# Sort array elements by frequency

Here given code implementation process.

``````//C++ Program
//Sort array elements by frequency
#include <iostream>
#include <map>
#include <vector>

using namespace std;

void occurrence(int arr[],int size)
{

map<int, int> mp;

map <int , vector<int> > result;

map<int, int> :: iterator element;

map<int, vector<int>> :: iterator output;

vector<int> :: iterator it;

int j = size-1;

//Find the frequency of array elements
for (int i = 0; i < size; ++i)
{
mp[arr[i]]++;
}

for (element = mp.begin(); element != mp.end(); ++element)
{
result[element->second].insert(result[element->second].begin(),element->first);
}

for (output = result.begin(); output != result.end(); ++output)
{

for(it = result[output->first].begin();it != result[output->first].end(); it++)
{
for(int i=0; i < output->first && j>=0;i++,j--)
{
//store the new result into array
arr[j]=*it;
}

}
}

}
void print_data(int arr[],int size)
{
for (int i = 0; i < size; ++i)
{
cout<<arr[i]<<"  " ;
}
cout<<endl;
}
int main()
{
//Array element
int arr[] = {1, 3, 7, 4, 7, 4, 3,7, 4, 8, 7};
//Get the size of array
int size = sizeof(arr)/sizeof(arr[0]);
print_data(arr,size);
occurrence(arr,size);
print_data(arr,size);
return 0;
}``````

#### Output

``````  1  3  7  4  7  4  3  7  4  8  7
7  7  7  7  4  4  4  3  3  1  8
``````
``````/*
Java program
Sort array elements by frequency
*/
import java.util.*;
public class Hashing
{
public static void occurrence(int []arr)
{
Map<Integer, Integer> map = new HashMap<>();
Map<Integer, List<Integer>> result   = new HashMap<>();
int j = arr.length-1;
for (int i=0;i<arr.length ; i++ )
{
if (map.containsKey(arr[i]))
{
//when key exist then put new key
map.put(arr[i],map.get(arr[i])+1);
}
else
{
map.put(arr[i],1);
}
}
for(int key : map.keySet())
{
if (result.containsKey(map.get(key)))
{
result.get(map.get(key)).add(0,key);
}
else
{
// map.get(current_sum).add(i);
result.put(map.get(key), new ArrayList<Integer> ());
result.get(map.get(key)).add(0,key);
}
}
for (int key : result.keySet())
{
for(int value: result.get(key))
{

for (int i=0;i<key && j >=0 ;i++,j-- )
{
arr[j]=value;
}
}
}

}
public static void  print_data(int []arr)
{
int size=arr.length;
for (int i = 0; i < size; ++i)
{
System.out.print(arr[i]+"  ") ;
}
System.out.println();
}
public static void main(String[] args)
{
//Array element
int []arr = {1, 3, 7, 4, 7, 4, 3,7, 4, 8, 7};
print_data(arr);
occurrence(arr);
print_data(arr);
}
}``````

#### Output

``````1  3  7  4  7  4  3  7  4  8  7
7  7  7  7  4  4  4  3  3  1  8 ``````
``````#Python program
#Sort array elements by frequency

class Hashing:

def occurrence(self,listData):

frequency={}
output={}

for key in listData:

if key in frequency:
frequency[key] += 1
else:
frequency[key] = 1

for key, value in frequency.items():

if value in output:
output[value].insert(0,key)
else:
output[value] = [key]

j=len(listData)-1

for key, value in output.items():

for element in value:
i=0
while i<key and j >=0 :
listData[j]=element
i+=1
j-=1

def main():
obj = Hashing()
#define list element
item =[1, 3, 7, 4, 7, 4, 3,7, 4, 8, 7]
print(item)
obj.occurrence(item)
print(item)

if __name__=="__main__":
main()``````

#### Output

``````[1, 3, 7, 4, 7, 4, 3, 7, 4, 8, 7]
[7, 7, 7, 7, 4, 4, 4, 3, 3, 8, 1]``````
``````/*
C# program
Sort array elements by frequency
*/
using System;
using System.Collections.Generic;
public class Hashing
{
public static void occurrence(int []arr)
{
SortedDictionary<int,int> map = new SortedDictionary<int,int>();
SortedDictionary<int,List<int>> result = new SortedDictionary<int,List<int>>();
int j = arr.Length - 1;
for (int i=0;i<arr.Length ; i++ )
{
if (map.ContainsKey(arr[i]))
{
//when key exist then put new key
map[arr[i]]=map[arr[i]]+1;
}
else
{
map.Add(arr[i],1);
}
}
foreach(KeyValuePair<int, int> entry in map)
{

if (result.ContainsKey(entry.Value))
{
//when value exist then add new element in list
result[entry.Value].Insert(0,entry.Key);
}
else
{
result[entry.Value]=new List<int>(){entry.Key};
}
}

foreach(KeyValuePair<int, List<int>> entry in result)
{

for (int i = 0; i < (entry.Value).Count;i++ )
{
for (int k = 0; k <entry.Key ; k++,j--)
{
arr[j]=entry.Value[i];
}
}

}

}
public static void  print_data(int []arr)
{
int size=arr.Length;
for (int i = 0; i < size; ++i)
{
Console.Write(arr[i]+"  ") ;
}
Console.WriteLine();
}
public static void Main(String[] args)
{
//Array element
int []arr = {1, 3, 7, 4, 7, 4, 3,7, 4, 8, 7};
print_data (arr);
occurrence(arr);
print_data (arr);
}
}``````

#### Output

``````1  3  7  4  7  4  3  7  4  8  7
7  7  7  7  4  4  4  3  3  1  8 ``````
``````<?php
/*
* PHP Program
Sort array elements by frequency
*/

class Hashing
{

public function occurrence(&\$arr)
{
\$size= count(\$arr);

\$frequency=array();
\$output=array();
for (\$i=0; \$i < \$size ; ++\$i)
{
if(array_key_exists(\$arr[\$i],\$frequency))
{
\$frequency[\$arr[\$i]]= \$frequency[\$arr[\$i]]+1;
}
else
{
\$frequency[\$arr[\$i]]=1;
}

}
ksort(\$frequency);
foreach (\$frequency as \$key => \$value)
{
if(array_key_exists(\$value,\$output))
{
array_unshift(\$output[\$value],\$key);
}
else
{
\$output[\$value]=array(\$key);
}
}
\$j=\$size-1;
ksort(\$output);

foreach (\$output as \$key => \$value)
{
foreach (\$value as \$location => \$data)
{
for (\$i=0; \$i <\$key && \$j>=0 ; \$i++,\$j--)
{
\$arr[\$j]=\$data;
}
}
}
}
public function  print_data( \$arr)
{
\$size= count(\$arr);
for ( \$i = 0; \$i < \$size; ++\$i)
{
echo \$arr[\$i]."  " ;
}
echo "\n";
}

}

function main()
{
\$obj =new Hashing();
\$arr=array(1, 3, 7, 4, 7, 4, 3,7, 4, 8, 7);
\$obj->print_data(\$arr);
\$obj->occurrence(\$arr);
\$obj->print_data(\$arr);
}
main();
?>``````

#### Output

``````1  3  7  4  7  4  3  7  4  8  7
7  7  7  7  4  4  4  3  3  1  8 ``````

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