CODEWITHSUNDEEP
hadoopmapreducehadoop2
user3928562
user3928562

Reputation: 55

How to sort data in descending order in map reduce?

My reducer gives this o/p 

Country-Year,Medals
India-2008,60
United States-2008,1237
Zimbabwe-2008, 2
Namibia-2009,22
China-2009,43
United States-2009,54

And i want this, sorting should happen based on medals and top three should be shown.

Country-Year,Medals
United States-2008,1237
India-2008,60
United States-2009,54

Someone suggested me to do this sorting in custom recordreader(understood that it is used in mapper part) and i browsed through some resources but couldn't find much on sorting. Please share any ideas or link to resources. Advance thanks !

Upvotes: 0

Views: 8395

Answers (2)

Elijah
Elijah

Reputation: 306

When you aggregate the result of mapper in Reducer class rather than writing it to output take it into a map then sort the map and display result accordingly.

Key = Country-Year , Value = Medals Dummy code to showcase how to implement

public class Medal_reducer extends Reducer<Text,IntWritable, Text , IntWritable> {
 // Change access modifier as per your need 
 public Map<String , Integer > map = new HashMap<String , Integer>();
 public void reduce(Text key , Iterable<IntWritable> values ,Context context )
   {
    // write logic for your reducer 
    // Enter reduced values in map for each key
    for (IntWritable value : values ){

        // calculate count
    }
     map.put(key.toString() , count); 



     }

   public void cleanup(Context context){ 
    //Cleanup is called once at the end to finish off anything for reducer
    //Here we will write our final output
     Map<String , Integer>  sortedMap = new HashMap<String , Integer>();
    sortedMap = sortMap(map);

    for (Map.Entry<String,Integer> entry = sortedMap.entrySet()){
        context.write(new Text(entry.getKey()),new IntWritable(entry.getValue()));
        }


   }
  public Map<String , Integer > sortMap (Map<String,Integer> unsortMap){

    Map<String ,Integer> hashmap = new HashMap<String,Integer>();
    int count=0;
    List<Map.Entry<String,Integer>> list = new LinkedList<Map.Entry<String,Integer>>(unsortMap.entrySet());
    //Sorting the list we created from unsorted Map
    Collections.sort(list , new Comparator<Map.Entry<String,Integer>>(){

        public int compare (Map.Entry<String , Integer> o1 , Map.Entry<String , Integer> o2 ){
            //sorting in descending order
            return o2.getValue().compareTo(o1.getValue());

        }


    });

    for(Map.Entry<String, Integer> entry : list){
        // only writing top 3 in the sorted map 
        if(count>2)
            break;

        hashmap.put(entry.getKey(),entry.getValue());


    }

    return hashmap ;

    }

  }

Hopefully this will help.

Upvotes: 0

Kiran Krishna Innamuri
Kiran Krishna Innamuri

Reputation: 1002

You can implement Map Reduce Top K Design pattern to achieve your objective.

Top K Design pattern will sort your records on values and picks the top k records.

You can go through this link for implementing Top K Design pattern on your data.

Upvotes: 2

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