KNN分类算法--python实现

一、kNN算法分析

K最近邻（k-Nearest Neighbor，KNN）分类算法可以说是最简单的机器学习算法了。它采用测量不同特征值之间的距离方法进行分类。它的思想很简单：如果一个样本在特征空间中的k个最相似（即特征空间中最邻近）的样本中的大多数属于某一个类别，则该样本也属于这个类别。

KNN算法中，所选择的邻居都是已经正确分类的对象。该方法在定类决策上只依据最邻近的一个或者几个样本的类别来决定待分样本所属的类别。由于KNN方法主要靠周围有限的邻近的样本，而不是靠判别类域的方法来确定所属类别的，因此对于类域的交叉或重叠较多的待分样本集来说，KNN方法较其他方法更为适合。

该算法在分类时有个主要的不足是，当样本不平衡时，如一个类的样本容量很大，而其他类样本容量很小时，有可能导致当输入一个新样本时，该样本的K个邻居中大容量类的样本占多数。因此可以采用权值的方法（和该样本距离小的邻居权值大）来改进。该方法的另一个不足之处是计算量较大，因为对每一个待分类的文本都要计算它到全体已知样本的距离，才能求得它的K个最近邻点。目前常用的解决方法是事先对已知样本点进行剪辑，事先去除对分类作用不大的样本。该算法比较适用于样本容量比较大的类域的自动分类，而那些样本容量较小的类域采用这种算法比较容易产生误分[参考机器学习十大算法]。

总的来说就是我们已经存在了一个带标签的数据库，然后输入没有标签的新数据后，将新数据的每个特征与样本集中数据对应的特征进行比较，然后算法提取样本集中特征最相似（最近邻）的分类标签。一般来说，只选择样本数据库中前k个最相似的数据。最后，选择k个最相似数据中出现次数最多的分类。其算法描述如下：

1）计算已知类别数据集中的点与当前点之间的距离；

2）按照距离递增次序排序；

3）选取与当前点距离最小的k个点；

4）确定前k个点所在类别的出现频率；

5）返回前k个点出现频率最高的类别作为当前点的预测分类。

代码：

#########################################
# kNN: k Nearest Neighbors

# Input:      inX: vector to compare to existing dataset (1xN)
#             dataSet: size m data set of known vectors (NxM)
#             labels: data set labels (1xM vector)
#             k: number of neighbors to use for comparison 

# Output:     the most popular class label
#########################################

from numpy import *
import operator
import os
from Canvas import Line

# classify using kNN
def kNNClassify(newInput, dataSet, labels, k):
    numSamples = dataSet.shape[0] # shape[0] stands for the num of row

    ## step 1: calculate Euclidean distance
    # tile(A, reps): Construct an array by repeating A reps times
    # the following copy numSamples rows for dataSet
    diff = tile(newInput, (numSamples, 1)) - dataSet # Subtract element-wise
    squaredDiff = diff ** 2 # squared for the subtract
    squaredDist = sum(squaredDiff, axis = 1) # sum is performed by row
    distance = squaredDist ** 0.5

    ## step 2: sort the distance
    # argsort() returns the indices that would sort an array in a ascending order
    sortedDistIndices = argsort(distance)

    classCount = {} # define a dictionary (can be append element)
    for i in xrange(k):
        ## step 3: choose the min k distance
        voteLabel = labels[sortedDistIndices[i]]

        ## step 4: count the times labels occur
        # when the key voteLabel is not in dictionary classCount, get()
        # will return 0
        classCount[voteLabel] = classCount.get(voteLabel, 0) + 1

    ## step 5: the max voted class will return
    maxCount = 0
    for key, value in classCount.items():
        if value > maxCount:
            maxCount = value
            maxIndex = key

    return maxIndex    

# convert image to vector
def  img2vector(filename):
     rows = 32
     cols = 32
     imgVector = zeros((1, rows * cols))

     fileIn = open(filename)
     for row in xrange(rows):
         lineStr = fileIn.readline()
         for col in xrange(cols):
             imgVector[0, row * 32 + col] = int(lineStr[col])

     return imgVector

# load dataSet
def loadDataSet():
    ## step 1: Getting training set
    print "---Getting training set..."
    dataSetDir = 'F:/eclipse/workspace/KnnTest/'
    trainingFileList = os.listdir(dataSetDir + 'trainingDigits') # load the training set
    numSamples = len(trainingFileList)

    train_x = zeros((numSamples, 1024))
    train_y = []
    for i in xrange(numSamples):
        filename = trainingFileList[i]

        # get train_x
        train_x[i, :] = img2vector(dataSetDir + 'trainingDigits/%s' % filename) 

        # get label from file name such as "1_18.txt"
        label = int(filename.split('_')[0]) # return 1
        train_y.append(label)

    ## step 2: Getting testing set
    print "---Getting testing set..."
    testingFileList = os.listdir(dataSetDir + 'testDigits') # load the testing set
    numSamples = len(testingFileList)
    test_x = zeros((numSamples, 1024))
    test_y = []
    for i in xrange(numSamples):
        filename = testingFileList[i]

        # get train_x
        test_x[i, :] = img2vector(dataSetDir + 'testDigits/%s' % filename) 

        # get label from file name such as "1_18.txt"
        label = int(filename.split('_')[0]) # return 1
        test_y.append(label)

    return train_x, train_y, test_x, test_y

# test hand writing class
def testHandWritingClass():
    ## step 1: load data
    print "step 1: load data..."
    train_x, train_y, test_x, test_y = loadDataSet()

    ## step 2: training...
    print "step 2: training..."
    pass

    ## step 3: testing
    print "step 3: testing..."
    numTestSamples = test_x.shape[0]
    matchCount = 0
    for i in xrange(numTestSamples):
        predict = kNNClassify(test_x[i], train_x, train_y, 3)
        if predict == test_y[i]:
            matchCount += 1
    accuracy = float(matchCount) / numTestSamples

    ## step 4: show the result
    print "step 4: show the result..."
    print 'The classify accuracy is: %.2f%%' % (accuracy * 100)

另外创建一个脚本knnTest.py

import KNN
KNN.testHandWritingClass()

其中数据集下载链接为：http://download.csdn.net/detail/zouxy09/6610571