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Replace every elements in the array by its frequency in the array
• Difficulty Level : Basic
• Last Updated : 28 May, 2021

Given an array of integers, replace every element by its frequency in the array.

Examples:

```Input : arr[] = { 1, 2, 5, 2, 2, 5 }
Output : 1 3 2 3 3 2

Input : arr[] = { 4 5 4 5 6 6 6 }
Output : 2 2 2 2 3 3 3```

Approach:

1. Take a hash map, which will store the frequency of all the elements in the array.
2. Now, traverse once again.
3. Now, replace all the elements by its frequency.
4. Print the modified array.

## C++

 `// C++ program to replace the elements``// by their frequency in the array.``#include "iostream"``#include "unordered_map"``using` `namespace` `std;` `void` `ReplaceElementsByFrequency(``int` `arr[], ``int` `n)``{``    ``// Hash map which will store the``    ``// frequency of the elements of the array.``    ``unordered_map<``int``, ``int``> mp;` `    ``for` `(``int` `i = 0; i < n; ++i) {` `        ``// Increment the frequency``        ``// of the element by 1.``        ``mp[arr[i]]++;``    ``}` `    ``// Replace every element by its frequency``    ``for` `(``int` `i = 0; i < n; ++i) {``        ``arr[i] = mp[arr[i]];``    ``}``}` `int` `main()``{``    ``int` `arr[] = { 1, 2, 5, 2, 2, 5 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr);` `    ``ReplaceElementsByFrequency(arr, n);` `    ``// Print the modified array.``    ``for` `(``int` `i = 0; i < n; ++i) {``        ``cout << arr[i] << ``" "``;``    ``}``    ``return` `0;``}`

## Java

 `import` `java.util.HashMap;` `// Java program to replace the elements``// by their frequency in the array.``class` `GFG {` `    ``static` `void` `ReplaceElementsByFrequency(``int` `arr[], ``int` `n) {``        ``// Hash map which will store the``        ``// frequency of the elements of the array.``        ``HashMap mp = ``new` `HashMap();` `        ``for` `(``int` `i = ``0``; i < n; ++i) {` `            ``// Increment the frequency``            ``// of the element by 1.``            ``if` `(mp.get(arr[i]) == ``null``) {``                ``mp.put(arr[i], ``1``);``            ``} ``else` `{``                ``mp.put(arr[i], (mp.get(arr[i]) + ``1``));``            ``}``            ``//mp[arr[i]]++;``        ``}` `        ``// Replace every element by its frequency``        ``for` `(``int` `i = ``0``; i < n; ++i) {``            ``if` `(mp.get(arr[i]) != ``null``) {``                ``arr[i] = mp.get(arr[i]);``            ``}``            ``//arr[i] = mp[arr[i]];``        ``}``    ``}` `    ``public` `static` `void` `main(String[] args) {``        ``int` `arr[] = {``1``, ``2``, ``5``, ``2``, ``2``, ``5``};``        ``int` `n = arr.length;` `        ``ReplaceElementsByFrequency(arr, n);` `        ``// Print the modified array.``        ``for` `(``int` `i = ``0``; i < n; ++i) {``            ``System.out.print(arr[i] + ``" "``);``        ``}``    ``}``}``// This code contributed by 29AJayKumar`

## Python3

 `# Python 3 program to replace the elements``# by their frequency in the array.` `def` `ReplaceElementsByFrequency(arr, n):``    ` `    ``# Hash map which will store the``    ``# frequency of the elements of the array.``    ``mp ``=` `{i:``0` `for` `i ``in` `range``(``len``(arr))}` `    ``for` `i ``in` `range``(n):``        ` `        ``# Increment the frequency of the``        ``# element by 1.``        ``mp[arr[i]] ``+``=` `1` `    ``# Replace every element by its frequency``    ``for` `i ``in` `range``(n):``        ``arr[i] ``=` `mp[arr[i]]` `# Driver Code``if` `__name__ ``=``=` `'__main__'``:``    ``arr ``=` `[``1``, ``2``, ``5``, ``2``, ``2``, ``5``]``    ``n ``=` `len``(arr)` `    ``ReplaceElementsByFrequency(arr, n);` `    ``# Print the modified array.``    ``for` `i ``in` `range``(n):``        ``print``(arr[i], end ``=` `" "``)` `# This code is contributed by``# Sahil_shelangia`

## C#

 `// C# program to replace the elements``// by their frequency in the array.``using` `System;``using` `System.Collections.Generic;``    ` `class` `GFG``{` `    ``static` `void` `ReplaceElementsByFrequency(``int` `[]arr, ``int` `n)``    ``{``        ``// Hash map which will store the``        ``// frequency of the elements of the array.``        ``Dictionary<``int``,``int``> mp = ``new` `Dictionary<``int``,``int``>();` `        ``for` `(``int` `i = 0; i < n; ++i)``        ``{` `            ``// Increment the frequency``            ``// of the element by 1.``            ``if` `(!mp.ContainsKey(arr[i]))``            ``{``                ``mp.Add(arr[i], 1);``            ``}``            ``else``            ``{``                ``var` `a = mp[arr[i]] + 1;``                ``mp.Remove(arr[i]);``                ``mp.Add(arr[i], a);``            ``}``        ``}` `        ``// Replace every element by its frequency``        ``for` `(``int` `i = 0; i < n; ++i)``        ``{``            ``if` `(mp[arr[i]] != 0)``            ``{``                ``arr[i] = mp[arr[i]];``            ``}``            ``//arr[i] = mp[arr[i]];``        ``}``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main(String[] args)``    ``{``        ``int` `[]arr = {1, 2, 5, 2, 2, 5};``        ``int` `n = arr.Length;` `        ``ReplaceElementsByFrequency(arr, n);` `        ``// Print the modified array.``        ``for` `(``int` `i = 0; i < n; ++i)``        ``{``            ``Console.Write(arr[i] + ``" "``);``        ``}``    ``}``}` `// This code contributed by Rajput-Ji`

## Javascript

 ``

Output :

`1 3 2 3 3 2`

Time Complexity – O(N)

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