什么时候只用join 什么时候用 join in

Posted 2023-04-19

1)加入某党派、某组织或某社会团体，用join，不可用join in。如：
①He will never forget the day when he joined the Party.
他永远也忘不了他入党的那一天。
②His brother joined the army three years ago.
他哥哥是三年前参军的。
join还可解释为“连接”。如：
①The railway joined the two cities.
铁路把两个城市连接起来了。
②The two clauses are joined by a conjunction.
两个分句由一个连词连接起来。
2)说参加某种活动用join in，如说“与其人一起做某事”，则用join sb. in sth. / doing sth. 。如：
①May I join in the game? 我可以参加这个游戏吗？
②Come and join us in the discussion. 来和我们一起讨论吧！
③We are having supper now.Would you like to join us?
我们正在吃晚饭，你也来和我们一起吃好吗？
join in多指参加小规模的活动如“球赛、游戏”等，常用于日常口语。如：
①Come along, and join in the ball game. 快，来参加球赛。
②Why didn\'t you join in the talk last night? 昨晚你为什么没参加座谈？ 参考技术A 本来就是及物动词，加in时一般后面跟party等组织之类的词 参考技术B 加入某人时用join，参加某活动时用join in追答

请采纳

参考技术C join+组织，join+人

mapreduce的join

一篇超级详细的文章：this one

读完之后感触颇深，什么时候在map的时候join什么时候在reducer的时候join

之前写两个输入的时候，写的多么可笑，效率极低。

先用了一遍这篇文章的分布式缓存，在reduce的时候读取，因为hadoop版本太低，所以又做了修改，结合好几篇文章结果：

版本：Hadoop0.20.203.0

package bjut.edu.ting;

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Hashtable;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.filecache.DistributedCache;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.NullWritable;
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;

import com.opencsv.CSVParser;

//有两个输入：GPS（数据量大）,Dictionary(数据量小)；
//两者通过属性bus_line在reduce过程中连接，将dictonary放在内存，读取之时用hashtable，存储检索
//其中mapper过程通过passtime计算date，并赋值给GPS数据


public  class DateLineJob{
    
    public static class JoinMapper extends Mapper<LongWritable,Text,Text,Text>{
        //处理GPS数据
        protected void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException{
            CSVParser parser = new CSVParser();
            String[] gpsData = parser.parseLine(value.toString());
            
            Integer date_label=null;
            try {
                date_label=getDateStamp(gpsData[2]);
            } catch (ParseException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            if(date_label==12&&date_label!=-1){//这儿的date_label需要修改
                    String outValue=date_label.toString()+","+gpsData[0]+","+gpsData[2]+","+gpsData[3]+","+gpsData[4]；
                    //key:bus_line　　value:0:date,1:vehicle,2:pass,3:lon,4:lat
                    context.write(new Text(gpsData[1]),new Text(outValue));
                }
        }
    }
    public static class JoinReducer extends Reducer<Text, Text, NullWritable, Text>{
        //定义HashTable存放缓存数据
        private Hashtable <String,String> table=new Hashtable<String,String>();
        
        /**
         * 获取分布式缓存文件
         */
        private Path[] modelPath;  
        private BufferedReader modelBR;   
        
        protected void setup(Context context) throws IOException {

            //返回本地文件路径
            Configuration conf = context.getConfiguration(); 
            modelPath =  DistributedCache.getLocalCacheFiles(conf); 
            if(modelPath.length==0){
                throw new FileNotFoundException("Distributed cache file not found");
            }
            modelBR = new BufferedReader(new FileReader(modelPath[0].toString()));  
           
            //按行读取并解析字典数据,
            String infoDic=null;
            while((infoDic=modelBR.readLine())!=null){
                String[] records=infoDic.split(",");
                //key为bus_line   value为line_code
                table.put(records[1],records[0]);//将相应的字段存入Hashtable里面
            }
            modelBR.close();
        }
        public void reduce(Text key,Iterable<Text> values, Context context) throws IOException, InterruptedException{
            //字典数据根据bus_line获取line_code
            String line_code=table.get(key.toString());//从Hashtable中获取line_code
            
            if(line_code!=null){//有些线路在字典中没有
                for(Text value:values){
                    String outValue=value.toString();
                    String[] valueData=outValue.split(",");
                
                    //0:date,1:vehicle,2:pass,3:lon,4:lat
                    String out=valueData[0]+","+line_code+","+valueData[1]+","+valueData[2]+","+valueData[3]+","+valueData[4];
                    context.write(null, new Text(out));
                }
            }
        }
    }

    public static void main(String[] args) throws Exception{
        Configuration conf = new Configuration();
        conf.set("mapreduce.admin.reduce.child.java.opts", "-Xmx512m");
        DistributedCache.addCacheFile(new Path("hdfs://172.18.49.17:8020/Anewday/line_route_dict_update08_nohead.csv").toUri(), conf);  
        DistributedCache.createSymlink(conf);
        
        Job job = new Job(conf,"join");
        job.setJarByClass(DateLineJob.class);

        //设置GPS作为输入
          FileInputFormat.addInputPath(job,new Path(args[0]));
          //输出目录
          FileOutputFormat.setOutputPath(job, new Path(args[1]));
          job.setMapperClass(JoinMapper.class);
          job.setReducerClass(JoinReducer.class);
         
          job.setMapOutputKeyClass(Text.class);
          job.setMapOutputValueClass(Text.class);
             
          job.setOutputKeyClass(NullWritable.class);
          job.setOutputValueClass(Text.class);
        
          System.exit(job.waitForCompletion(true)?0:1);
    }
    private static int getDateStamp(String timeStr) throws ParseException{
        if(timeStr.length()==19){//若不是这个形式，则不考虑
            SimpleDateFormat formatter=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            Date data=formatter.parse(timeStr);
            int dateStamp=-1;
            dateStamp=data.getDate();
            //这个地方会出现日期不存在的情况错误提示，但不影响运行，而且这样的数据量特别少。
            return dateStamp;
        }else{//返回－１
            return -1;
        }
    }
}