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Length of Longest Prime Subsequence in an Array
• Last Updated : 20 May, 2021

Given an array arr containing non-negative integers, the task is to print the length of the longest subsequence of prime numbers in the array.
Examples:

Input: arr[] = { 3, 4, 11, 2, 9, 21 }
Output:
Longest Prime Subsequence is {3, 2, 11} and hence the answer is 3.
Input: arr[] = { 6, 4, 10, 13, 9, 25 }
Output:

Approach:

• Traverse the given array.
• For each element in the array, check if it prime or not.
• If the element is prime, it will be in Longest Prime Subsequence. Hence, increment the required length of Longest Prime Subsequence by 1

Below is the implementation of the above approach:

## C++

 `// C++ program to find the length of``// Longest Prime Subsequence in an Array` `#include ``using` `namespace` `std;``#define N 100005` `// Function to create Sieve``// to check primes``void` `SieveOfEratosthenes(``    ``bool` `prime[], ``int` `p_size)``{` `    ``// False here indicates``    ``// that it is not prime``    ``prime = ``false``;``    ``prime = ``false``;` `    ``for` `(``int` `p = 2; p * p <= p_size; 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 <= p_size;``                 ``i += p)``                ``prime[i] = ``false``;``        ``}``    ``}``}` `// Function to find the longest subsequence``// which contain all prime numbers``int` `longestPrimeSubsequence(``int` `arr[], ``int` `n)``{``    ``bool` `prime[N + 1];``    ``memset``(prime, ``true``, ``sizeof``(prime));` `    ``// Precompute N primes``    ``SieveOfEratosthenes(prime, N);` `    ``int` `answer = 0;` `    ``// Find the length of``    ``// longest prime subsequence``    ``for` `(``int` `i = 0; i < n; i++) {``        ``if` `(prime[arr[i]]) {``            ``answer++;``        ``}``    ``}` `    ``return` `answer;``}` `// Driver code``int` `main()``{``    ``int` `arr[] = { 3, 4, 11, 2, 9, 21 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr);` `    ``// Function call``    ``cout << longestPrimeSubsequence(arr, n)``         ``<< endl;` `    ``return` `0;``}`

## Java

 `// Java program to find the length of``// Longest Prime Subsequence in an Array``import` `java.util.*;` `class` `GFG``{``static` `final` `int` `N = ``100005``;`` ` `// Function to create Sieve``// to check primes``static` `void` `SieveOfEratosthenes(``    ``boolean` `prime[], ``int` `p_size)``{`` ` `    ``// False here indicates``    ``// that it is not prime``    ``prime[``0``] = ``false``;``    ``prime[``1``] = ``false``;`` ` `    ``for` `(``int` `p = ``2``; p * p <= p_size; 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 <= p_size;``                 ``i += p)``                ``prime[i] = ``false``;``        ``}``    ``}``}`` ` `// Function to find the longest subsequence``// which contain all prime numbers``static` `int` `longestPrimeSubsequence(``int` `arr[], ``int` `n)``{``    ``boolean` `[]prime = ``new` `boolean``[N + ``1``];``    ``Arrays.fill(prime, ``true``);`` ` `    ``// Precompute N primes``    ``SieveOfEratosthenes(prime, N);`` ` `    ``int` `answer = ``0``;`` ` `    ``// Find the length of``    ``// longest prime subsequence``    ``for` `(``int` `i = ``0``; i < n; i++) {``        ``if` `(prime[arr[i]]) {``            ``answer++;``        ``}``    ``}`` ` `    ``return` `answer;``}`` ` `// Driver code``public` `static` `void` `main(String[] args)``{``    ``int` `arr[] = { ``3``, ``4``, ``11``, ``2``, ``9``, ``21` `};``    ``int` `n = arr.length;`` ` `    ``// Function call``    ``System.out.print(longestPrimeSubsequence(arr, n)``         ``+``"\n"``);``}``}` `// This code is contributed by 29AjayKumar`

## Python3

 `# Python 3 program to find the length of``# Longest Prime Subsequence in an Array``N ``=` `100005`` ` `# Function to create Sieve``# to check primes``def` `SieveOfEratosthenes(prime,  p_size):`` ` `    ``# False here indicates``    ``# that it is not prime``    ``prime[``0``] ``=` `False``    ``prime[``1``] ``=` `False`` ` `    ``p ``=` `2``    ``while`  `p ``*` `p <``=` `p_size:`` ` `        ``# 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``, p_size ``+` `1``, p):``                ``prime[i] ``=` `False` `        ``p ``+``=` `1``      ` `# Function to find the longest subsequence``# which contain all prime numbers``def` `longestPrimeSubsequence( arr, n):``    ``prime ``=` `[``True``]``*``(N ``+` `1``)`` ` `    ``# Precompute N primes``    ``SieveOfEratosthenes(prime, N)`` ` `    ``answer ``=` `0`` ` `    ``# Find the length of``    ``# longest prime subsequence``    ``for` `i ``in` `range` `(n):``        ``if` `(prime[arr[i]]):``            ``answer ``+``=` `1`` ` `    ``return` `answer`` ` `# Driver code``if` `__name__ ``=``=` `"__main__"``:``    ``arr ``=` `[ ``3``, ``4``, ``11``, ``2``, ``9``, ``21` `]``    ``n ``=` `len``(arr)`` ` `    ``# Function call``    ``print` `(longestPrimeSubsequence(arr, n))` `# This code is contributed by chitranayal`

## C#

 `// C# program to find the length of``// longest Prime Subsequence in an Array``using` `System;` `class` `GFG``{``static` `readonly` `int` `N = 100005;``  ` `// Function to create Sieve``// to check primes``static` `void` `SieveOfEratosthenes(``    ``bool` `[]prime, ``int` `p_size)``{``  ` `    ``// False here indicates``    ``// that it is not prime``    ``prime = ``false``;``    ``prime = ``false``;``  ` `    ``for` `(``int` `p = 2; p * p <= p_size; 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 <= p_size;``                 ``i += p)``                ``prime[i] = ``false``;``        ``}``    ``}``}``  ` `// Function to find the longest subsequence``// which contain all prime numbers``static` `int` `longestPrimeSubsequence(``int` `[]arr, ``int` `n)``{``    ``bool` `[]prime = ``new` `bool``[N + 1];``    ``for` `(``int` `i = 0; i < N+1; i++)``        ``prime[i] = ``true``;``  ` `    ``// Precompute N primes``    ``SieveOfEratosthenes(prime, N);``  ` `    ``int` `answer = 0;``  ` `    ``// Find the length of``    ``// longest prime subsequence``    ``for` `(``int` `i = 0; i < n; i++) {``        ``if` `(prime[arr[i]]) {``            ``answer++;``        ``}``    ``}``  ` `    ``return` `answer;``}``  ` `// Driver code``public` `static` `void` `Main(String[] args)``{``    ``int` `[]arr = { 3, 4, 11, 2, 9, 21 };``    ``int` `n = arr.Length;``  ` `    ``// Function call``    ``Console.Write(longestPrimeSubsequence(arr, n)``         ``+``"\n"``);``}``}`` ` `// This code is contributed by 29AjayKumar`

## Javascript

 ``
Output:
`3`

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