2、k-means聚类算法sklearn与手动实现

本文将对k-means聚类算法原理和实现过程进行简述

算法原理

k-means算法原理较简单，基本步骤如下：

1、假定我们要对N个样本观测做聚类，要求聚为K类，首先选择K个点作为初始中心点；
2、接下来，按照距离初始中心点最小的原则，把所有观测分到各中心点所在的类中；
3、每类中有若干个观测，计算K个类中所有样本点的均值，作为第二次迭代的K个中心点；
4、然后根据这个中心重复第2、3步，直到收敛（中心点不再改变或达到指定的迭代次数），聚类过程结束。

聚类过程示意图：

算法实践

下面对一个具体场景做聚类分析：500x500px的地图上，随机生成60个城市，要求生成10个聚类中心。

Sklearn实现

下面是调取sklearn相关的函数进行实现：

import matplotlib.pyplot as plt
import numpy as np
import warnings

from sklearn.cluster import KMeans

Num_dots = 60  # 城市总数
Num_gas = 10  # 聚类中心总数
Size_map = 500  # 500x500地图


colors = ['#FF3838', '#FF9D97', '#FF701F', '#FFB21D', '#CFD231', '#48F90A', '#92CC17', '#3DDB86', '#1A9334', '#00D4BB',
          '#2C99A8', '#00C2FF', '#344593', '#6473FF', '#0018EC', '8438FF', '#520085', '#CB38FF', '#FF95C8', '#FF37C7']

warnings.filterwarnings("ignore")

# 生成随机点
def generate():
    dots = []
    for i in range(Num_dots):
        dots.append(np.random.uniform([Size_map, Size_map]))
    # dots_sorted_x = sorted(dots, key=lambda dot: dot[0])
    return dots

# 计算两点之间欧式距离
def cal_dist(x, y):
    return ((x[0] - y[0]) ** 2 + (x[1] - y[1]) ** 2) ** 0.5

# 统计数组中各种相同元素个数
def num_same(dots_labels):
    num_labels = []
    key = np.unique(dots_labels)
    for k in key:
        mask = (dots_labels == k)
        y_new = dots_labels[mask]
        v = y_new.size
        num_labels.append(v)
    return num_labels

def cal_center_dist(center, dots):
    distance = 0
    for i in range(len(dots)):
        distance += cal_dist(center, dots[i])
    return distance

# K-Means聚类
def k_means(dots):
    cluster = KMeans(n_clusters=Num_gas)
    dots_labels = cluster.fit_predict(dots)
    centers = cluster.cluster_centers_
    return dots_labels, centers


# 绘制图像
def plot_dots(dots, dots_labels, centers):
    # 绘制点
    for i in range(len(dots_labels)):
        plt.scatter(dots[i][0], dots[i][1], color=colors[dots_labels[i]])
    # 绘制聚类中心
    for i in range(len(centers)):
        plt.scatter(centers[i][0], centers[i][1], marker='x', color="#000000", s=50)
    plt.show()


if __name__ == '__main__':
    np.random.seed(250)
    dots = generate()
    dots_labels, centers = k_means(dots)
    num_labels = num_same(dots_labels)
    # 输出结果
    distance = 0
    for i in range(len(centers)):
        print("聚类中心", i+1, "坐标为", np.round(centers[i], 2))
        index = np.argwhere(dots_labels == i)
        print("属于该聚类中心的城市标号为", [int(x)+1 for x in index])
        mark = [int(x) for x in index]
        distance += cal_center_dist(centers[i], [dots[i] for i in mark])
        print("所有聚类中心和所辖城市的距离之和为", np.round(distance,2))

    # 绘图
    plot_dots(dots, dots_labels, centers)

输出总距离：所有聚类中心和所辖城市的距离之和为 2860.48.

手动实现

下面根据算法的理解，进行手动实现：

import numpy as np
from matplotlib import pyplot as plt

Num_dots = 60  # 城市总数
Num_gas = 10  # 聚类中心总数
Size_map = 500  # 500x500地图

colors = ['#FF3838', '#FF9D97', '#FF701F', '#FFB21D', '#CFD231', '#48F90A', '#92CC17', '#3DDB86', '#1A9334', '#00D4BB',
          '#2C99A8', '#00C2FF', '#344593', '#6473FF', '#0018EC', '8438FF', '#520085', '#CB38FF', '#FF95C8', '#FF37C7']

# 生成随机点
def generate():
    dots = []
    for i in range(Num_dots):
        dots.append(np.random.uniform([Size_map, Size_map]))
    # dots_sorted_x = sorted(dots, key=lambda dot: dot[0])
    return dots

# 计算两点之间欧式距离
def cal_dist(x, y):
    return ((x[0] - y[0]) ** 2 + (x[1] - y[1]) ** 2) ** 0.5

# 计算中心点距离它所负责的所有点之和
def cal_center_dist(center, dots):
    distance = 0
    for i in range(len(dots)):
        distance += cal_dist(center, dots[i])
    return distance

# 根据城市坐标搜索城市序号
def search_city(value, dots):
    for i, item in enumerate(dots):
        if (item == value).any():
            return i


class K_Means:
    # k是分组数；tolerance‘中心点误差’；max_iter是迭代次数
    def __init__(self, k=2, tolerance=0.0001, max_iter=300):
        self.k_ = k
        self.tolerance_ = tolerance
        self.max_iter_ = max_iter
        self.distance = 0

    def fit(self, data):
        self.centers_ = {}
        for i in range(self.k_):
            self.centers_[i] = data[i]
            # print(self.centers_[i])  # {0: array([256.5, 542. ]), 1: array([586.5, 261.5]), 2: array([869. , 449.5])}

        for iter in range(self.max_iter_):
            self.clf_ = {}
            for i in range(self.k_):
                self.clf_[i] = []
            for feature in data:
                distances = []
                for center in self.centers_:
                    distances.append(cal_dist(feature, self.centers_[center]))

                classification = distances.index(min(distances))
                self.clf_[classification].append(feature)

            # 记录总路程
            self.distance = np.sum(distances)

            # 记录上一阶段中心点位置
            prev_centers = dict(self.centers_)

            # 移动每一个center到所辖城市的中心位置
            for c in self.clf_:
                self.centers_[c] = np.average(self.clf_[c], axis=0)

            # 若center的移动空间在误差范围内，跳出循环得到结果
            optimized = True
            for center in self.centers_:
                org_centers = prev_centers[center]
                cur_centers = self.centers_[center]
                if np.sum((cur_centers - org_centers) / org_centers * 100.0) > self.tolerance_:
                    optimized = False
            if optimized:
                break


if __name__ == '__main__':
    np.random.seed(250)
    dots = generate()
    k_means = K_Means(Num_gas)
    k_means.fit(dots)

    # 输出结果
    for i in range(Num_gas):
        print("聚类中心", i + 1, "坐标为", np.round(k_means.centers_[i], 2))
        city_index = []
        for x in k_means.clf_[i]:
            city_index.append(search_city(x, dots))
        print("属于该聚类中心的城市标号为", city_index)

    print("所有聚类中心和所辖城市的距离之和为", np.round(k_means.distance, 2))

    # 绘制中心点
    for center in k_means.centers_:
        plt.scatter(k_means.centers_[center][0], k_means.centers_[center][1], marker='x', color="#000000", s=50)

    # 绘制城市点
    for cat in k_means.clf_:
        for point in k_means.clf_[cat]:
            plt.scatter(point[0], point[1], c=colors[cat])

    plt.show()

输出总距离：所有聚类中心和所辖城市的距离之和为 2816.76

结论

聚类的常规标准是让聚类中心和所辖城市的距离之和，在本实验中，手动实现的k-means算法的结果要优于sklearn的结果。

这主要是由于k-means算法本身并不是非常稳定，容易受到初始点、离群点的影响，因此，所求解不一定是最优解。

附录：sklearn K-means参数/属性/接口

下面是sklearn中K-means算法的常用接口参数，数据来自菜菜的机器学习sklearn