该案例来自吴恩达深度学习系列课程一《神经网络和深度学习》第三周编程作业，作业内容是设计带有一个隐藏层的平面数据分类。作业提供的资料包括测试实例（testCases.py）和任务功能包（planar_utils.py），下载请移步参考链接一。

1 介绍

1.1 案例应用核心公式

1.2 涉及库和主要接口

numpy：用Python进行科学计算的基本软件包。

numpy.round：均匀地四舍五入到给定的小数位数。

numpy.random.seed：设置随机数生成器的种子，可以使随机数的生成具有可重复性。

sklearn：为数据挖掘和数据分析提供的机器学习库。

sklearn.linear_model.LogisticRegressionCV：逻辑回归CV（又名logit，MaxEnt）分类器。

matplotlib：用于在Python中绘制图表的库。

matplotlib.pyplot.scatter：具有不同标记大小和/或颜色的y与x的散点图。

2 编码

2.1 查看数据集相关参数

check_data.py

import matplotlib.pyplot as plt
from planar_utils import load_planar_dataset

X, Y = load_planar_dataset()  # 加载数据

# 查看数据散点图
plt.scatter(X[0, :], X[1, :], c=Y, s=40, cmap=plt.cm.Spectral)
plt.show()

# 计算和打印相关参数
print("X的维度为：" + str(X.shape))
print("Y的维度为：" + str(Y.shape))
print("数据集里的数据个数为：" + str(Y.shape[1]))

2.2 简单逻辑回归应用效果

logic_nn.py

import numpy as np
import matplotlib.pyplot as plt
import sklearn.linear_model
from planar_utils import plot_decision_boundary, load_planar_dataset

X, Y = load_planar_dataset()

# 搭建模型并训练
clf = sklearn.linear_model.LogisticRegressionCV()
clf.fit(X.T, Y.T)

# 应用模型预测结果
predictions = clf.predict(X.T)
correct_predictions = ((np.dot(Y, predictions.T) + np.dot(1 - Y, 1 - predictions.T)) / float(Y.size)).reshape(1,)
print('准确率: %.2f' % (correct_predictions[0] * 100) + '%')

# 绘制颜色块边界
plot_decision_boundary(lambda x: clf.predict(x), X, Y)
plt.title("Logistic Regression")
plt.show()

经过测试，准确性只有47%，原因是数据集不是线性可分的，所以逻辑回归表现不佳。

线性可分：指在特征空间中，存在一个超平面能够将不同类别的数据点完全分开，在二维空间中超平面表现为一条直线。

2.3 搭建神经网络

double_layer_nn.py

import numpy as np
import matplotlib.pyplot as plt
from planar_utils import plot_decision_boundary, sigmoid, load_planar_dataset


# 层单元数量
def layer_sizes(X, Y):
    n_x = X.shape[0]
    n_h = 4
    n_y = Y.shape[0]
    return n_x, n_h, n_y


# 初始化模型参数
def initialize_parameters(n_x, n_h, n_y):
    W1 = np.random.rand(n_h, n_x) * 0.01
    b1 = np.random.rand(n_h, 1)
    W2 = np.random.rand(n_y, n_h) * 0.01
    b2 = np.random.rand(n_y, 1)

    parameters = {"W1": W1, "b1": b1, "W2": W2, "b2": b2}
    return parameters


# 前向传播
def forward_propagation(X, parameters):
    W1 = parameters["W1"]
    b1 = parameters["b1"]
    W2 = parameters["W2"]
    b2 = parameters["b2"]

    Z1 = np.dot(W1, X) + b1
    A1 = np.tanh(Z1)
    Z2 = np.dot(W2, A1) + b2
    A2 = sigmoid(Z2)

    cache = {"Z1": Z1, "A1": A1, "Z2": Z2, "A2": A2}
    return cache


# 计算代价函数
def compute_cost(A2, Y):
    m = Y.shape[1]
    cost = (-1 / m) * np.sum(Y * np.log(A2) + (1 - Y) * np.log(1 - A2))
    cost = float(np.squeeze(cost))
    return cost


# 反向传播
def backward_propagation(parameters, cache, X, Y):
    m = X.shape[1]
    W1 = parameters["W1"]
    W2 = parameters["W2"]
    A1 = cache["A1"]
    A2 = cache["A2"]

    dZ2 = A2 - Y
    dW2 = (1 / m) * np.dot(dZ2, A1.T)
    db2 = (1 / m) * np.sum(dZ2, axis=1, keepdims=True)
    dZ1 = np.dot(W2.T, dZ2) * (1 - np.power(A1, 2))
    dW1 = (1 / m) * np.dot(dZ1, X.T)
    db1 = (1 / m) * np.sum(dZ1, axis=1, keepdims=True)

    grads = {"dW1": dW1, "db1": db1, "dW2": dW2, "db2": db2}
    return grads


# 更新参数
def update_parameters(parameters, grads, learning_rate=1.2):
    W1 = parameters["W1"]
    b1 = parameters["b1"]
    W2 = parameters["W2"]
    b2 = parameters["b2"]

    dW1 = grads["dW1"]
    db1 = grads["db1"]
    dW2 = grads["dW2"]
    db2 = grads["db2"]

    W1 = W1 - learning_rate * dW1
    b1 = b1 - learning_rate * db1
    W2 = W2 - learning_rate * dW2
    b2 = b2 - learning_rate * db2

    parameters = {"W1": W1, "b1": b1, "W2": W2, "b2": b2}
    return parameters


# 构建神经网络
def nn_model(X, Y, n_h, num_iterations, learning_rate=0.5, print_cost=False):
    n_x = layer_sizes(X, Y)[0]
    n_y = layer_sizes(X, Y)[2]

    parameters = initialize_parameters(n_x, n_h, n_y)

    for i in range(num_iterations):
        cache = forward_propagation(X, parameters)
        cost = compute_cost(cache["A2"], Y)
        grads = backward_propagation(parameters, cache, X, Y)
        parameters = update_parameters(parameters, grads, learning_rate)
        if print_cost and (i % 1000 == 0):
            print("第 %i 次循环，成本为: %f" % (i, cost))

    return parameters


# 预测函数
def predict(parameters, X):
    cache = forward_propagation(X, parameters)
    predictions = np.round(cache["A2"])
    return predictions


# 进行深度学习
X, Y = load_planar_dataset()
n_h = 4
parameters = nn_model(X, Y, n_h, num_iterations=10000, learning_rate=0.5, print_cost=True)
predictions = predict(parameters, X)
correct_predictions = ((np.dot(Y, predictions.T) + np.dot(1 - Y, 1 - predictions.T)) / float(Y.size)).reshape(1,)
print('准确率: %.2f' % (correct_predictions[0] * 100) + '%')

# 绘制颜色块边界
plot_decision_boundary(lambda x: predict(parameters, x.T), X, Y)
plt.title("Decision Boundary for hidden layer size %i : %.2f " % (n_h, correct_predictions[0] * 100) + '%')
plt.show()

2.4 隐藏层单元数的变换

变换隐藏层的单元数量，观察对预测结果是否产生哪些影响。

3 调试

3.1 运行时警告

E:\pythonPrograming\DLHomework\course1-week3\double_layer_nn.py:53: RuntimeWarning: divide by zero encountered in log

logprobs= np.multiply(np.log(A2), Y) + np.multiply((1 - Y), np.log(1 - A2))

E:\pythonPrograming\DLHomework\course1-week3\planar_utils.py:25: RuntimeWarning: overflow encountered in exp

s = 1/(1+np.exp(-x))

3.2 弃用警告

E:\pythonPrograming\DLHomework\course1-week3\double_layer_nn.py:135: DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)

print ('准确率: %d' % float((np.dot(Y, predictions.T) + np.dot(1 - Y, 1 - predictions.T)) / float(Y.size) * 100) + '%')

3.3 数据转换警告

E:\pythonPrograming\DLHomework\course1-week3.venv\Lib\site-packages\sklearn\utils\validation.py:1339: DataConversionWarning: A column-vector y was passed when a 1d array was expected. Please change the shape of y to (n_samples, ), for example using ravel().
y = column_or_1d(y, warn=True)

clf.fit(X.T, Y.T)

此处问题出现在逻辑回归效果 logic_nn.py 中，由于提供的 Y 值是一个二维数组，与函数所需的一维数组存在数据转换问题，可以人工使用ravel() 或 flatten() 将 Y 转换为一维数组。

4 参考

【中文】【吴恩达课后编程作业】Course 1 - 神经网络和深度学习 - 第三周作业-CSDN博客

NumPy reference — NumPy v2.1 Manual

scikit-learn 1.5.2 documentation

API Reference — Matplotlib 3.9.2 documentation