聚类算法——K-means（下）

K-means的源码实现

　　一般情况下，我们通过C++/Matlab/Python等语言进行实现K-means算法，结合近期我刚刚学的C++，先从C++实现谈起，C++里面我们一般采用的是OpenCV库中写好的K-means函数，即cvKmeans2，首先来看函数原型：

　　从OpenCV manual看到的是：

int cvKMeans2(const CvArr* samples, int nclusters,

　　　　　　　 CvArr* labels, CvTermCriteria termcrit,

　　　　　　　 int attempts=1, CvRNG* rng=0,int flags=0,

　　　　　　　 CvArr* centers=0,double* compactness=0);

由于除去已经确定的参数，我们自己需要输入的为：

void cvKMeans2(

　　const CvArr* samples, //输入样本的浮点矩阵，每个样本一行。

　　int cluster_count, //所给定的聚类数目

　　* labels, //输出整数向量：每个样本对应的类别标识

　　CvTermCriteria termcrit //指定聚类的最大迭代次数和／或精度（两次迭代引起的聚类中心的移动距离）

);

其使用例程为：

1 #ifdef _CH_
2 #pragma package <opencv>
3 #endif
4
5 #define CV_NO_BACKWARD_COMPATIBILITY
6
7 #ifndef _EiC
8 #include "cv.h"
9 #include "highgui.h"
10 #include <stdio.h>
11 #endif
12
13 int main( int argc, char** argv )
14 {
15     #define MAX_CLUSTERS 5    //设置类别的颜色，个数（《=5）
16     CvScalar color_tab[MAX_CLUSTERS];
17     IplImage* img = cvCreateImage( cvSize( 500, 500 ), 8, 3 );
18     CvRNG rng = cvRNG(-1);
19     CvPoint ipt;
20
21     color_tab[0] = CV_RGB(255,0,0);
22     color_tab[1] = CV_RGB(0,255,0);
23     color_tab[2] = CV_RGB(100,100,255);
24     color_tab[3] = CV_RGB(255,0,255);
25     color_tab[4] = CV_RGB(255,255,0);
26
27     cvNamedWindow( "clusters", 1 );
28
29     for(;;)
30     {
31         char key;
32         int k, cluster_count = cvRandInt(&rng)%MAX_CLUSTERS + 1;
33         int i, sample_count = cvRandInt(&rng)%1000 + 1;
34         CvMat* points = cvCreateMat( sample_count, 1, CV_32FC2 );
35         CvMat* clusters = cvCreateMat( sample_count, 1, CV_32SC1 );
36         cluster_count = MIN(cluster_count, sample_count);
37
38         /** generate random sample from multigaussian distribution */
39         for( k = 0; k < cluster_count; k++ )
40         {
41             CvPoint center;
42             CvMat point_chunk;
43             center.x = cvRandInt(&rng)%img->width;
44             center.y = cvRandInt(&rng)%img->height;
45             cvGetRows( points, &point_chunk, k*sample_count/cluster_count,
46                        k == cluster_count - 1 ? sample_count :
47                        (k+1)*sample_count/cluster_count, 1 );
48
49             cvRandArr( &rng, &point_chunk, CV_RAND_NORMAL,
50                        cvScalar(center.x,center.y,0,0),
51                        cvScalar(img->width*0.1,img->height*0.1,0,0));
52         }
53
54         /** shuffle samples */
55         for( i = 0; i < sample_count/2; i++ )
56         {
57             CvPoint2D32f* pt1 = (CvPoint2D32f*)points->data.fl + cvRandInt(&rng)%sample_count;
58             CvPoint2D32f* pt2 = (CvPoint2D32f*)points->data.fl + cvRandInt(&rng)%sample_count;
59             CvPoint2D32f temp;
60             CV_SWAP( *pt1, *pt2, temp );
61         }
62
63         printf( "iterations=%d\n", cvKMeans2( points, cluster_count, clusters,
64                 cvTermCriteria( CV_TERMCRIT_EPS+CV_TERMCRIT_ITER, 10, 1.0 ),
65                 5, 0, 0, 0, 0 ));
66
67         cvZero( img );
68
69         for( i = 0; i < sample_count; i++ )
70         {
71             int cluster_idx = clusters->data.i[i];
72             ipt.x = (int)points->data.fl[i*2];
73             ipt.y = (int)points->data.fl[i*2+1];
74             cvCircle( img, ipt, 2, color_tab[cluster_idx], CV_FILLED, CV_AA, 0 );
75         }
76
77         cvReleaseMat( &points );
78         cvReleaseMat( &clusters );
79
80         cvShowImage( "clusters", img );
81
82         key = (char) cvWaitKey(0);
83         if( key == 27 || key == 'q' || key == 'Q' ) // 'ESC'
84             break;
85     }
86
87     cvDestroyWindow( "clusters" );
88     return 0;
89 }
90
91 #ifdef _EiC
92 main(1,"kmeans.c");
93 #endif

　　至于cvKmeans2函数的具体实现细节，可参见OpenCV源码

　　下面是Python的实现代码（网上所找）：

1  #!/usr/bin/python
2
3 from __future__ import with_statement
4 import cPickle as pickle
5 from matplotlib import pyplot
6 from numpy import zeros, array, tile
7 from scipy.linalg import norm
8 import numpy.matlib as ml
9 import random
10
11 def kmeans(X, k, observer=None, threshold=1e-15, maxiter=300):
12     N = len(X)
13     labels = zeros(N, dtype=int)
14     centers = array(random.sample(X, k))
15     iter = 0
16
17     def calc_J():
18         sum = 0
19         for i in xrange(N):
20             sum += norm(X[i]-centers[labels[i]])
21         return sum
22
23     def distmat(X, Y):
24         n = len(X)
25         m = len(Y)
26         xx = ml.sum(X*X, axis=1)
27         yy = ml.sum(Y*Y, axis=1)
28         xy = ml.dot(X, Y.T)
29
30         return tile(xx, (m, 1)).T+tile(yy, (n, 1)) - 2*xy
31
32     Jprev = calc_J()
33     while True:
34         # notify the observer
35         if observer is not None:
36             observer(iter, labels, centers)
37
38         # calculate distance from x to each center
39         # distance_matrix is only available in scipy newer than 0.7
40         # dist = distance_matrix(X, centers)
41         dist = distmat(X, centers)
42         # assign x to nearst center
43         labels = dist.argmin(axis=1)
44         # re-calculate each center
45         for j in range(k):
46             idx_j = (labels == j).nonzero()
47             centers[j] = X[idx_j].mean(axis=0)
48
49         J = calc_J()
50         iter += 1
51
52         if Jprev-J < threshold:
53             break
54         Jprev = J
55         if iter >= maxiter:
56             break
57
58     # final notification
59     if observer is not None:
60         observer(iter, labels, centers)
61
62 if __name__ == '__main__':
63     # load previously generated points
64     with open('cluster.pkl') as inf:
65         samples = pickle.load(inf)
66     N = 0
67     for smp in samples:
68         N += len(smp[0])
69     X = zeros((N, 2))
70     idxfrm = 0
71     for i in range(len(samples)):
72         idxto = idxfrm + len(samples[i][0])
73         X[idxfrm:idxto, 0] = samples[i][0]
74         X[idxfrm:idxto, 1] = samples[i][1]
75         idxfrm = idxto
76
77     def observer(iter, labels, centers):
78         print "iter %d." % iter
79         colors = array([[1, 0, 0], [0, 1, 0], [0, 0, 1]])
80         pyplot.plot(hold=False)  # clear previous plot
81         pyplot.hold(True)
82
83         # draw points
84         data_colors=[colors[lbl] for lbl in labels]
85         pyplot.scatter(X[:, 0], X[:, 1], c=data_colors, alpha=0.5)
86         # draw centers
87         pyplot.scatter(centers[:, 0], centers[:, 1], s=200, c=colors)
88
89         pyplot.savefig('kmeans/iter_%02d.png' % iter, format='png')
90
91     kmeans(X, 3, observer=observer)

　　matlab的kmeans实现代码可直接参照其kmeans(X,k)函数的实现源码。