数据挖掘笔记-分类-KNN-原理与简单实现

KNN算法又称为k近邻分类(k-nearest neighbor classification)算法。最简单平凡的分类器。KNN算法则是从训练集中找到和新数据最接近的k条记录，然后根据他们的主要分类来决定新数据的类别。该算法涉及3个主要因素：训练集、距离、k的大小。可用于客户流失预测、欺诈侦测等（更适合于稀有事件的分类问题）。 

计算步骤如下：
    1）计算距离：给定测试对象，计算它与训练集中的每个对象的距离。距离计算可以选择欧氏距离、曼哈顿距离、余弦距离等。计算距离之前最好对数据进行规范化处理，以便于更好的计算。
    2）寻找邻居：圈定距离最近的k个训练对象，作为测试对象的近邻。K值的选择可以通过若干试验，选取分类误差最小的K值。
    3）判断分类：根据这k个近邻归属的主要类别，来对测试对象分类。判定方式主要是投票决定，少数服从多数，近邻中哪个类别的点最多就分为该类。也可以通过加权投票方法来决定。


优点
简单，易于理解，易于实现，无需估计参数，无需训练
适合对稀有事件进行分类（例如当流失率很低时，比如低于0.5%，构造流失预测模型）
特别适合于多分类问题(multi-modal,对象具有多个类别标签)，例如根据基因特征来判断其功能分类，KNN比SVM的表现要好

缺点

计算开销大，需要有效的存储技术和并行硬件的支撑。

可解释性较差，无法给出决策树那样的规则。

下面是基于MapReduce简单实现KNN的代码：

public class KNNClassifier {private static void configureJob(Job job) {job.setJarByClass(KNNClassifier.class);job.setMapperClass(KNNMapper.class);job.setMapOutputKeyClass(Text.class);job.setMapOutputValueClass(PointWritable.class);job.setReducerClass(KNNReducer.class);job.setOutputKeyClass(Text.class);job.setOutputValueClass(Text.class);job.setInputFormatClass(TextInputFormat.class);job.setOutputFormatClass(TextOutputFormat.class);}public static void main(String[] args) {long start = System.currentTimeMillis();Configuration configuration = new Configuration();try {String[] inputArgs = new GenericOptionsParser(configuration, args).getRemainingArgs();if (inputArgs.length != 4) {System.out.println("error, please input three path.");System.out.println("1 train set path.");System.out.println("2 test set path.");System.out.println("3 output path.");System.out.println("4 k value.");System.exit(2);}DistributedCache.addCacheFile(new Path(inputArgs[0]).toUri(), configuration);configuration.set("k", inputArgs[3]);Job job = new Job(configuration, "KNN Classifier");FileInputFormat.setInputPaths(job, new Path(inputArgs[1]));FileOutputFormat.setOutputPath(job, new Path(inputArgs[2]));configureJob(job);System.out.println(job.waitForCompletion(true) ? 0 : 1);long end = System.currentTimeMillis();System.out.println("spend time: " + (end - start) / 1000);} catch (Exception e) {e.printStackTrace();}}}class KNNMapper extends Mapper<LongWritable, Text, Text, PointWritable> {private List<Point> trainPoints = new ArrayList<Point>();@Overrideprotected void setup(Context context) throws IOException, InterruptedException {super.setup(context);Configuration conf = context.getConfiguration();FileSystem fs = FileSystem.get(conf);URI[] uris = DistributedCache.getCacheFiles(conf);Path[] paths = HDFSUtils.getPathFiles(fs, new Path(uris[0]));for(Path path : paths) {FSDataInputStream in = fs.open(path);BufferedReader reader = new BufferedReader(new InputStreamReader(in));String line = reader.readLine();while (null != line && !"".equals(line)) {String[] datas = line.split(" ");trainPoints.add(new Point(Double.parseDouble(datas[0]), Double.parseDouble(datas[1]), datas[2]));line = reader.readLine();}IOUtils.closeQuietly(in);IOUtils.closeQuietly(reader);}}@Overrideprotected void map(LongWritable key, Text value, Context context)throws IOException, InterruptedException {String line = value.toString();String[] datas = line.split(" ");double x = Double.parseDouble(datas[0]);double y = Double.parseDouble(datas[1]);Point testPoint = new Point(x, y);String outputKey = x + "-" + y;for (Point trainPoint : trainPoints) {double distance = distance(testPoint, trainPoint);context.write(new Text(outputKey), new PointWritable(trainPoint, distance));}}public double distance(Point point1, Point point2) {return Math.sqrt(Math.pow((point1.getX() - point2.getX()), 2)+ Math.pow((point1.getY() - point2.getY()), 2));}@Overrideprotected void cleanup(Context context) throws IOException, InterruptedException {super.cleanup(context);}}class KNNReducer extends Reducer<Text, PointWritable, Text, Text> {private int k = 0;@Overrideprotected void setup(Context context) throws IOException, InterruptedException {super.setup(context);Configuration conf = context.getConfiguration();k = Integer.parseInt(conf.get("k", "0"));}@Overrideprotected void reduce(Text key, Iterable<PointWritable> values,Context context) throws IOException, InterruptedException {System.out.println(key);List<PointWritable> points = new ArrayList<PointWritable>();for (PointWritable point : values) {points.add(new PointWritable(point));}Collections.sort(points, new Comparator<PointWritable>() {@Overridepublic int compare(PointWritable o1, PointWritable o2) {return o1.getDistance().compareTo(o2.getDistance());}});Map<String, Integer> map = new HashMap<String, Integer>();k = points.size() < k ? points.size() : k;for (int i = 0; i < k; i++) {PointWritable point = points.get(i);String category = point.getCategory().toString();Integer count = map.get(category);map.put(category, null == count ? 1 : count + 1);}List<Map.Entry<String, Integer>> list = new ArrayList<Map.Entry<String, Integer>>(map.entrySet());Collections.sort(list, new Comparator<Map.Entry<String, Integer>>(){@Overridepublic int compare(Entry<String, Integer> o1,Entry<String, Integer> o2) {return o2.getValue().compareTo(o1.getValue());}});context.write(key, new Text(list.get(0).getKey()));}@Overrideprotected void cleanup(Context context) throws IOException,InterruptedException {super.cleanup(context);}}

附：Spark学习笔记-KNN算法实现

代码托管:https://github.com/fighting-one-piece/repository-datamining.git