http://blog.csdn.net/zouxy09/article/details/17590137
机器学习算法与Python实践之(六)二分k均值聚类
机器学习算法与Python实践这个系列主要是参考《机器学习实战》这本书。因为自己想学习Python,然后也想对一些机器学习算法加深下了解,所以就想通过Python来实现几个比较常用的机器学习算法。恰好遇见这本同样定位的书籍,所以就参考这本书的过程来学习了。
在上一个博文中,我们聊到了k-means算法。但k-means算法有个比较大的缺点就是对初始k个质心点的选取比较敏感。有人提出了一个二分k均值(bisecting k-means)算法,它的出现就是为了一定情况下解决这个问题的。也就是说它对初始的k个质心的选择不太敏感。那下面我们就来了解和实现下这个算法。
一、二分k均值(bisecting k-means)算法
二分k均值(bisecting k-means)算法的主要思想是:首先将所有点作为一个簇,然后将该簇一分为二。之后选择能最大程度降低聚类代价函数(也就是误差平方和)的簇划分为两个簇。以此进行下去,直到簇的数目等于用户给定的数目k为止。
以上隐含着一个原则是:因为聚类的误差平方和能够衡量聚类性能,该值越小表示数据点月接近于它们的质心,聚类效果就越好。所以我们就需要对误差平方和最大的簇进行再一次的划分,因为误差平方和越大,表示该簇聚类越不好,越有可能是多个簇被当成一个簇了,所以我们首先需要对这个簇进行划分。
二分k均值算法的伪代码如下:
***************************************************************
将所有数据点看成一个簇
当簇数目小于k时
对每一个簇
计算总误差
在给定的簇上面进行k-均值聚类(k=2)
计算将该簇一分为二后的总误差
选择使得误差最小的那个簇进行划分操作
***************************************************************
二、Python实现
我使用的Python是2.7.5版本的。附加的库有Numpy和Matplotlib。具体的安装和配置见前面的博文。在代码中已经有了比较详细的注释了。不知道有没有错误的地方,如果有,还望大家指正(每次的运行结果都有可能不同)。里面我写了个可视化结果的函数,但只能在二维的数据上面使用。直接贴代码:
biKmeans.py
- #################################################
- # kmeans: k-means cluster
- # Author : zouxy
- # Date : 2013-12-25
- # HomePage : http://blog.csdn.net/zouxy09
- # Email : zouxy09@qq.com
- #################################################
- from numpy import *
- import time
- import matplotlib.pyplot as plt
- # calculate Euclidean distance
- def euclDistance(vector1, vector2):
- return sqrt(sum(power(vector2 - vector1, 2)))
- # init centroids with random samples
- def initCentroids(dataSet, k):
- numSamples, dim = dataSet.shape
- centroids = zeros((k, dim))
- for i in range(k):
- index = int(random.uniform(0, numSamples))
- centroids[i, :] = dataSet[index, :]
- return centroids
- # k-means cluster
- def kmeans(dataSet, k):
- numSamples = dataSet.shape[0]
- # first column stores which cluster this sample belongs to,
- # second column stores the error between this sample and its centroid
- clusterAssment = mat(zeros((numSamples, 2)))
- clusterChanged = True
- ## step 1: init centroids
- centroids = initCentroids(dataSet, k)
- while clusterChanged:
- clusterChanged = False
- ## for each sample
- for i in xrange(numSamples):
- minDist = 100000.0
- minIndex = 0
- ## for each centroid
- ## step 2: find the centroid who is closest
- for j in range(k):
- distance = euclDistance(centroids[j, :], dataSet[i, :])
- if distance < minDist:
- minDist = distance
- minIndex = j
- ## step 3: update its cluster
- if clusterAssment[i, 0] != minIndex:
- clusterChanged = True
- clusterAssment[i, :] = minIndex, minDist**2
- ## step 4: update centroids
- for j in range(k):
- pointsInCluster = dataSet[nonzero(clusterAssment[:, 0].A == j)[0]]
- centroids[j, :] = mean(pointsInCluster, axis = 0)
- print 'Congratulations, cluster using k-means complete!'
- return centroids, clusterAssment
- # bisecting k-means cluster
- def biKmeans(dataSet, k):
- numSamples = dataSet.shape[0]
- # first column stores which cluster this sample belongs to,
- # second column stores the error between this sample and its centroid
- clusterAssment = mat(zeros((numSamples, 2)))
- # step 1: the init cluster is the whole data set
- centroid = mean(dataSet, axis = 0).tolist()[0]
- centList = [centroid]
- for i in xrange(numSamples):
- clusterAssment[i, 1] = euclDistance(mat(centroid), dataSet[i, :])**2
- while len(centList) < k:
- # min sum of square error
- minSSE = 100000.0
- numCurrCluster = len(centList)
- # for each cluster
- for i in range(numCurrCluster):
- # step 2: get samples in cluster i
- pointsInCurrCluster = dataSet[nonzero(clusterAssment[:, 0].A == i)[0], :]
- # step 3: cluster it to 2 sub-clusters using k-means
- centroids, splitClusterAssment = kmeans(pointsInCurrCluster, 2)
- # step 4: calculate the sum of square error after split this cluster
- splitSSE = sum(splitClusterAssment[:, 1])
- notSplitSSE = sum(clusterAssment[nonzero(clusterAssment[:, 0].A != i)[0], 1])
- currSplitSSE = splitSSE + notSplitSSE
- # step 5: find the best split cluster which has the min sum of square error
- if currSplitSSE < minSSE:
- minSSE = currSplitSSE
- bestCentroidToSplit = i
- bestNewCentroids = centroids.copy()
- bestClusterAssment = splitClusterAssment.copy()
- # step 6: modify the cluster index for adding new cluster
- bestClusterAssment[nonzero(bestClusterAssment[:, 0].A == 1)[0], 0] = numCurrCluster
- bestClusterAssment[nonzero(bestClusterAssment[:, 0].A == 0)[0], 0] = bestCentroidToSplit
- # step 7: update and append the centroids of the new 2 sub-cluster
- centList[bestCentroidToSplit] = bestNewCentroids[0, :]
- centList.append(bestNewCentroids[1, :])
- # step 8: update the index and error of the samples whose cluster have been changed
- clusterAssment[nonzero(clusterAssment[:, 0].A == bestCentroidToSplit), :] = bestClusterAssment
- print 'Congratulations, cluster using bi-kmeans complete!'
- return mat(centList), clusterAssment
- # show your cluster only available with 2-D data
- def showCluster(dataSet, k, centroids, clusterAssment):
- numSamples, dim = dataSet.shape
- if dim != 2:
- print "Sorry! I can not draw because the dimension of your data is not 2!"
- return 1
- mark = ['or', 'ob', 'og', 'ok', '^r', '+r', 'sr', 'dr', '<r', 'pr']
- if k > len(mark):
- print "Sorry! Your k is too large! please contact Zouxy"
- return 1
- # draw all samples
- for i in xrange(numSamples):
- markIndex = int(clusterAssment[i, 0])
- plt.plot(dataSet[i, 0], dataSet[i, 1], mark[markIndex])
- mark = ['Dr', 'Db', 'Dg', 'Dk', '^b', '+b', 'sb', 'db', '<b', 'pb']
- # draw the centroids
- for i in range(k):
- plt.plot(centroids[i, 0], centroids[i, 1], mark[i], markersize = 12)
- plt.show()
三、测试结果
测试数据是二维的,共80个样本。有4个类。具体见上一个博文。
测试代码:
test_biKmeans.py
- #################################################
- # kmeans: k-means cluster
- # Author : zouxy
- # Date : 2013-12-25
- # HomePage : http://blog.csdn.net/zouxy09
- # Email : zouxy09@qq.com
- #################################################
- from numpy import *
- import time
- import matplotlib.pyplot as plt
- ## step 1: load data
- print "step 1: load data..."
- dataSet = []
- fileIn = open('E:/Python/Machine Learning in Action/testSet.txt')
- for line in fileIn.readlines():
- lineArr = line.strip().split('\t')
- dataSet.append([float(lineArr[0]), float(lineArr[1])])
- ## step 2: clustering...
- print "step 2: clustering..."
- dataSet = mat(dataSet)
- k = 4
- centroids, clusterAssment = biKmeans(dataSet, k)
- ## step 3: show the result
- print "step 3: show the result..."
- showCluster(dataSet, k, centroids, clusterAssment)
这里贴出两次的运行结果:
不同的类用不同的颜色来表示,其中的大菱形是对应类的均值质心点。
事实上,这个算法在初始质心选择不同时运行效果也会不同。我没有看初始的论文,不确定它究竟是不是一定会收敛到全局最小值。《机器学习实战》这本书说是可以的,但因为每次运行的结果不同,所以我有点怀疑,自己去找资料也没找到相关的说明。对这个算法有了解的还望您不吝指点下,谢谢。