给定N个正整数的数组arr []。任务是编写一个程序以计算给定数组中素数的数量。
例子：
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
天真的方法：一个简单的解决方案是遍历数组并继续检查每个元素是否为素数，并同时保留素数的计数。
高效方法：使用Eratosthenes筛子生成所有素数，直到数组的最大元素，并将它们存储在哈希中。现在遍历数组，并使用哈希表查找那些素数的元素的计数。
下面是上述方法的实现：
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[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
// PrinciRaj1992Python3
# 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_SharmaC#
// 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 29AjayKumarPHP
输出：
4
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