python实现线性回归之岭回归

岭回归与多项式回归的最大区别就是损失函数上的区别。岭回归的代价函数如下：

为了方便计算导数，通常也会写成以下形式：

上述式子中w为长度为n的向量，不包括偏置项的系数 θ0，θ是长度为n+1的向量，包括偏置项系数θ0；m为样本数，n为特征数。

岭回归的代价函数仍然是凸函数，因此可以利用梯度等于0的方式求得全局最优解：

上述方程与一般线性回归方程相比多了一项λI，其中I表示单位矩阵，加入XTX是一个奇异矩阵（不满秩），添加这一项之后可以保证该项可逆，由于单位矩阵上的对角元素均为1，其余元素都为0，看起来像是一条山岭，因此而得名。

还可以使用随机梯度下降算法来求解：

参数更新就可以如下表示：

上述解释摘自：https://www.cnblogs.com/Belter/p/8536939.html 

接下来是实现代码，代码来源： https://github.com/eriklindernoren/ML-From-Scratch

首先还是定义一个基类，各种线性回归都需要继承该基类：

class Regression(object):
    """ Base regression model. Models the relationship between a scalar dependent variable y and the independent 
    variables X. 
    Parameters:
    -----------
    n_iterations: float
        The number of training iterations the algorithm will tune the weights for.
    learning_rate: float
        The step length that will be used when updating the weights.
    """
    def __init__(self, n_iterations, learning_rate):
        self.n_iterations = n_iterations
        self.learning_rate = learning_rate

    def initialize_weights(self, n_features):
        """ Initialize weights randomly [-1/N, 1/N] """
        limit = 1 / math.sqrt(n_features)
        self.w = np.random.uniform(-limit, limit, (n_features, ))

    def fit(self, X, y):
        # Insert constant ones for bias weights
        X = np.insert(X, 0, 1, axis=1)
        self.training_errors = []
        self.initialize_weights(n_features=X.shape[1])

        # Do gradient descent for n_iterations
        for i in range(self.n_iterations):
            y_pred = X.dot(self.w)
            # Calculate l2 loss
            mse = np.mean(0.5 * (y - y_pred)**2 + self.regularization(self.w))
            self.training_errors.append(mse)
            # Gradient of l2 loss w.r.t w
            grad_w = -(y - y_pred).dot(X) + self.regularization.grad(self.w)
            # Update the weights
            self.w -= self.learning_rate * grad_w

    def predict(self, X):
        # Insert constant ones for bias weights
        X = np.insert(X, 0, 1, axis=1)
        y_pred = X.dot(self.w)
        return y_pred

岭回归的核心就是l2正则化项：

class l2_regularization():
    """ Regularization for Ridge Regression """
    def __init__(self, alpha):
        self.alpha = alpha
    
    def __call__(self, w):
        return self.alpha * 0.5 *  w.T.dot(w)

    def grad(self, w):
        return self.alpha * w

然后是岭回归的核心代码：

class PolynomialRidgeRegression(Regression):
    """Similar to regular ridge regression except that the data is transformed to allow
    for polynomial regression.
    Parameters:
    -----------
    degree: int
        The degree of the polynomial that the independent variable X will be transformed to.
    reg_factor: float
        The factor that will determine the amount of regularization and feature
        shrinkage. 
    n_iterations: float
        The number of training iterations the algorithm will tune the weights for.
    learning_rate: float
        The step length that will be used when updating the weights.
    """
    def __init__(self, degree, reg_factor, n_iterations=3000, learning_rate=0.01, gradient_descent=True):
        self.degree = degree
        self.regularization = l2_regularization(alpha=reg_factor)
        super(PolynomialRidgeRegression, self).__init__(n_iterations, 
                                                        learning_rate)

    def fit(self, X, y):
        X = normalize(polynomial_features(X, degree=self.degree))
        super(PolynomialRidgeRegression, self).fit(X, y)

    def predict(self, X):
        X = normalize(polynomial_features(X, degree=self.degree))
        return super(PolynomialRidgeRegression, self).predict(X)

其中的一些具体函数的用法可参考：https://www.cnblogs.com/xiximayou/p/12802868.html

最后是主函数：

from __future__ import print_function
import matplotlib.pyplot as plt
import sys
sys.path.append("/content/drive/My Drive/learn/ML-From-Scratch/")
import numpy as np
import pandas as pd
# Import helper functions
from mlfromscratch.supervised_learning import PolynomialRidgeRegression
from mlfromscratch.utils import k_fold_cross_validation_sets, normalize, Plot
from mlfromscratch.utils import train_test_split, polynomial_features, mean_squared_error


def main():

    # Load temperature data
    data = pd.read_csv('mlfromscratch/data/TempLinkoping2016.txt', sep="	")

    time = np.atleast_2d(data["time"].values).T
    temp = data["temp"].values

    X = time # fraction of the year [0, 1]
    y = temp

    X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.4)

    poly_degree = 15

    # Finding regularization constant using cross validation
    lowest_error = float("inf")
    best_reg_factor = None
    print ("Finding regularization constant using cross validation:")
    k = 10
    for reg_factor in np.arange(0, 0.1, 0.01):
        cross_validation_sets = k_fold_cross_validation_sets(
            X_train, y_train, k=k)
        mse = 0
        for _X_train, _X_test, _y_train, _y_test in cross_validation_sets:
            model = PolynomialRidgeRegression(degree=poly_degree, 
                                            reg_factor=reg_factor,
                                            learning_rate=0.001,
                                            n_iterations=10000)
            model.fit(_X_train, _y_train)
            y_pred = model.predict(_X_test)
            _mse = mean_squared_error(_y_test, y_pred)
            mse += _mse
        mse /= k

        # Print the mean squared error
        print ("	Mean Squared Error: %s (regularization: %s)" % (mse, reg_factor))

        # Save reg. constant that gave lowest error
        if mse < lowest_error:
            best_reg_factor = reg_factor
            lowest_error = mse

    # Make final prediction
    model = PolynomialRidgeRegression(degree=poly_degree, 
                                    reg_factor=reg_factor,
                                    learning_rate=0.001,
                                    n_iterations=10000)
    model.fit(X_train, y_train)

    y_pred = model.predict(X_test)
    mse = mean_squared_error(y_test, y_pred)
    print ("Mean squared error: %s (given by reg. factor: %s)" % (mse, reg_factor))

    y_pred_line = model.predict(X)

    # Color map
    cmap = plt.get_cmap('viridis')

    # Plot the results
    m1 = plt.scatter(366 * X_train, y_train, color=cmap(0.9), s=10)
    m2 = plt.scatter(366 * X_test, y_test, color=cmap(0.5), s=10)
    plt.plot(366 * X, y_pred_line, color='black', linewidth=2, label="Prediction")
    plt.suptitle("Polynomial Ridge Regression")
    plt.title("MSE: %.2f" % mse, fontsize=10)
    plt.xlabel('Day')
    plt.ylabel('Temperature in Celcius')
    plt.legend((m1, m2), ("Training data", "Test data"), loc='lower right')
    plt.savefig("test1.png")
    plt.show()

if __name__ == "__main__":
    main()

结果：

Finding regularization constant using cross validation:
    Mean Squared Error: 13.812293192023807 (regularization: 0.0)
    Mean Squared Error: 13.743127176668661 (regularization: 0.01)
    Mean Squared Error: 13.897319799448272 (regularization: 0.02)
    Mean Squared Error: 13.755294291853932 (regularization: 0.03)
    Mean Squared Error: 13.864603077117456 (regularization: 0.04)
    Mean Squared Error: 14.13017742349847 (regularization: 0.05)
    Mean Squared Error: 14.031692893193021 (regularization: 0.06)
    Mean Squared Error: 14.12160512870597 (regularization: 0.07)
    Mean Squared Error: 14.462275871359097 (regularization: 0.08)
    Mean Squared Error: 14.155492625301093 (regularization: 0.09)
Mean squared error: 9.743831581107068 (given by reg. factor: 0.09)