Count number of primes in an array

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

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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++

 // CPP program to find count of // primes in given array. #include using namespace std;    // Function to find count of prime int 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 prime(max_val + 1, true);        // Remaining part of SIEVE     prime = false;     prime = 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 int main() {        int arr[] = { 1, 2, 3, 4, 5, 6, 7 };     int n = sizeof(arr) / sizeof(arr);        cout << primeCount(arr, n);        return 0; }

Java

 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 = false;         prime = 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

Python3

 # Python 3 program to find count of # primes in given array. from math import sqrt    # Function to find count of prime def primeCount(arr, n):            # Find maximum value in the array     max_val = arr;     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 = False     prime = 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 code if __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#

 // 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 = false;         prime = 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



Output:

4

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