MapReduce sort by value in descending order - sorting

Question

I'm trying to write in pseudo code a MapReduce task that returns the items sorted in descending order. For example: for the wordcount task, instead of getting:
apple 1
banana 3
mango 2
I want the output to be:
banana 3
mango 2
apple 1
Any ideas of how to do it? I know how to do it in ascending order (replace the keys and value in the mapper job) but not in descending order.

Answer 1

Here you can take help of below reducer code to achieve sorting in descending order .
Assuming you have written mapper and driver code where mapper will produce output as (Banana,1) etc
In reducer we will sum all values for a particular key and put final result in a map then sort the map on the basis of values and write final result in cleanup function of reduce.
Please see below code for further understadnind:
public class Word_Reducer extends Reducer<Text, IntWritable, Text, IntWritable> {
// Change access modifier as per your need
public Map<String , Integer > map = new LinkedHashMap<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" associated with each word
}
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 LinkedHashMap<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 ;
}