在Python中,如何通过自适应学习率的梯度下降法优化线性回归模型?请结合黄金分割法给出具体的实现方法和代码示例。
时间: 2024-11-20 15:50:43 浏览: 40
梯度下降法是一种常用的优化算法,尤其在机器学习领域,用于找到多元函数的最小值点。为了提高梯度下降法的性能,可以使用黄金分割法来自适应地调整学习率。下面是如何在Python中结合黄金分割法实现线性回归模型的梯度下降法优化的具体步骤:
参考资源链接:[Python实现梯度下降法:多维无约束极值优化与可视化](https://wenku.csdn.net/doc/645307f3ea0840391e76c6ce?spm=1055.2569.3001.10343)
首先,我们需要定义线性回归的目标函数,通常是一个凸函数。在线性回归中,目标函数通常是损失函数,如均方误差(MSE):
```python
import numpy as np
# 目标函数 - 线性回归的均方误差
def objective_function(weights, X, y):
predictions = np.dot(X, weights)
errors = predictions - y
return (1/len(y)) * np.dot(errors.T, errors)
# 目标函数的梯度
def gradient(weights, X, y):
predictions = np.dot(X, weights)
errors = predictions - y
return (2/len(y)) * np.dot(X.T, errors)
```
接下来,我们使用黄金分割法来寻找最佳的学习率。黄金分割法是一种寻找一维函数最小值的方法,它基于黄金分割比例来缩小搜索区间。我们将目标函数对学习率求导,然后使用黄金分割法找到导数为零时的学习率:
```python
# 黄金分割法寻找最佳学习率
def golden_section_search(objective, gradient, alpha_low, alpha_high, tol=1e-5):
# 定义黄金分割比例
ratio = (np.sqrt(5) - 1) / 2
# 初始化变量
alpha1 = alpha_high - ratio * (alpha_high - alpha_low)
alpha2 = alpha_low + ratio * (alpha_high - alpha_low)
f1 = gradient(alpha1)
f2 = gradient(alpha2)
# 迭代寻找最佳学习率
while (alpha_high - alpha_low) > tol:
if f1 > f2:
alpha_low = alpha1
alpha1 = alpha2
f1 = f2
alpha2 = alpha_low + ratio * (alpha_high - alpha_low)
f2 = gradient(alpha2)
else:
alpha_high = alpha2
alpha2 = alpha1
f2 = f1
alpha1 = alpha_high - ratio * (alpha_high - alpha_low)
f1 = gradient(alpha1)
return (alpha_low + alpha_high) / 2
```
现在我们可以定义梯度下降法的迭代过程,并使用黄金分割法找到的学习率来更新权重:
```python
# 梯度下降法
def gradient_descent(X, y, initial_weights, max_iterations, tolerance):
weights = initial_weights
for _ in range(max_iterations):
grad = gradient(weights, X, y)
alpha = golden_section_search(lambda alpha: objective_function(weights - alpha * grad, X, y),
lambda alpha: -gradient(weights - alpha * grad, X, y),
0, 1)
if np.linalg.norm(grad) < tolerance:
break
weights = weights - alpha * grad
return weights
# 示例数据
X = np.array([[1, 2], [3, 4], [5, 6], [7, 8]])
y = np.array([1, 2, 3, 4])
# 初始权重
initial_weights = np.zeros(X.shape[1])
# 执行梯度下降法
optimal_weights = gradient_descent(X, y, initial_weights, max_iterations=1000, tolerance=1e-5)
print(
参考资源链接:[Python实现梯度下降法:多维无约束极值优化与可视化](https://wenku.csdn.net/doc/645307f3ea0840391e76c6ce?spm=1055.2569.3001.10343)
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