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例子：计算一个给定文档集中单词的相对频度。目标是建立一个N*N矩阵M，其中N为所有给定文档的单词量，每个单元Mij包含一个特定上下文单词Wi与Wj共同出现的次数。为简单起见，将这个上下文定义为Wi的邻域。例如：给定以下单词：W1，W2，W3，W4，W5，W6

如果定义一个单词的邻域为这个单词的前两个单词和后两个单词，那么这6个单词的邻域如下：

单词    领域+-2

W1    W2，W3

W2    W1，W3，W4

W3    W1，W2，W4，W5

W4    W2，W3，W5，W6

W5    W3，W4，W6

W6    W4，W5

反转排序的MapReduce/Hadoop实现：

我们要生成两个数据序列。第一个序列三单词的总领域计数（一个单词的总共出现次数），用组合键（W，*）表示，其中W表示该单词。

第二个序列是这个单词和其他特定单词出现次数，用组合键（W，W2）表示

Hadoop的定制分区器实现

定制分区器必须确保含有相同左词（即自然键）的所有词对要发送到相同的规约器。例如，组合键{(man,tall),(man,strong),(man,moon)...}都要发送到同一个规约器。要把这些键发送到相同的规约器，必须定义一个定制分区器，它只关心左词，如例中man。

相对频度映射器

以W1，W2，W3，W4，W5，W6为例：

键    值

(W1,W2) 1

(W1,W3) 1

(W1,*) 2

(W2,W1) 1

(W2,W3) 1

(W2,W4) 1

(W2,*) 3

(W3,W1) 1

(W3,W2) 1

(W3,W4) 1

(W3,W5) 1

(W3,*) 4

(W4,W2) 1

(W4,W3) 1

(W4,W5) 1

(W4,W6) 1

(W4,*) 4

(W5,W3) 1

(W5,W4) 1

(W5,W6) 1

(W5,*) 3

(W6,W4) 1

(W6,W6) 1

(W6,*) 1

相对频度规约器

规约器的输入：

键    值

(W1,*)，(W1,W2) ，(W1,W3)    2，1，1

(W2，*)，（W2，W1），（W2，W3），（W2，W4） 3，1，1，1

....

代码实现：

自定义组合键

package fanzhuanpaixu; import java.io.DataInput;import java.io.DataOutput;import java.io.IOException;//import org.apache.hadoop.io.Text;import org.apache.hadoop.io.WritableComparable;import org.apache.hadoop.io.WritableComparator;import org.apache.hadoop.io.WritableUtils;/** * Adapted from Cloud9: A Hadoop toolkit for working with big data * edu.umd.cloud9.io.pair.PairOfStrings  * * WritableComparable representing a pair of Strings.  * The elements in the pair are referred to as the left (word)  * and right (neighbor) elements. The natural sort order is: * first by the left element, and then by the right element. *  * @author Mahmoud Parsian (added few new convenient methods) * */public class PairOfWords implements WritableComparable
   
     {    private String leftElement;    private String rightElement;    /**     * Creates a pair.     */    public PairOfWords() {    }    /**     * Creates a pair.     *     * @param left the left element     * @param right the right element     */    public PairOfWords(String left, String right) {        set(left, right);    }    /**     * Deserializes the pair.     *     * @param in source for raw byte representation     */    @Override    public void readFields(DataInput in) throws IOException {        leftElement = Text.readString(in);        rightElement = Text.readString(in);    }    /**     * Serializes this pair.     *     * @param out where to write the raw byte representation     */    @Override    public void write(DataOutput out) throws IOException {        Text.writeString(out, leftElement);        Text.writeString(out, rightElement);    }    public void setLeftElement(String leftElement) {        this.leftElement = leftElement;    }    public void setWord(String leftElement) {        setLeftElement(leftElement);    }    /**     * Returns the left element.     *     * @return the left element     */    public String getWord() {        return leftElement;    }    /**     * Returns the left element.     *     * @return the left element     */    public String getLeftElement() {        return leftElement;    }    public void setRightElement(String rightElement) {        this.rightElement = rightElement;    }    public void setNeighbor(String rightElement) {        setRightElement(rightElement);    }    /**     * Returns the right element.     *     * @return the right element     */    public String getRightElement() {        return rightElement;    }    public String getNeighbor() {        return rightElement;    }    /**     * Returns the key (left element).     *     * @return the key     */    public String getKey() {        return leftElement;    }    /**     * Returns the value (right element).     *     * @return the value     */    public String getValue() {        return rightElement;    }    /**     * Sets the right and left elements of this pair.     *     * @param left the left element     * @param right the right element     */    public void set(String left, String right) {        leftElement = left;        rightElement = right;    }    /**     * Checks two pairs for equality.     *     * @param obj object for comparison     * @return 
    true if 
    obj is equal to this object, 
    false otherwise     */    @Override    public boolean equals(Object obj) {        if (obj == null) {            return false;        }        //        if (!(obj instanceof PairOfWords)) {            return false;        }        //        PairOfWords pair = (PairOfWords) obj;        return leftElement.equals(pair.getLeftElement())                && rightElement.equals(pair.getRightElement());    }    /**     * Defines a natural sort order for pairs. Pairs are sorted first by the left element, and then by the right     * element.     *     * @return a value less than zero, a value greater than zero, or zero if this pair should be sorted before, sorted     * after, or is equal to 
    obj.     */    @Override    public int compareTo(PairOfWords pair) {        String pl = pair.getLeftElement();        String pr = pair.getRightElement();        if (leftElement.equals(pl)) {            return rightElement.compareTo(pr);        }        return leftElement.compareTo(pl);    }    /**     * Returns a hash code value for the pair.     *     * @return hash code for the pair     */    @Override    public int hashCode() {        return leftElement.hashCode() + rightElement.hashCode();    }    /**     * Generates human-readable String representation of this pair.     *     * @return human-readable String representation of this pair     */    @Override    public String toString() {        return "(" + leftElement + ", " + rightElement + ")";    }    /**     * Clones this object.     *     * @return clone of this object     */    @Override    public PairOfWords clone() {        return new PairOfWords(this.leftElement, this.rightElement);    }    /**     * Comparator optimized for 
    PairOfWords.     */    public static class Comparator extends WritableComparator {        /**         * Creates a new Comparator optimized for 
    PairOfWords.         */        public Comparator() {            super(PairOfWords.class);        }        /**         * Optimization hook.         */        @Override        public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) {            try {                int firstVIntL1 = WritableUtils.decodeVIntSize(b1[s1]);                int firstVIntL2 = WritableUtils.decodeVIntSize(b2[s2]);                int firstStrL1 = readVInt(b1, s1);                int firstStrL2 = readVInt(b2, s2);                int cmp = compareBytes(b1, s1 + firstVIntL1, firstStrL1, b2, s2 + firstVIntL2, firstStrL2);                if (cmp != 0) {                    return cmp;                }                int secondVIntL1 = WritableUtils.decodeVIntSize(b1[s1 + firstVIntL1 + firstStrL1]);                int secondVIntL2 = WritableUtils.decodeVIntSize(b2[s2 + firstVIntL2 + firstStrL2]);                int secondStrL1 = readVInt(b1, s1 + firstVIntL1 + firstStrL1);                int secondStrL2 = readVInt(b2, s2 + firstVIntL2 + firstStrL2);                return compareBytes(b1, s1 + firstVIntL1 + firstStrL1 + secondVIntL1, secondStrL1, b2,                        s2 + firstVIntL2 + firstStrL2 + secondVIntL2, secondStrL2);            } catch (IOException e) {                throw new IllegalArgumentException(e);            }        }    }    static { // register this comparator        WritableComparator.define(PairOfWords.class, new Comparator());    }}
   

映射器

package fanzhuanpaixu;import org.apache.hadoop.io.IntWritable;import org.apache.hadoop.io.LongWritable;import org.apache.hadoop.io.Text;import org.apache.hadoop.mapreduce.Mapper;//import org.apache.commons.lang.StringUtils;//import java.io.IOException;/** * RelativeFrequencyMapper implements the map() function for Relative Frequency of words. * * @author Mahmoud Parsian * */public class RelativeFrequencyMapper        extends Mapper
   
     {    private int neighborWindow = 2;    // pair = (leftElement, rightElement)    private final PairOfWords pair = new PairOfWords();    private final IntWritable totalCount = new IntWritable();    private static final IntWritable ONE = new IntWritable(1);    @Override    public void setup(Context context) {        this.neighborWindow = context.getConfiguration().getInt("neighbor.window", 2);    }    @Override    protected void map(LongWritable key, Text value, Context context)            throws IOException, InterruptedException {        String[] tokens = StringUtils.split(value.toString(), " ");        //String[] tokens = StringUtils.split(value.toString(), "\\s+");        if ((tokens == null) || (tokens.length < 2)) {            return;        }        for (int i = 0; i < tokens.length; i++) {            tokens[i] = tokens[i].replaceAll("\\W+", "");            if (tokens[i].equals("")) {                continue;            }            pair.setWord(tokens[i]);            int start = (i - neighborWindow < 0) ? 0 : i - neighborWindow;            int end = (i + neighborWindow >= tokens.length) ? tokens.length - 1 : i + neighborWindow;            for (int j = start; j <= end; j++) {                if (j == i) {                    continue;                }                pair.setNeighbor(tokens[j].replaceAll("\\W", ""));                context.write(pair, ONE);            }            //            pair.setNeighbor("*");            totalCount.set(end - start);            context.write(pair, totalCount);        }    }}
   

分区器

package fanzhuanpaixu;import org.apache.hadoop.io.IntWritable;import org.apache.hadoop.mapreduce.Partitioner;/**  * This is an plug-in class. * The OrderInversionPartitioner class indicates how to partition data * based on the word only (the natural key). *  * @author Mahmoud Parsian * */public class OrderInversionPartitioner    extends Partitioner
   
     {    @Override    public int getPartition(PairOfWords key,                             IntWritable value,                             int numberOfPartitions) {        // key = (leftWord, rightWord) = (word, neighbor)        String leftWord = key.getLeftElement();        return Math.abs( ((int) hash(leftWord)) % numberOfPartitions);    }        /**     * Return a hashCode() of a given String object.        * This is adapted from String.hashCode()     *     * @param str a string object     *     */    private static long hash(String str) {       long h = 1125899906842597L; // prime       int length = str.length();       for (int i = 0; i < length; i++) {          h = 31*h + str.charAt(i);       }       return h;    }	    }
   

组合器

package fanzhuanpaixu;import org.apache.hadoop.io.IntWritable;import org.apache.hadoop.mapreduce.Reducer;import java.io.IOException;/**  * RelativeFrequencyCombiner implements the combine() [in Hadoop,  * we use reduce() for implementing the combine() function] function  * for MapReduce/Hadoop. * * @author Mahmoud Parsian * */public class RelativeFrequencyCombiner        extends Reducer
   
     {    @Override    protected void reduce(PairOfWords key, Iterable
    
      values, Context context)            throws IOException, InterruptedException {        //        int partialSum = 0;        for (IntWritable value : values) {            partialSum += value.get();        }        //        context.write(key, new IntWritable(partialSum));    }}
    
   

规约器

package fanzhuanpaixu;import org.apache.hadoop.io.IntWritable;import org.apache.hadoop.io.DoubleWritable;import org.apache.hadoop.mapreduce.Reducer;import java.io.IOException;/** * RelativeFrequencyReducer implements the reduce() function for Relative Frequency of words. * * @author Mahmoud Parsian * */public class RelativeFrequencyReducer        extends Reducer
   
     {    private double totalCount = 0;    private final DoubleWritable relativeCount = new DoubleWritable();    private String currentWord = "NOT_DEFINED";    @Override    protected void reduce(PairOfWords key, Iterable
    
      values, Context context)            throws IOException, InterruptedException {        if (key.getNeighbor().equals("*")) {            if (key.getWord().equals(currentWord)) {                totalCount += totalCount + getTotalCount(values);            } else {                currentWord = key.getWord();                totalCount = getTotalCount(values);            }        } else {            int count = getTotalCount(values);            relativeCount.set((double) count / totalCount);            context.write(key, relativeCount);        }    }    private int getTotalCount(Iterable
     
       values) {        int sum = 0;        for (IntWritable value : values) {            sum += value.get();        }        return sum;    }}
     
    
   

驱动

package fanzhuanpaixu;import org.apache.hadoop.conf.Configured;import org.apache.hadoop.conf.Configuration;import org.apache.hadoop.fs.Path;import org.apache.hadoop.fs.FileSystem;import org.apache.hadoop.io.IntWritable;import org.apache.hadoop.io.DoubleWritable;import org.apache.hadoop.mapreduce.Job;import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;import org.apache.hadoop.util.Tool;import org.apache.hadoop.util.ToolRunner;//import org.apache.log4j.Logger;/** * RelativeFrequencyDriver is driver class for computing relative frequency of words. * * @author Mahmoud Parsian * */public class RelativeFrequencyDriver        extends Configured implements Tool {    private static final Logger THE_LOGGER = Logger.getLogger(RelativeFrequencyDriver.class);    /**     * Dispatches command-line arguments to the tool by the ToolRunner.     */    public static void main(String[] args) throws Exception {    	args = new String[1];    	args[0] = "2";        int status = ToolRunner.run(new RelativeFrequencyDriver(), args);        System.exit(status);    }    @Override    public int run(String[] args) throws Exception {        int neighborWindow = Integer.parseInt(args[0]);        Path inputPath = new Path("input/paper.txt");        Path outputPath = new Path("output/fanzhuanpaixu");        Job job = new Job(new Configuration(), "RelativeFrequencyDriver");        job.setJarByClass(RelativeFrequencyDriver.class);        job.setJobName("RelativeFrequencyDriver");        // Delete the output directory if it exists already        FileSystem.get(getConf()).delete(outputPath, true);        job.getConfiguration().setInt("neighbor.window", neighborWindow);        FileInputFormat.setInputPaths(job, inputPath);        FileOutputFormat.setOutputPath(job, outputPath);        // (key,value) generated by map()        job.setMapOutputKeyClass(PairOfWords.class);        job.setMapOutputValueClass(IntWritable.class);        // (key,value) generated by reduce()        job.setOutputKeyClass(PairOfWords.class);        job.setOutputValueClass(DoubleWritable.class);        job.setMapperClass(RelativeFrequencyMapper.class);        job.setReducerClass(RelativeFrequencyReducer.class);        job.setCombinerClass(RelativeFrequencyCombiner.class);        job.setPartitionerClass(OrderInversionPartitioner.class);        job.setNumReduceTasks(3);        long startTime = System.currentTimeMillis();        job.waitForCompletion(true);        THE_LOGGER.info("Job Finished in milliseconds: " + (System.currentTimeMillis() - startTime));        return 0;    }}

运行结果：