K近鄰(kNN,k-NearestNeighbor)分類算法基本思想是:如果一個樣本在特徵空間中的k個最相似,也就是特徵空間中k個最鄰近的樣本大多數屬於某一個類別,則該樣本也屬於這個類別。類似與古話:近朱者赤,近墨者黑,背後自然也蘊藏着物以類聚,人以羣分的思想!
算法步驟:
1.對數據進行歸一化處理
2.求每個測試樣本基於訓練樣本的k個最近臨樣本
3.k個最近臨樣本所屬類別中最大的一個即位測試樣本的類別。
優點:
1.容易理解,易於分類
2.適合多類別的分類問題
缺點:
1.每個測試樣本需要與所有訓練樣本進行求距離,計算量大
2.當各類樣本不平衡時,測試結果可能會趨向與樣本數量多的那一類。
k值的選擇:
k值過小,得到的近臨數太少,使得分類精度低,同時放大了噪聲的干擾;k值過大,當各類樣本不平衡時,測試結果可能會趨向與樣本數量多的那一類。k值的選擇一般小於訓練樣本的平方根。
實例:
《機器學習實戰》一書中手寫數字識別例子:
訓練樣本: 32*32的0,1文本,每個文本代表一個手寫數字,文本名中包含該文本所屬的數字類別。點擊下載
測試樣本:同訓練樣本
代碼:
from numpy import *
import operator
import os
def ReadData(trainDir, testDir):
trainFileList = os.listdir(trainDir)
testFileList = os.listdir(testDir)
numSamples = len(trainFileList)
trainX = zeros((numSamples, 1024))
trainY = []
for i in xrange(numSamples):
fileName = trainFileList[i]
trainX[i, :] = ReadImgData(trainDir + fileName)
label = int(fileName.split('_')[0])
trainY.append(label)
numSamples = len(testFileList)
testX = zeros((numSamples, 1024))
testY = []
for i in xrange(numSamples):
fileName = testFileList[i]
testX[i, :] = ReadImgData(testDir + fileName)
label = int(fileName.split('_')[0])
testY.append(label)
return trainX, trainY, testX, testY
#ReadImgData讀取每個文本內容
def ReadImgData(fileName):
row = 32
col = 32
fileX = zeros((1, row*col))
fileFp = open(fileName)
for i in xrange(row):
lineTemp = fileFp.readline()
for j in xrange(col):
fileX[0, i*row + j] = int(lineTemp[j])
return fileX
def knn(testX, trainX, trainY, k):
numSamples = trainX.shape[0]
diff = tile(testX, (numSamples, 1)) - trainX
squareDiff = diff ** 2
squareDist = sum(squareDiff, axis = 1)
dist = squareDist ** 0.5
sortedDist = argsort(dist)
classCount = {}
for i in xrange(k):
voteLabel = trainY[sortedDist[i]]
classCount[voteLabel] = classCount.get(voteLabel, 0) + 1
maxCount = 0
for key, value in classCount.items():
if value > maxCount:
maxCount = value
maxIndex = key
return maxIndex
print "******start******"
trainDir = './trainingDigits/'
testDir = './testDigits/'
trainX, trainY, testX, testY = ReadData(trainDir, testDir)
print "******Data End******"
sumSamples = testX.shape[0]
right = 0
for i in xrange(sumSamples):
label = knn(testX[i], trainX, trainY, 3)
#print "label = %d" % label
if label == testY[i]:
right += 1
print "*****Test End******"
print 'right = %d' % right
rate = float(right) / sumSamples
print 'rate = %f' % rate
代碼中ReadData讀取樣本目錄中的數據,ReadImgData讀取單個文本的數據,knn實現測試算法。