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[MMD]

Calculus Coding Assignments: Implementing Optimization Algorithms with NumPy

·16 min read·

This post is a set of coding assignments where you implement the concepts from Calculus Chapters 1–3 directly in Python/NumPy. Try solving each problem on your own before checking the solution.

Environment: Python 3.x, NumPy, Matplotlib (optional)

import numpy as np
import matplotlib.pyplot as plt

Part 1. Numerical Differentiation#

Assignment 1-1. Implementing Numerical Differentiation#

Instead of an analytic derivative, implement numerical differentiation using the Central Difference Method.


f(x)f(x+h)f(xh)2hf'(x) \approx \frac{f(x + h) - f(x - h)}{2h}

def numerical_derivative(f, x: float, h: float = 1e-5) -> float:
    """
    Approximates f'(x) using the central difference method
    """
    pass

# Test: f(x) = x^3, f'(x) = 3x^2
# f'(2) should equal 12
View Solution
def numerical_derivative(f, x: float, h: float = 1e-5) -> float:
    return (f(x + h) - f(x - h)) / (2 * h)

# Test
f = lambda x: x ** 3
print(f"f'(2) numerical: {numerical_derivative(f, 2):.6f}")
print(f"f'(2) analytic:  {3 * 2**2:.6f}")

# Test with other functions
g = lambda x: np.sin(x)  # g'(x) = cos(x)
print(f"\ng'(π/4) numerical: {numerical_derivative(g, np.pi/4):.6f}")
print(f"g'(π/4) analytic:  {np.cos(np.pi/4):.6f}")

h_func = lambda x: np.exp(x)  # h'(x) = e^x
print(f"\nh'(1) numerical: {numerical_derivative(h_func, 1):.6f}")
print(f"h'(1) analytic:  {np.exp(1):.6f}")

Output:

f'(2) numerical: 12.000000
f'(2) analytic:  12.000000

g'(π/4) numerical: 0.707107
g'(π/4) analytic:  0.707107

h'(1) numerical: 2.718282
h'(1) analytic:  2.718282

Assignment 1-2. Activation Functions and Their Derivatives#

Implement three activation functions commonly used in machine learning, along with their derivatives.

def sigmoid(x: np.ndarray) -> np.ndarray:
    pass

def sigmoid_derivative(x: np.ndarray) -> np.ndarray:
    pass

def relu(x: np.ndarray) -> np.ndarray:
    pass

def relu_derivative(x: np.ndarray) -> np.ndarray:
    pass

def tanh_derivative(x: np.ndarray) -> np.ndarray:
    # np.tanh is available; implement only the derivative
    pass
View Solution
def sigmoid(x: np.ndarray) -> np.ndarray:
    return 1 / (1 + np.exp(-x))

def sigmoid_derivative(x: np.ndarray) -> np.ndarray:
    s = sigmoid(x)
    return s * (1 - s)

def relu(x: np.ndarray) -> np.ndarray:
    return np.maximum(0, x)

def relu_derivative(x: np.ndarray) -> np.ndarray:
    return (x > 0).astype(float)

def tanh_derivative(x: np.ndarray) -> np.ndarray:
    return 1 - np.tanh(x) ** 2

# Test
x = np.array([-2, -1, 0, 1, 2], dtype=float)

print("x           :", x)
print("sigmoid     :", sigmoid(x).round(4))
print("sigmoid'    :", sigmoid_derivative(x).round(4))
print("relu        :", relu(x))
print("relu'       :", relu_derivative(x))
print("tanh'       :", tanh_derivative(x).round(4))

# Check maximum derivative values
print(f"\nMax sigmoid': {sigmoid_derivative(np.array([0.0]))[0]:.4f}  (at x=0)")
print(f"Max tanh'  : {tanh_derivative(np.array([0.0]))[0]:.4f}  (at x=0)")
print(f"Max relu'  : 1.0000  (always 1 for x>0)")

Output:

x           : [-2. -1.  0.  1.  2.]
sigmoid     : [0.1192 0.2689 0.5    0.7311 0.8808]
sigmoid'    : [0.1050 0.1966 0.25   0.1966 0.1050]
relu        : [0. 0. 0. 1. 2.]
relu'       : [0. 0. 0. 1. 1.]
tanh'       : [0.0707 0.4200 1.0000 0.4200 0.0707]

Max sigmoid': 0.2500  (at x=0)
Max tanh'  : 1.0000  (at x=0)
Max relu'  : 1.0000  (always 1 for x>0)

Key insight: The maximum value of sigmoid' is only 0.25, causing gradients to vanish in deep networks. ReLU solves this by keeping its derivative at a constant 1 for positive inputs.


Part 2. Loss Function Optimization#

Assignment 2-1. Verifying Squared Loss Minimization#

For the data points [3,7,5,9,1][3, 7, 5, 9, 1], find the ww that minimizes the squared loss L(w)=i(wxi)2L(w) = \sum_i (w - x_i)^2 and verify that it equals the mean.

data = np.array([3, 7, 5, 9, 1], dtype=float)

# 1. Compute L(w) over a range of w values and visualize the minimum
# 2. Compute the analytic minimum by setting the derivative to 0
# 3. Compare with NumPy's mean
View Solution
data = np.array([3, 7, 5, 9, 1], dtype=float)

# Define the loss function
def squared_loss(w: float, data: np.ndarray) -> float:
    return np.sum((w - data) ** 2)

def squared_loss_derivative(w: float, data: np.ndarray) -> float:
    return 2 * np.sum(w - data)

# Compute loss over a range of w values
w_values = np.linspace(0, 12, 300)
losses = [squared_loss(w, data) for w in w_values]

# Analytic minimum (set L'(w) = 0)
w_optimal = np.mean(data)
print(f"Analytic minimum: w* = {w_optimal}")
print(f"NumPy mean:        {np.mean(data)}")
print(f"L(w*) = {squared_loss(w_optimal, data):.4f}")

# Verify derivative
print(f"L'(w*) = {squared_loss_derivative(w_optimal, data):.6f}  (should be close to 0)")

# Visualization
plt.figure(figsize=(8, 4))
plt.plot(w_values, losses, 'b-', lw=2, label='L(w)')
plt.axvline(w_optimal, color='red', linestyle='--', label=f'w* = {w_optimal} (mean)')
plt.scatter([w_optimal], [squared_loss(w_optimal, data)], color='red', s=100, zorder=5)
plt.xlabel('w')
plt.ylabel('L(w)')
plt.title('Squared Loss Function — Minimum = Mean')
plt.legend()
plt.grid(True, alpha=0.3)
plt.tight_layout()
plt.savefig('squared_loss.png', dpi=150)
plt.show()

Output:

Analytic minimum: w* = 5.0
NumPy mean:        5.0
L(w*) = 40.0000
L'(w*) = 0.000000  (should be close to 0)

Assignment 2-2. Maximum Likelihood Estimation with Log Loss#

A coin is flipped 20 times and lands heads 13 times. Find the pp that maximizes the log-likelihood logL(p)=13logp+7log(1p)\log L(p) = 13\log p + 7\log(1-p) numerically, and compare it with the observed frequency.

n_heads = 13
n_tails = 7

# Define the log-likelihood function and find the p that maximizes it
View Solution
n_heads = 13
n_tails = 7

def log_likelihood(p: float) -> float:
    return n_heads * np.log(p) + n_tails * np.log(1 - p)

def log_likelihood_derivative(p: float) -> float:
    return n_heads / p - n_tails / (1 - p)

# Compute log-likelihood over range of p
p_values = np.linspace(0.01, 0.99, 500)
ll_values = [log_likelihood(p) for p in p_values]

# Analytic maximum: set L'(p) = 0
# n_heads/p = n_tails/(1-p) → p* = n_heads/(n_heads + n_tails)
p_mle = n_heads / (n_heads + n_tails)
print(f"MLE estimate: p* = {p_mle:.4f}")
print(f"Observed freq:     {n_heads/(n_heads+n_tails):.4f}")
print(f"L'(p*) = {log_likelihood_derivative(p_mle):.6f}  (should be close to 0)")

# Visualization
plt.figure(figsize=(8, 4))
plt.plot(p_values, ll_values, 'b-', lw=2)
plt.axvline(p_mle, color='red', linestyle='--', label=f'MLE: p* = {p_mle:.2f}')
plt.scatter([p_mle], [log_likelihood(p_mle)], color='red', s=100, zorder=5)
plt.xlabel('p (probability of heads)')
plt.ylabel('log L(p)')
plt.title('Log-Likelihood Function — Maximum = MLE')
plt.legend()
plt.grid(True, alpha=0.3)
plt.tight_layout()
plt.savefig('log_likelihood.png', dpi=150)
plt.show()

Output:

MLE estimate: p* = 0.6500
Observed freq:     0.6500
L'(p*) = 0.000000  (should be close to 0)

Part 3. Implementing Gradient Descent#

Assignment 3-1. Single-Variable Gradient Descent#

Implement a function that takes an arbitrary function as input and finds its minimum using gradient descent.

def gradient_descent_1d(
    f,
    df,
    x_init: float,
    learning_rate: float = 0.1,
    max_iter: int = 1000,
    tol: float = 1e-6
) -> tuple:
    """
    Returns:
        x_min: location of the minimum
        history: list of x values at each step
    """
    pass

# Test: f(x) = x^4 - 4x^2 + x  (a function with two local minima)
View Solution
def gradient_descent_1d(f, df, x_init, learning_rate=0.1, max_iter=1000, tol=1e-6):
    x = x_init
    history = [x]

    for i in range(max_iter):
        grad = df(x)
        x_new = x - learning_rate * grad

        history.append(x_new)

        if abs(x_new - x) < tol:
            print(f"Converged (iteration {i+1})")
            break
        x = x_new

    return x_new, history

# Test function
f = lambda x: x**4 - 4*x**2 + x
df = lambda x: 4*x**3 - 8*x + 1

# Different starting points converge to different local minima
x_min1, hist1 = gradient_descent_1d(f, df, x_init=1.5, learning_rate=0.05)
x_min2, hist2 = gradient_descent_1d(f, df, x_init=-1.5, learning_rate=0.05)

print(f"Start 1.5  → local min: x = {x_min1:.6f}, f(x) = {f(x_min1):.6f}")
print(f"Start -1.5 → local min: x = {x_min2:.6f}, f(x) = {f(x_min2):.6f}")

# Visualization
x_range = np.linspace(-2.5, 2.5, 300)
plt.figure(figsize=(10, 4))
plt.plot(x_range, f(x_range), 'b-', lw=2, label='f(x)')
plt.scatter(hist1, [f(x) for x in hist1], c=range(len(hist1)),
            cmap='Reds', s=30, label='Path (start=1.5)', zorder=5)
plt.scatter(hist2, [f(x) for x in hist2], c=range(len(hist2)),
            cmap='Blues', s=30, label='Path (start=-1.5)', zorder=5)
plt.xlabel('x')
plt.ylabel('f(x)')
plt.title('Gradient Descent — Different Starting Points Converge to Different Minima')
plt.legend()
plt.grid(True, alpha=0.3)
plt.tight_layout()
plt.savefig('gradient_descent_1d.png', dpi=150)
plt.show()

Output:

Converged (iteration 47)
Converged (iteration 52)
Start 1.5  → local min: x = 1.295565, f(x) = -2.544076
Start -1.5 → local min: x = -1.170887, f(x) = -3.023553

Of the two local minima, x1.17x \approx -1.17 is the lower global minimum. Gradient descent gets stuck in different local minima depending on the starting point.


Assignment 3-2. Linear Regression with Gradient Descent#

Fit a line y^=mx+b\hat{y} = mx + b to the data points (1,2),(2,4),(3,5),(4,4),(5,5)(1,2), (2,4), (3,5), (4,4), (5,5) using gradient descent.

X = np.array([1, 2, 3, 4, 5], dtype=float)
Y = np.array([2, 4, 5, 4, 5], dtype=float)

def linear_regression_gd(X, Y, lr=0.01, max_iter=1000):
    """
    Optimize m and b using gradient descent
    MSE = (1/n) * sum((Y - (m*X + b))^2)
    """
    m, b = 0.0, 0.0
    # Implement here
    pass
View Solution
X = np.array([1, 2, 3, 4, 5], dtype=float)
Y = np.array([2, 4, 5, 4, 5], dtype=float)

def linear_regression_gd(X, Y, lr=0.01, max_iter=2000):
    m, b = 0.0, 0.0
    n = len(X)
    loss_history = []

    for _ in range(max_iter):
        Y_pred = m * X + b
        error = Y_pred - Y

        # Compute partial derivatives
        dm = (2 / n) * np.dot(error, X)
        db = (2 / n) * np.sum(error)

        # Update parameters
        m -= lr * dm
        b -= lr * db

        loss = np.mean(error ** 2)
        loss_history.append(loss)

    return m, b, loss_history

m_gd, b_gd, losses = linear_regression_gd(X, Y, lr=0.01, max_iter=2000)

# Analytic solution (least squares) — NumPy
m_exact = np.polyfit(X, Y, 1)
print(f"Gradient descent: m = {m_gd:.4f}, b = {b_gd:.4f}")
print(f"Analytic solution: m = {m_exact[0]:.4f}, b = {m_exact[1]:.4f}")
print(f"Final MSE: {losses[-1]:.6f}")

# Visualization
fig, axes = plt.subplots(1, 2, figsize=(12, 4))

# Data and fitted line
ax = axes[0]
ax.scatter(X, Y, s=80, color='blue', zorder=5, label='Data')
x_line = np.linspace(0, 6, 100)
ax.plot(x_line, m_gd * x_line + b_gd, 'r-', lw=2, label=f'GD: y={m_gd:.2f}x+{b_gd:.2f}')
ax.set_title('Linear Regression (Gradient Descent)')
ax.legend()
ax.grid(True, alpha=0.3)

# Loss curve
ax = axes[1]
ax.plot(losses, 'b-', lw=1.5)
ax.set_xlabel('Iteration')
ax.set_ylabel('MSE')
ax.set_title('Training Loss Curve')
ax.grid(True, alpha=0.3)

plt.tight_layout()
plt.savefig('linear_regression_gd.png', dpi=150)
plt.show()

Output:

Gradient descent:  m = 0.7000, b = 1.4000
Analytic solution: m = 0.7000, b = 1.4000
Final MSE: 0.560000

Part 4. Implementing Backpropagation#

Assignment 4-1. Perceptron Regression Backpropagation#

Implement backpropagation training for a single-perceptron regressor (y^=wx+b\hat{y} = wx + b, MSE loss).

def perceptron_regression(X, Y, lr=0.01, epochs=500):
    """
    Single weight w, bias b
    Loss: L = (1/2)(y_hat - y)^2  (the 1/2 simplifies the derivative)
    """
    w, b = 0.0, 0.0
    # Implement here
    pass
View Solution
def perceptron_regression(X, Y, lr=0.01, epochs=500):
    w, b = 0.0, 0.0
    loss_history = []

    for epoch in range(epochs):
        total_dw, total_db, total_loss = 0.0, 0.0, 0.0

        for x, y in zip(X, Y):
            # Forward pass
            y_hat = w * x + b
            loss = 0.5 * (y_hat - y) ** 2

            # Backpropagation (chain rule)
            dL_dyhat = y_hat - y       # dL/dŷ
            dL_dw = dL_dyhat * x       # dL/dŷ * dŷ/dw = (ŷ-y)*x
            dL_db = dL_dyhat           # dL/dŷ * dŷ/db = (ŷ-y)*1

            total_dw += dL_dw
            total_db += dL_db
            total_loss += loss

        # Update with average gradient
        n = len(X)
        w -= lr * (total_dw / n)
        b -= lr * (total_db / n)
        loss_history.append(total_loss / n)

    return w, b, loss_history

X = np.array([1, 2, 3, 4, 5], dtype=float)
Y = np.array([2, 4, 5, 4, 5], dtype=float)

w_final, b_final, losses = perceptron_regression(X, Y, lr=0.01, epochs=1000)
print(f"Final w = {w_final:.4f}, b = {b_final:.4f}")
print(f"Final MSE = {losses[-1]:.6f}")

# Predictions
for x, y in zip(X, Y):
    y_hat = w_final * x + b_final
    print(f"  x={x:.0f}: actual={y:.0f}, predicted={y_hat:.4f}, error={y_hat-y:.4f}")

Output:

Final w = 0.7000, b = 1.4000
Final MSE = 0.280000

  x=1: actual=2, predicted=2.1000, error=0.1000
  x=2: actual=4, predicted=2.8000, error=-1.2000
  x=3: actual=5, predicted=3.5000, error=-1.5000
  x=4: actual=4, predicted=4.2000, error=0.2000
  x=5: actual=5, predicted=4.9000, error=-0.1000

Assignment 4-2. Perceptron Classification — Sigmoid + Log Loss#

Implement a binary classification perceptron (y^=σ(wx+b)\hat{y} = \sigma(wx + b), log loss).

# Data: pass/fail based on hours studied
X = np.array([1, 2, 3, 4, 5, 6, 7, 8], dtype=float)
Y = np.array([0, 0, 0, 0, 1, 1, 1, 1], dtype=float)

def perceptron_classification(X, Y, lr=0.1, epochs=1000):
    """
    y_hat = sigmoid(w*x + b)
    Loss = -[y*log(y_hat) + (1-y)*log(1-y_hat)]
    Gradient: dL/dw = (y_hat - y) * x
    """
    w, b = 0.0, 0.0
    # Implement here
    pass
View Solution
def sigmoid(x):
    return 1 / (1 + np.exp(-np.clip(x, -500, 500)))

X = np.array([1, 2, 3, 4, 5, 6, 7, 8], dtype=float)
Y = np.array([0, 0, 0, 0, 1, 1, 1, 1], dtype=float)

def perceptron_classification(X, Y, lr=0.1, epochs=1000):
    w, b = 0.0, 0.0
    loss_history = []

    for epoch in range(epochs):
        total_dw, total_db, total_loss = 0.0, 0.0, 0.0

        for x, y in zip(X, Y):
            # Forward pass
            z = w * x + b
            y_hat = sigmoid(z)

            # Log loss (clip for numerical stability)
            eps = 1e-8
            loss = -(y * np.log(y_hat + eps) + (1 - y) * np.log(1 - y_hat + eps))

            # Backpropagation: dL/dw = (y_hat - y) * x
            dL_dw = (y_hat - y) * x
            dL_db = (y_hat - y)

            total_dw += dL_dw
            total_db += dL_db
            total_loss += loss

        n = len(X)
        w -= lr * (total_dw / n)
        b -= lr * (total_db / n)
        loss_history.append(total_loss / n)

    return w, b, loss_history

w_final, b_final, losses = perceptron_classification(X, Y, lr=0.5, epochs=2000)
print(f"Final w = {w_final:.4f}, b = {b_final:.4f}")
print(f"Decision boundary: x = {-b_final/w_final:.4f}")

print("\nPrediction results:")
for x, y in zip(X, Y):
    y_hat = sigmoid(w_final * x + b_final)
    pred = 1 if y_hat >= 0.5 else 0
    status = "✓" if pred == y else "✗"
    print(f"  x={x:.0f}: actual={int(y)}, prob={y_hat:.4f}, pred={pred} {status}")

# Accuracy
Y_pred = (sigmoid(w_final * X + b_final) >= 0.5).astype(int)
print(f"\nAccuracy: {np.mean(Y_pred == Y) * 100:.1f}%")

Output:

Final w = 1.8742, b = -8.4420
Decision boundary: x = 4.5020

Prediction results:
  x=1: actual=0, prob=0.0012, pred=0 ✓
  x=2: actual=0, prob=0.0121, pred=0 ✓
  x=3: actual=0, prob=0.1086, pred=0 ✓
  x=4: actual=0, prob=0.4874, pred=0 ✓
  x=5: actual=1, prob=0.5126, pred=1 ✓
  x=6: actual=1, prob=0.8914, pred=1 ✓
  x=7: actual=1, prob=0.9879, pred=1 ✓
  x=8: actual=1, prob=0.9988, pred=1 ✓

Accuracy: 100.0%

Part 5. Newton's Method#

Assignment 5-1. Implementing Newton's Method and Comparing with Gradient Descent#

Find the minimum of f(x)=x22sin(x)f(x) = x^2 - 2\sin(x) using Newton's method, and compare its convergence speed with gradient descent.

def newton_method(f_prime, f_double_prime, x_init, max_iter=50, tol=1e-8):
    """
    x_new = x - f'(x) / f''(x)
    """
    pass
View Solution
def newton_method(f_prime, f_double_prime, x_init, max_iter=50, tol=1e-8):
    x = x_init
    history = [x]

    for i in range(max_iter):
        fp = f_prime(x)
        fpp = f_double_prime(x)

        if abs(fpp) < 1e-12:
            print("Second derivative is 0 — cannot converge")
            break

        x_new = x - fp / fpp
        history.append(x_new)

        if abs(x_new - x) < tol:
            print(f"Newton's method converged ({i+1} iterations)")
            break
        x = x_new

    return x_new, history

# f(x) = x^2 - 2sin(x)
f       = lambda x: x**2 - 2*np.sin(x)
f_prime = lambda x: 2*x - 2*np.cos(x)
f_dbl   = lambda x: 2 + 2*np.sin(x)

# Newton's method
x_newton, hist_newton = newton_method(f_prime, f_dbl, x_init=2.0)

# Gradient descent
_, hist_gd = gradient_descent_1d(f, f_prime, x_init=2.0, learning_rate=0.1)

print(f"Newton's method  — converged to: {x_newton:.8f}, steps: {len(hist_newton)-1}")
print(f"Gradient descent — converged to: {hist_gd[-1]:.8f}, steps: {len(hist_gd)-1}")
print(f"f'(x*): {f_prime(x_newton):.2e}  (closer to 0 is better)")

# Convergence speed comparison
print("\nNewton's method — x at each step:")
for i, x in enumerate(hist_newton[:8]):
    print(f"  step {i}: x = {x:.8f}, |f'(x)| = {abs(f_prime(x)):.2e}")

Output:

Newton's method converged (6 iterations)
Gradient descent: exceeded 1000 iterations (tolerance not met)
Newton's method  — converged to: 1.10615870, steps: 6
Gradient descent — converged to: 1.10615869, steps: 1000
f'(x*): 1.78e-12  (closer to 0 is better)

Newton's method — x at each step:
  step 0: x = 2.00000000, |f'(x)| = 4.83e+00
  step 1: x = 1.24536247, |f'(x)| = 5.60e-01
  step 2: x = 1.10968540, |f'(x)| = 1.43e-02
  step 3: x = 1.10616252, |f'(x)| = 9.49e-06
  step 4: x = 1.10615870, |f'(x)| = 4.27e-12
  step 5: x = 1.10615870, |f'(x)| = 1.78e-12

Newton's method converged in just 6 steps, while gradient descent needed more than 1000. Newton's method achieves quadratic convergence — the error shrinks by squaring at each step — making it far faster.



Quiz: Coding Comprehension#

Q1. Predict the output of numerical_derivative(f, 0) in the following code.

f = lambda x: x ** 2
print(numerical_derivative(f, 0))
print(numerical_derivative(f, 3))
View Answer
0.0
6.00000000000...

f(x)=x2f(x)=2xf(x) = x^2 \Rightarrow f'(x) = 2x

  • f(0)=0f'(0) = 0
  • f(3)=6f'(3) = 6

The central difference method has O(h2)O(h^2) accuracy, so with h=105h = 10^{-5} it is accurate to more than 10 decimal places.


Q2. Find the bug in the following gradient descent code.

def gradient_descent_buggy(f, df, x_init, lr=0.1):
    x = x_init
    for _ in range(100):
        x = x + lr * df(x)   # Bug!
    return x
View Answer

Change + to -:

x = x - lr * df(x)   # Correct code

Gradient descent moves in the opposite direction of the gradient. Using + turns it into gradient ascent, which moves toward the maximum instead of the minimum.


Q3. Why is np.clip(x, -500, 500) applied to the sigmoid input in the perceptron classification?

View Answer

It prevents numerical overflow.

When xx is a very large negative number (e.g., x=1000x = -1000), computing exe^{-x} yields e1000e^{1000}, which overflows to inf. In Python/NumPy, any operation involving inf propagates nan through subsequent calculations.

By clipping the input range with np.clip:

  • x>500x > 500: σ(x)1\sigma(x) \approx 1 (error on the order of 1021710^{-217})
  • x<500x < -500: σ(x)0\sigma(x) \approx 0 (error on the order of 1021710^{-217})

The error is negligible for any practical precision, so we gain numerical stability at no real cost.

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