Count number of primes in an array

21 May, 2021
Given an array arr[] of N positive integers. The task is to write a program to count the number of prime elements in the given array.Examples:  
Input: arr[] = {1, 3, 4, 5, 7}
Output: 3
There are three primes, 3, 5 and 7

Input: arr[] = {1, 2, 3, 4, 5, 6, 7}
Output: 4
 
Naive Approach: A simple solution is to traverse the array and keep checking for every element if it is prime or not and keep the count of the prime elements at the same time.Efficient Approach: Generate all primes upto maximum element of the array using sieve of Eratosthenes and store them in a hash. Now traverse the array and find the count of those elements which are prime using the hash table. Below is the implementation of above approach:  
C++
Java
Python3
C#
PHP
Javascript
// CPP program to find count of// primes in given array.#include <bits/stdc++.h>using namespace std; // Function to find count of primeint primeCount(int arr[], int n){    // Find maximum value in the array    int max_val = *max_element(arr, arr+n);     // USE SIEVE TO FIND ALL PRIME NUMBERS LESS    // THAN OR EQUAL TO max_val    // Create a boolean array "prime[0..n]". A    // value in prime[i] will finally be false    // if i is Not a prime, else true.    vector<bool> prime(max_val + 1, true);     // Remaining part of SIEVE    prime[0] = false;    prime[1] = false;    for (int p = 2; p * p <= max_val; p++) {         // If prime[p] is not changed, then        // it is a prime        if (prime[p] == true) {             // Update all multiples of p            for (int i = p * 2; i <= max_val; i += p)                prime[i] = false;        }    }     // Find all primes in arr[]    int count = 0;    for (int i = 0; i < n; i++)        if (prime[arr[i]])            count++;        return count;} // Driver codeint main(){     int arr[] = { 1, 2, 3, 4, 5, 6, 7 };    int n = sizeof(arr) / sizeof(arr[0]);     cout << primeCount(arr, n);     return 0;}
import java.util.Arrays;import java.util.Vector; // Java program to find count of// primes in given array.class GFG{     // Function to find count of prime    static int primeCount(int arr[], int n)    {        // Find maximum value in the array        //.*max_element(arr, arr+n);        int max_val = Arrays.stream(arr).max().getAsInt();         // USE SIEVE TO FIND ALL PRIME NUMBERS LESS        // THAN OR EQUAL TO max_val        // Create a boolean array "prime[0..n]". A        // value in prime[i] will finally be false        // if i is Not a prime, else true.        Boolean[] prime = new Boolean[max_val + 1];        for (int i = 0; i < max_val + 1; i++)        {            prime[i] = true;        }         // Remaining part of SIEVE        prime[0] = false;        prime[1] = false;        for (int p = 2; p * p <= max_val; p++)        {             // If prime[p] is not changed, then            // it is a prime            if (prime[p] == true)            {                 // Update all multiples of p                for (int i = p * 2; i <= max_val; i += p)                {                    prime[i] = false;                }            }        }         // Find all primes in arr[]        int count = 0;        for (int i = 0; i < n; i++)        {            if (prime[arr[i]])            {                count++;            }        }         return count;    }     // Driver code    public static void main(String[] args)    {        int arr[] = {1, 2, 3, 4, 5, 6, 7};        int n = arr.length;        System.out.println(primeCount(arr, n));    }} // This code is contributed by// PrinciRaj1992
# Python 3 program to find count of# primes in given array.from math import sqrt # Function to find count of primedef primeCount(arr, n):         # Find maximum value in the array    max_val = arr[0];    for i in range(len(arr)):        if(arr[i] > max_val):            max_val = arr[i]     # USE SIEVE TO FIND ALL PRIME NUMBERS    # LESS THAN OR EQUAL TO max_val    # Create a boolean array "prime[0..n]".    # A value in prime[i] will finally be    # false if i is Not a prime, else true.    prime =[ True for i in range(max_val + 1)]     # Remaining part of SIEVE    prime[0] = False    prime[1] = False    k = int(sqrt(max_val)) + 1    for p in range(2, k, 1):                 # If prime[p] is not changed,        # then it is a prime        if (prime[p] == True):                         # Update all multiples of p            for i in range(p * 2, max_val + 1, p):                prime[i] = False     # Find all primes in arr[]    count = 0    for i in range(0, n, 1):        if (prime[arr[i]]):            count += 1     return count # Driver codeif __name__ == '__main__':    arr = [1, 2, 3, 4, 5, 6, 7]    n = len(arr)     print(primeCount(arr, n)) # This code is contributed by# Shashank_Sharma
// C# program to find count of// primes in given array.using System;using System.Linq; class GFG{     // Function to find count of prime    static int primeCount(int []arr, int n)    {                 // Find maximum value in the array        //.*max_element(arr, arr+n);        int max_val = arr.Max();         // USE SIEVE TO FIND ALL PRIME NUMBERS LESS        // THAN OR EQUAL TO max_val        // Create a boolean array "prime[0..n]". A        // value in prime[i] will finally be false        // if i is Not a prime, else true.        Boolean[] prime = new Boolean[max_val + 1];        for (int i = 0; i < max_val + 1; i++)        {            prime[i] = true;        }         // Remaining part of SIEVE        prime[0] = false;        prime[1] = false;        for (int p = 2; p * p <= max_val; p++)        {             // If prime[p] is not changed, then            // it is a prime            if (prime[p] == true)            {                 // Update all multiples of p                for (int i = p * 2; i <= max_val; i += p)                {                    prime[i] = false;                }            }        }         // Find all primes in arr[]        int count = 0;        for (int i = 0; i < n; i++)        {            if (prime[arr[i]])            {                count++;            }        }        return count;    }     // Driver code    public static void Main()    {        int []arr = {1, 2, 3, 4, 5, 6, 7};        int n = arr.Length;        Console.WriteLine(primeCount(arr, n));    }} //This code is contributed by 29AjayKumar
<?php// PHP program to find count// of primes in given array. // Function to find count of primefunction primeCount($arr, $n){    // Find maximum value in the array    $max_val = max($arr);     // Use Sieve to find all Prime Numbers    // less than or equal to max_val    // Create a boolean array "prime[0..n]". A    // value in prime[i] will finally be false    // if i is Not a prime, else true.    $prime = array_fill(0, $max_val + 1, true);     // Remaining part of SIEVE    $prime[0] = false;    $prime[1] = false;    for ($p = 2; $p * $p <= $max_val; $p++)    {         // If prime[p] is not changed,        // then it is a prime        if ($prime[$p] == true)        {             // Update all multiples of p            for ($i = $p * 2;                 $i <= $max_val; $i += $p)                $prime[$i] = false;        }    }     // Find all primes in arr[]    $count = 0;    for ($i = 0; $i < $n; $i++)        if ($prime[$arr[$i]])            $count++;     return $count;} // Driver code$arr = array(1, 2, 3, 4, 5, 6, 7 );$n = sizeof($arr); echo primeCount($arr, $n); // This code is contributed by mits?>
<script>// Javascript program to find count// of primes in given array. // Function to find count of primefunction primeCount(arr, n){    // Find maximum value in the array    let max_val = arr.sort((a, b) => b - a)[0];     // Use Sieve to find all Prime Numbers    // less than or equal to max_val    // Create a boolean array "prime[0..n]". A    // value in prime[i] will finally be false    // if i is Not a prime, else true.    let prime = new Array(max_val + 1).fill(true);     // Remaining part of SIEVE    prime[0] = false;    prime[1] = false;    for (let p = 2; p * p <= max_val; p++)    {         // If prime[p] is not changed,        // then it is a prime        if (prime[p] == true)        {             // Update all multiples of p            for (let i = p * 2;                i <= max_val; i += p)                prime[i] = false;        }    }     // Find all primes in arr[]    let count = 0;    for (let i = 0; i < n; i++)        if (prime[arr[i]])            count++;     return count;} // Driver codelet arr = new Array(1, 2, 3, 4, 5, 6, 7 );let n = arr.length; document.write(primeCount(arr, n)); // This code is contributed by _saurabh_jaiswal</script>
4
 
Mithun Kumar
Shashank_Sharma
princiraj1992
29AjayKumar
nidhi_biet
_saurabh_jaiswal
Prime Number
sieve
Arrays
Competitive Programming
Arrays
Prime Number
sieve
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