Count number of primes in an array
Last Updated :
01 Sep, 2022
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++
#include <bits/stdc++.h>
using namespace std;
int primeCount(int arr[], int n)
{
int max_val = *max_element(arr, arr+n);
vector<bool> prime(max_val + 1, true);
prime[0] = false;
prime[1] = false;
for (int p = 2; p * p <= max_val; p++) {
if (prime[p] == true) {
for (int i = p * 2; i <= max_val; i += p)
prime[i] = false;
}
}
int count = 0;
for (int i = 0; i < n; i++)
if (prime[arr[i]])
count++;
return count;
}
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;
class GFG
{
static int primeCount(int arr[], int n)
{
int max_val = Arrays.stream(arr).max().getAsInt();
Boolean[] prime = new Boolean[max_val + 1];
for (int i = 0; i < max_val + 1; i++)
{
prime[i] = true;
}
prime[0] = false;
prime[1] = false;
for (int p = 2; p * p <= max_val; p++)
{
if (prime[p] == true)
{
for (int i = p * 2; i <= max_val; i += p)
{
prime[i] = false;
}
}
}
int count = 0;
for (int i = 0; i < n; i++)
{
if (prime[arr[i]])
{
count++;
}
}
return count;
}
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));
}
}
|
Python3
from math import sqrt
def primeCount(arr, n):
max_val = arr[0];
for i in range(len(arr)):
if(arr[i] > max_val):
max_val = arr[i]
prime =[ True for i in range(max_val + 1)]
prime[0] = False
prime[1] = False
k = int(sqrt(max_val)) + 1
for p in range(2, k, 1):
if (prime[p] == True):
for i in range(p * 2, max_val + 1, p):
prime[i] = False
count = 0
for i in range(0, n, 1):
if (prime[arr[i]]):
count += 1
return count
if __name__ == '__main__':
arr = [1, 2, 3, 4, 5, 6, 7]
n = len(arr)
print(primeCount(arr, n))
|
C#
using System;
using System.Linq;
class GFG
{
static int primeCount(int []arr, int n)
{
int max_val = arr.Max();
Boolean[] prime = new Boolean[max_val + 1];
for (int i = 0; i < max_val + 1; i++)
{
prime[i] = true;
}
prime[0] = false;
prime[1] = false;
for (int p = 2; p * p <= max_val; p++)
{
if (prime[p] == true)
{
for (int i = p * 2; i <= max_val; i += p)
{
prime[i] = false;
}
}
}
int count = 0;
for (int i = 0; i < n; i++)
{
if (prime[arr[i]])
{
count++;
}
}
return count;
}
public static void Main()
{
int []arr = {1, 2, 3, 4, 5, 6, 7};
int n = arr.Length;
Console.WriteLine(primeCount(arr, n));
}
}
|
PHP
<?php
function primeCount($arr, $n)
{
$max_val = max($arr);
$prime = array_fill(0, $max_val + 1, true);
$prime[0] = false;
$prime[1] = false;
for ($p = 2; $p * $p <= $max_val; $p++)
{
if ($prime[$p] == true)
{
for ($i = $p * 2;
$i <= $max_val; $i += $p)
$prime[$i] = false;
}
}
$count = 0;
for ($i = 0; $i < $n; $i++)
if ($prime[$arr[$i]])
$count++;
return $count;
}
$arr = array(1, 2, 3, 4, 5, 6, 7 );
$n = sizeof($arr);
echo primeCount($arr, $n);
?>
|
Javascript
<script>
function primeCount(arr, n)
{
let max_val = arr.sort((a, b) => b - a)[0];
let prime = new Array(max_val + 1).fill(true);
prime[0] = false;
prime[1] = false;
for (let p = 2; p * p <= max_val; p++)
{
if (prime[p] == true)
{
for (let i = p * 2;
i <= max_val; i += p)
prime[i] = false;
}
}
let count = 0;
for (let i = 0; i < n; i++)
if (prime[arr[i]])
count++;
return count;
}
let arr = new Array(1, 2, 3, 4, 5, 6, 7 );
let n = arr.length;
document.write(primeCount(arr, n));
</script>
|
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