# Fill an array based on frequency where elements are in range from 0 to n-1

Given an array of positive integers with duplicate allowed. The array contains elements from range 0 to n-1. The task is to fill the array such that arr[i] contains the frequency of i.

Examples :

```Input  : arr[] = {1, 4, 3, 4, 1, 1, 4, 4, 4, 7}
Output : arr[] = {0, 3, 0, 1, 5, 0, 0, 1, 0, 0}
Here 0 appears 0 times, so arr[0] is 0
1 appears 3 times
2 appears 0 times
3 appears 0 times
4 appears 5 times
..........

Input  : arr[] = {1, 2, 3, 4, 1, 1, 4, 5, 6, 7}
Output : arr[] = {0, 3, 1, 1, 2, 1, 1, 1, 0, 0} ```

A simple solution of this problem is to run two loops. The outer loop picks elements one by one. The inner loop counts frequency of the picked element and stores frequency in final array.

An efficient solution is to use an auxiliary array of size n.

```1) Create an array temp[0..n-1] and
initialize it as 0.
2) Traverse given array and do following
for every element arr[i].
temp[arr[i]]++
3) Copy temp[] to arr[].```

Below is the implementation of above steps.

## C++

 `// C++ program to fill an array with frequencies. ``#include ``using` `namespace` `std; `` ` `// Fills arr[] with frequencies of elements ``void` `fillWithFreq(``int` `arr[], ``int` `n) ``{ ``    ``int` `temp[n]; ``    ``memset``(temp, 0, ``sizeof``(temp)); `` ` `    ``// Fill temp with frequencies ``    ``for` `(``int` `i=0; i

## Java

 `// Java program to fill an array with frequencies. ``import` `java.util.Arrays; `` ` `class` `GFG { ``     ` `    ``// Fills arr[] with frequencies of elements ``    ``static` `void` `fillWithFreq(``int` `arr[], ``int` `n) ``    ``{ ``         ` `        ``int` `temp[]=``new` `int``[n]; ``        ``Arrays.fill(temp, ``0``); ``     ` `        ``// Fill temp with frequencies ``        ``for` `(``int` `i = ``0``; i < n; i++) ``            ``temp[arr[i]] += ``1``; ``     ` `        ``// Copy temp to array ``        ``for` `(``int` `i = ``0``; i < n; i++) ``            ``arr[i] = temp[i]; ``    ``} ``     ` `    ``// Driver method ``    ``public` `static` `void` `main(String[] args) ``    ``{ ``         ` `        ``int` `arr[] = {``5``, ``2``, ``3``, ``4``, ``5``, ``5``, ``4``, ``5``, ``6``, ``7``}; ``        ``int` `n = arr.length; ``         ` `        ``fillWithFreq(arr, n); ``         ` `        ``for` `(``int` `i=``0``; i

## Python3

 `# Python3 program to fill an ``# array with frequencies. `` ` `# Fills arr[] with frequencies of elements ``def` `fillWithFreq(arr, n): `` ` `    ``temp ``=` `[``0` `for` `i ``in` `range``(n)] `` ` `    ``# Fill temp with frequencies ``    ``for` `i ``in` `range``(n): ``        ``temp[arr[i]] ``+``=` `1`` ` `    ``# Copy temp to array ``    ``for` `i ``in` `range``(n): ``        ``arr[i] ``=` `temp[i] `` ` `# Driver Code ``arr ``=` `[``5``, ``2``, ``3``, ``4``, ``5``, ``5``, ``4``, ``5``, ``6``, ``7``] ``n ``=` `len``(arr) ``fillWithFreq(arr, n) ``for` `i ``in` `range``(n): ``    ``print``(arr[i], end ``=` `" "``) ``     ` `# This code is contributed by Anant Agarwal. `

## C#

 `// C# program to fill an  ``// array with frequencies. ``using` `System; `` ` `class` `GFG { ``     ` `    ``// Fills arr[] with frequencies of elements ``    ``static` `void` `fillWithFreq(``int` `[]arr, ``int` `n) ``    ``{ ``        ``int` `[]temp = ``new` `int``[n]; ``        ``for``(``int` `i = 0; i < n; i++) ``         ``temp[i] = 0; ``          ` `        ``// Fill temp with frequencies ``        ``for` `(``int` `i = 0; i < n; i++) ``            ``temp[arr[i]] += 1; ``     ` `        ``// Copy temp to array ``        ``for` `(``int` `i = 0; i < n; i++) ``            ``arr[i] = temp[i]; ``    ``} ``     ` `    ``// Driver method ``    ``public` `static` `void` `Main() ``    ``{ ``        ``int` `[]arr = {5, 2, 3, 4, 5, ``                     ``5, 4, 5, 6, 7}; ``        ``int` `n = arr.Length; ``         ` `        ``// Function calling ``        ``fillWithFreq(arr, n); ``         ` `        ``for` `(``int` `i = 0; i < n; i++) ``        ``Console.Write(arr[i] + ``" "``); ``    ``} ``} `` ` `// This code is contributed by nitin mittal. `

## PHP

 ` `

## Javascript

 ``

Output
`0 0 1 1 2 4 1 1 0 0 `

Time Complexity : O(n)
Auxiliary Space : O(n)

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