矩阵相乘能够查看百度百科的解释http://baike.baidu.com/view/2455255.htm?fr=aladdin
有a和b两个矩阵
a: 1 2 3
4 5 0
7 8 9
10 11 12
b: 10 15
0 2
11 9
c = a*b
1:将矩阵存到HDFS中：
矩阵a：
| 行 | 列 | 值 | hdfs存储 |
| 1 | 1 | 1 | 1,1,1 |
| 1 | 2 | 2 | 1,2,2 |
| 1 | 3 | 3 | 1,3,3 |
| 2 | 1 | 4 | 2,1,4 |
| 2 | 2 | 5 | 2,2,5 |
| 2 | 3 | 0 | 0不存储 |
| 3 | 1 | 7 | 3,1,7 |
| 3 | 2 | 8 | 3,2,8 |
| 3 | 3 | 9 | 3,3,9 |
| 4 | 1 | 10 | 4,1,10 |
| 4 | 2 | 11 | 4,2,11 |
| 4 | 3 | 12 | 4,3,12 |
矩阵b：
| 行 | 列 | 值 | hdfs存储 |
| 1 | 1 | 10 | 1,1,10 |
| 1 | 2 | 15 | 1,2,15 |
| 2 | 1 | 0 | 0不存储 |
| 2 | 2 | 2 | 2,2,2 |
| 3 | 1 | 11 | 3,1,11 |
| 3 | 2 | 9 | 3,2,9 |
2:a的map读取
读取第一个值是1,1,1。它是矩阵a的第一行第一列。那么它要在计算c(1,1) c(1,2)的时候使用（这里c仅仅用2列，假设用n列，那么它的值要在计算c(1,1),c(1,2),c(1,3)...c(1,n)的时候使用）。我们就以 key = 1,1 value = a,1,1 ， key= 1,2 value = a,1,1输出两条数据(1,1) (1,2)是 c(1,1) ,c(1,2)的坐标。
b的map读取
读取第一个值是1,1,10。它是矩阵b的第一行第一列。那么它要在计算c(1,1) c(2,1) c(3,1) c(4,1)的时候使用（这里c仅仅用4行，假设用m行，那么它的值要在计算c(1,1),c(2,1),c(3,1)...c(m,1)的时候使用）。我们就以 key = 1,1 value = b,1,10 , key= 2,1 value = b,1,10 ， key = 3,1 value = b,1,10 , key= 4,1 value = b,1,10输出
3:reduce读取计算
通过mapA和mapB的输出能够得到 key = 1,1 ， value=a,1,1 value=b,1,10 value=a,2,2 value=a,3,3 value = b,3,11l来计算c(1,1)的值
代码例如以下：
package MyMatrix;
import java.io.IOException;
import java.util.Iterator;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.MultipleInputs;
import org.apache.hadoop.mapreduce.lib.input.TextInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
public class MartrixMultiply{
/**
* 最后得到的矩阵的列数
*/
public static final int COL_COUNT = 2;
/**
* 最后得到的矩阵的行数
*/
public static final int ROW_COUNT = 4;
/**
* A矩阵的列数或者是B矩阵的行数
*/
public static final int BROW_ACOL= 3;
public static class MartrixMaperA extends Mapper<LongWritable, Text, Text, Text>{
@Override
protected void map(LongWritable key, Text value, Context context)
throws IOException, InterruptedException {
String valueStr = value.toString();
String[] items = valueStr.split(",");
int rowIndex = Integer.parseInt(items[0]);
int colIndex = Integer.parseInt(items[1]);
int valueInt = Integer.parseInt(items[2]);
Text outKey = null;
Text outValue = null;
for(int i=0;i<COL_COUNT;i++){
outKey = new Text(rowIndex + "," + (i+1));
outValue = new Text("a,"+colIndex+","+valueInt);
context.write(outKey, outValue);
}
}
}
public static class MartrixMaperB extends Mapper<LongWritable, Text, Text, Text>{
@Override
protected void map(LongWritable key, Text value, Context context)
throws IOException, InterruptedException {
String valueStr = value.toString();
String[] items = valueStr.split(",");
int rowIndex = Integer.parseInt(items[0]);
int colIndex = Integer.parseInt(items[1]);
int valueInt = Integer.parseInt(items[2]);
Text outKey = null;
Text outValue = null;
for(int i=0;i<ROW_COUNT;i++){
outKey = new Text((i+1) + "," + colIndex);
outValue = new Text("b,"+rowIndex+","+valueInt);
context.write(outKey, outValue);
}
}
}
public static class MartrixReducer extends Reducer<Text,Text,Text,IntWritable>{
@Override
protected void reduce(Text key, Iterable<Text> values,Context context)
throws IOException, InterruptedException {
String[] items = new String[3];
int[] valueA = new int[BROW_ACOL];
int[] valueB = new int[BROW_ACOL];
Iterator<Text> it = values.iterator();
while(it.hasNext()){
items = it.next().toString().split(",");
if(items[0].equals("a")){
valueA[Integer.parseInt(items[1])-1] = Integer.parseInt(items[2]);
}else if(items[0].equals("b")){
valueB[Integer.parseInt(items[1])-1] = Integer.parseInt(items[2]);
}
}
int result = 0;
for(int i=0;i<BROW_ACOL;i++){
result += valueA[i]*valueB[i];
}
context.write(key, new IntWritable(result));
}
}
@SuppressWarnings("deprecation")
public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
Path pathA = new Path("hdfs://localhost:9000/Martrix/a.txt");
Path pathB = new Path("hdfs://localhost:9000/Martrix/b.txt");
Path pathOut = new Path("hdfs://localhost:9000/Martrix/out");
Configuration conf = new Configuration();
Job job = new Job(conf,"MartrixMultiply");
job.setJarByClass(MartrixMultiply.class);
MultipleInputs.addInputPath(job, pathA, TextInputFormat.class, MartrixMaperA.class);
MultipleInputs.addInputPath(job, pathB, TextInputFormat.class, MartrixMaperB.class);
job.setReducerClass(MartrixReducer.class);
job.setMapOutputKeyClass(Text.class);
job.setMapOutputValueClass(Text.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);
FileOutputFormat.setOutputPath(job, pathOut);
if(job.waitForCompletion(true)){
System.exit(0);
}else{
System.exit(1);
}
}
}