [0, 9], and find its (absolute) distance to the target.
def loss(predictions, targets):
predictions = predictions.sigmoid() * 10
return (targets - predictions).abs().mean()

# Normalize every prediction to [0, 9]; a prediction is correct if it's within 1.0 of the target.
def accuracy(predictions, targets):
predictions = predictions.sigmoid() * 10
return ((targets - predictions).abs().float() < 1.).float().mean()

# 3. Train