Chapter 146 · Advanced & Applied Topics
Deep Learning Primer · Challenge Solutions
Worked solutions: a neuron by hand, the linear baseline, training the network, the no-nonlinearity ablation, and the decision-boundary plot.
In [1]:
import numpy as np, pandas as pd
import matplotlib.pyplot as plt
from sklearn.linear_model import LogisticRegression
from sklearn.model_selection import train_test_split
from sklearn.metrics import accuracy_score
import torch, torch.nn as nn
np.random.seed(46); torch.manual_seed(46)
plt.rcParams.update({"figure.dpi":110,"axes.grid":True,"grid.alpha":0.25,"font.size":11})
FU, BL, GR, RD = "#a21caf", "#2563eb", "#16a34a", "#dc2626"
BASE = "https://raw.githubusercontent.com/johnfisher-ai/Statistics-Data-Science-AI-Visual-Book/main/data/"
fn = "deep-learning-primer--sensor-qc.xlsx"
try:
df = pd.read_excel("../../data/" + fn)
except FileNotFoundError:
df = pd.read_excel(BASE + fn)
print(df.shape); print(df.head())
X = df[["sensor_a","sensor_b"]].values; y = df["passed_qc"].values
Xtr, Xte, ytr, yte = train_test_split(X, y, test_size=0.3, random_state=1, stratify=y)
Xtr_t = torch.tensor(Xtr, dtype=torch.float32); ytr_t = torch.tensor(ytr, dtype=torch.float32).unsqueeze(1)
Xte_t = torch.tensor(Xte, dtype=torch.float32); yte_t = torch.tensor(yte, dtype=torch.float32).unsqueeze(1)
(1200, 3) sensor_a sensor_b passed_qc 0 2.434 -0.347 0 1 -2.537 -2.739 0 2 -1.365 2.489 0 3 0.731 1.895 1 4 2.604 -0.033 0
Challenge 1 · Neuron by hand¶
One neuron = weighted sum + bias, then a sigmoid.
In [2]:
w = np.array([0.7, -0.4]); b = 0.1; x = np.array([1.5, 2.0])
print(f"output = {1/(1+np.exp(-(w@x+b))):.3f}")
output = 0.587
Challenge 2 · Linear baseline¶
A straight line cannot enclose a ring, so logistic regression stalls near 60%.
In [3]:
lr = LogisticRegression().fit(Xtr, ytr)
print(f"logistic regression accuracy = {accuracy_score(yte, lr.predict(Xte)):.3f}")
logistic regression accuracy = 0.614
Challenge 3 · Train the network¶
Forward, loss, backward, step, repeat.
In [4]:
def train(model, epochs=300, lr=0.02):
opt = torch.optim.Adam(model.parameters(), lr=lr); lf = nn.BCEWithLogitsLoss(); L=[]
for _ in range(epochs):
opt.zero_grad(); loss = lf(model(Xtr_t), ytr_t); loss.backward(); opt.step(); L.append(loss.item())
return L
torch.manual_seed(46)
mlp = nn.Sequential(nn.Linear(2,16), nn.ReLU(), nn.Linear(16,16), nn.ReLU(), nn.Linear(16,1))
L = train(mlp)
plt.figure(figsize=(7,3.2)); plt.plot(L, color=FU, lw=2); plt.xlabel("epoch"); plt.ylabel("loss"); plt.title("training loss"); plt.tight_layout(); plt.show()
with torch.no_grad():
print(f"MLP accuracy = {((torch.sigmoid(mlp(Xte_t))>0.5).float()==yte_t).float().mean().item():.3f}")
MLP accuracy = 0.911
Challenge 4 · Remove the nonlinearity¶
Delete the ReLUs: stacked linear layers are still ONE linear map, so accuracy collapses back to the linear level.
In [5]:
torch.manual_seed(46)
linear_only = nn.Sequential(nn.Linear(2,16), nn.Linear(16,16), nn.Linear(16,1)) # no ReLU
train(linear_only)
with torch.no_grad():
print(f"linear-only network accuracy = {((torch.sigmoid(linear_only(Xte_t))>0.5).float()==yte_t).float().mean().item():.3f}")
print("Same as logistic regression: without a nonlinearity, depth adds nothing.")
linear-only network accuracy = 0.614 Same as logistic regression: without a nonlinearity, depth adds nothing.
Challenge 5 · Draw the boundary¶
Predict over a grid and contour the 0.5 line: the trained network traces the ring.
In [6]:
gx, gy = np.meshgrid(np.linspace(-3.2,3.2,300), np.linspace(-3.2,3.2,300))
grid = torch.tensor(np.c_[gx.ravel(), gy.ravel()], dtype=torch.float32)
with torch.no_grad(): zz = torch.sigmoid(mlp(grid)).numpy().reshape(gx.shape)
plt.figure(figsize=(5.2,5))
plt.contourf(gx, gy, zz, levels=[0,0.5,1], colors=["#fde8e8","#e3f9ec"], alpha=0.9)
plt.contour(gx, gy, zz, levels=[0.5], colors=[FU], linewidths=2)
plt.scatter(X[y==0,0], X[y==0,1], s=6, color=RD, alpha=0.4); plt.scatter(X[y==1,0], X[y==1,1], s=6, color=GR, alpha=0.5)
plt.gca().set_aspect("equal"); plt.title("learned ring boundary"); plt.tight_layout(); plt.show()
