Sort prime numbers of an array in descending order

11 May, 2021
Given an array of integers ‘arr’, the task is to sort all the prime numbers from the array in descending order in their relative positions i.e. other positions of the other elements must not be affected.Examples:  
Input: arr[] = {2, 5, 8, 4, 3}
Output: 5 3 8 4 2

Input: arr[] = {10, 12, 2, 6, 5}
Output: 10 12 5 6 2
 
Approach:  
Create a sieve to check whether an element is prime or not in O(1).
Traverse the array and check if the number is prime. If it is prime, store it in a vector.
Then, sort the vector in descending order.
Again traverse the array and replace the prime numbers with the vector elements one by one.
Below is the implementation of the above approach: 
C++
Java
Python3
C#
Javascript
// C++ implementation of the approach#include <bits/stdc++.h>using namespace std; bool prime[100005]; void SieveOfEratosthenes(int n){     memset(prime, true, sizeof(prime));     // false here indicates    // that it is not prime    prime[1] = false;     for (int p = 2; p * p <= n; p++) {         // If prime[p] is not changed,        // then it is a prime        if (prime[p]) {             // Update all multiples of p,            // set them to non-prime            for (int i = p * 2; i <= n; i += p)                prime[i] = false;        }    }} // Function that sorts// all the prime numbers// from the array in descendingvoid sortPrimes(int arr[], int n){    SieveOfEratosthenes(100005);     // this vector will contain    // prime numbers to sort    vector<int> v;     for (int i = 0; i < n; i++) {         // if the element is prime        if (prime[arr[i]])            v.push_back(arr[i]);    }     sort(v.begin(), v.end(), greater<int>());     int j = 0;     // update the array elements    for (int i = 0; i < n; i++) {        if (prime[arr[i]])            arr[i] = v[j++];    }} // Driver codeint main(){     int arr[] = { 4, 3, 2, 6, 100, 17 };    int n = sizeof(arr) / sizeof(arr[0]);     sortPrimes(arr, n);     // print the results.    for (int i = 0; i < n; i++) {        cout << arr[i] << " ";    }     return 0;}
// Java implementation of the approachimport java.util.*; class GFG{     static boolean prime[] = new boolean[100005];     static void SieveOfEratosthenes(int n)    {         Arrays.fill(prime, true);         // false here indicates        // that it is not prime        prime[1] = false;         for (int p = 2; p * p <= n; p++)        {             // If prime[p] is not changed,            // then it is a prime            if (prime[p]) {                 // Update all multiples of p,                // set them to non-prime                for (int i = p * 2; i < n; i += p)                {                    prime[i] = false;                }            }        }    }     // Function that sorts    // all the prime numbers    // from the array in descending    static void sortPrimes(int arr[], int n)    {        SieveOfEratosthenes(100005);         // this vector will contain        // prime numbers to sort        Vector<Integer> v = new Vector<Integer>();         for (int i = 0; i < n; i++)        {             // if the element is prime            if (prime[arr[i]])            {                v.add(arr[i]);            }        }        Comparator comparator = Collections.reverseOrder();        Collections.sort(v, comparator);         int j = 0;         // update the array elements        for (int i = 0; i < n; i++)        {            if (prime[arr[i]])            {                arr[i] = v.get(j++);            }        }    }     // Driver code    public static void main(String[] args)    {        int arr[] = {4, 3, 2, 6, 100, 17};        int n = arr.length;         sortPrimes(arr, n);         // print the results.        for (int i = 0; i < n; i++)        {            System.out.print(arr[i] + " ");        }    }} // This code is contributed by 29AjayKumar
# Python3 implementation of the approach def SieveOfEratosthenes(n):     # false here indicates    # that it is not prime    prime[1] = False    p = 2    while p * p <= n:         # If prime[p] is not changed,        # then it is a prime        if prime[p]:             # Update all multiples of p,            # set them to non-prime            for i in range(p * 2, n + 1, p):                prime[i] = False                 p += 1 # Function that sorts all the prime# numbers from the array in descendingdef sortPrimes(arr, n):     SieveOfEratosthenes(100005)     # This vector will contain    # prime numbers to sort    v = []    for i in range(0, n):         # If the element is prime        if prime[arr[i]]:            v.append(arr[i])     v.sort(reverse = True)    j = 0     # update the array elements    for i in range(0, n):        if prime[arr[i]]:            arr[i] = v[j]            j += 1                 return arr     # Driver codeif __name__ == "__main__":     arr = [4, 3, 2, 6, 100, 17]    n = len(arr)         prime = [True] * 100006    arr = sortPrimes(arr, n)     # print the results.    for i in range(0, n):        print(arr[i], end = " ")     # This code is contributed by Rituraj Jain
// C# implementation of the approachusing System;using System.Collections.Generic; class GFG{     static bool []prime = new bool[100005];     static void SieveOfEratosthenes(int n)    {         for(int i = 0; i < 100005; i++)            prime[i] = true;         // false here indicates        // that it is not prime        prime[1] = false;         for (int p = 2; p * p <= n; p++)        {             // If prime[p] is not changed,            // then it is a prime            if (prime[p])            {                 // Update all multiples of p,                // set them to non-prime                for (int i = p * 2; i < n; i += p)                {                    prime[i] = false;                }            }        }    }     // Function that sorts    // all the prime numbers    // from the array in descending    static void sortPrimes(int []arr, int n)    {        SieveOfEratosthenes(100005);         // this vector will contain        // prime numbers to sort        List<int> v = new List<int>();         for (int i = 0; i < n; i++)        {             // if the element is prime            if (prime[arr[i]])            {                v.Add(arr[i]);            }        }        v.Sort();        v.Reverse();         int j = 0;         // update the array elements        for (int i = 0; i < n; i++)        {            if (prime[arr[i]])            {                arr[i] = v[j++];            }        }    }     // Driver code    public static void Main(String[] args)    {        int []arr = {4, 3, 2, 6, 100, 17};        int n = arr.Length;         sortPrimes(arr, n);         // print the results.        for (int i = 0; i < n; i++)        {            Console.Write(arr[i] + " ");        }    }} // This code contributed by Rajput-Ji
<script> // Javascript implementation of the approach var prime = Array(100005).fill(true); function SieveOfEratosthenes( n){     // false here indicates    // that it is not prime    prime[1] = false;     for (var p = 2; p * p <= n; p++) {         // If prime[p] is not changed,        // then it is a prime        if (prime[p]) {             // Update all multiples of p,            // set them to non-prime            for (var i = p * 2; i <= n; i += p)                prime[i] = false;        }    }} // Function that sorts// all the prime numbers// from the array in descendingfunction sortPrimes(arr, n){    SieveOfEratosthenes(100005);     // this vector will contain    // prime numbers to sort    var v = [];     for (var i = 0; i < n; i++) {         // if the element is prime        if (prime[arr[i]])            v.push(arr[i]);    }     v.sort((a,b)=>b-a)     var j = 0;     // update the array elements    for (var i = 0; i < n; i++) {        if (prime[arr[i]])            arr[i] = v[j++];    }} // Driver codevar arr = [4, 3, 2, 6, 100, 17 ];var n = arr.length;sortPrimes(arr, n);// print the results.for (var i = 0; i < n; i++) {    document.write( arr[i] + " ");}  </script>
4 17 3 6 100 2
 
rituraj_jain
29AjayKumar
Rajput-Ji
rrrtnx
Prime Number
sieve
Arrays
Competitive Programming
Mathematical
Sorting
Arrays
Mathematical
Sorting
Prime Number
sieve
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.