Get the best out of our app
GeeksforGeeks App
Open App
Browser
Continue

# 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```

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<``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 code``int` `main()``{` `    ``int` `arr[] = { 1, 2, 3, 4, 5, 6, 7 };``    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr[0]);` `    ``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[``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`

## 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[``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 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[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

 ``

## Javascript

 ``

Output

`4`

My Personal Notes arrow_drop_up