MapReduce案例2——求学生平均成绩

题目：计算学生考试平均成绩 源数据：
张三 98
李四 96
王五 95
张三 90
李四 92
王五 99
张三 80
李四 90
王五 94
张三 82
李四 92

最终答案：
张三 98
李四 87
王五 86
上面的答案仅是格式，非正确数据思路：姓名作为key，成绩作为value，在map分组，在reduce中求平均值
类似于SQL中的select name, avg(score) from studentscore group by name;
代码：/**
* @author: lpj
* @date: 2018年3月16日 下午7:16:47
* @Description:
*/
package lpj.reduceWork;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
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.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
/**
*
*/
public class AverageSocreMR {

public static void main(String[] args) throws Exception {
Configuration conf = new Configuration();
// conf.addResource("hdfs-site.xml");//使用配置文件
// System.setProperty("HADOOP_USER_NAME", "hadoop");//使用集群
FileSystem fs = FileSystem.get(conf);//默认使用本地

Job job = Job.getInstance(conf);
job.setJarByClass(AverageSocreMR.class);
job.setMapperClass(AverageSocreMR_Mapper.class);
job.setReducerClass(AverageSocreMR_Reducer.class);

job.setMapOutputKeyClass(Text.class);
job.setMapOutputValueClass(Text.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(Text.class);
//
// String inputpath = args[0];
// String outpath = args[1];

Path inputPath = new Path("d:/a/homework2.txt");
Path outputPath = new Path("d:/a/homework2");
if (fs.exists(inputPath)) {
fs.delete(outputPath, true);
}

FileInputFormat.setInputPaths(job, inputPath);
FileOutputFormat.setOutputPath(job, outputPath);
boolean isdone = job.waitForCompletion(true);
System.exit(isdone ? 0 : 1);
}

public static class AverageSocreMR_Mapper extends Mapper<LongWritable, Text, Text, Text>{
Text kout = new Text();
Text valueout = new Text();
@Override
protected void map(LongWritable key, Text value,Context context)throws IOException, InterruptedException {
//李四 92
String [] reads = value.toString().trim().split(" ");
String kk = reads[0];
String vv = reads[1];
kout.set(kk);
valueout.set(vv);
context.write(kout, valueout);

}
}
public static class AverageSocreMR_Reducer extends Reducer<Text, Text, Text, Text>{
Text kout = new Text();
Text valueout = new Text();
@Override
protected void reduce(Text key, Iterable<Text> values, Context context)throws IOException, InterruptedException {
int sum = 0;
int count = 0;
int avg = 0;
for(Text text : values){
sum += Integer.parseInt(text.toString());
count++;
}
avg = sum / count;
valueout.set(avg + "");
context.write(key, valueout);

}

}

}
结果：张三 87
李四 92
王五 96