给定数字N。如果数字不是素数，则任务是打印最接近的素数，方法是从2开始依次添加素数以使其成为素数。
例子：
Input: N = 8
Output: 13
8 is not prime, so add the first prime to it to get 10
10 is not prime, hence add the second prime, i.e., 3 to get 13 which is prime.
Input: N = 45
Output: 47
方法使用Eratosthenes筛，在isprime []列表中以1标记质数索引，并将所有质数存储在列表prime []中。继续按顺序将质数加到N，直到它变成质数为止。
下面是上述方法的实现：
C++
// C++ program to print the
// nearest prime number by
// sequentially adding the
// prime numbers
#include
using namespace std;
// Function to store prime
// numbers using prime sieve
void prime_sieve(int MAX, vector &isprime,
vector &prime)
{
// iterate for all
// the numbers
int i = 2;
while (i * i <= MAX)
{
// If prime[p] is not changed,
// then it is a prime
if (isprime[i] == 1)
{
// append the prime
// to the list
prime.push_back(i);
// Update all multiples of p
for (int j = i * 2; j < MAX; j += i)
{
isprime[j] = 0;
}
}
i += 1;
}
}
// Function to print
// the nearest prime
int printNearest(int N)
{
int MAX = 1e6;
// store all the
// index with 1
vector isprime(MAX, 1);
// 0 and 1 are not prime
isprime[0] = isprime[1] = 0;
// list to store
// prime numbers
vector prime;
// variable to
// add primes
int i = 0;
// call the sieve function
prime_sieve(MAX, isprime, prime);
// Keep on adding prime
// numbers till the nearest
// prime number is achieved
while (!isprime[N])
{
N += prime[i];
i += 1;
}
// return the
// nearest prime
return N ;
}
// Driver Code
int main()
{
int N = 8;
printf("%d", printNearest(N));
return 0;
}
// This code is contributed
// by Harshit Saini Java
// Java program to print the
// nearest prime number by
// sequentially adding the
// prime numbers
import java.util.*;
class GFG
{
// Function to store prime
// numbers using prime sieve
static void prime_sieve(int MAX, int []isprime,
Vector prime)
{
// iterate for all
// the numbers
int i = 2;
while (i * i <= MAX)
{
// If prime[p] is not changed,
// then it is a prime
if (isprime[i] == 1)
{
// append the prime
// to the list
prime.add(i);
// Update all multiples of p
for (int j = i * 2;
j < MAX; j += i)
{
isprime[j] = 0;
}
}
i += 1;
}
}
// Function to print
// the nearest prime
static int printNearest(int N)
{
int MAX = (int) 1e6;
// store all the
// index with 1 except 0,1 index
int [] isprime = new int[MAX];
for(int i = 2; i < MAX; i++)
isprime[i] = 1;
// list to store
// prime numbers
Vector prime = new Vector();
// variable to add primes
int i = 0;
// call the sieve function
prime_sieve(MAX, isprime, prime);
// Keep on adding prime
// numbers till the nearest
// prime number is achieved
while (isprime[N] == 0)
{
N += prime.get(i);
i += 1;
}
// return the
// nearest prime
return N ;
}
// Driver Code
public static void main(String[] args)
{
int N = 8;
System.out.printf("%d", printNearest(N));
}
}
// This code is contributed by Rajput-Ji Python3
# Python3 program to print the nearest prime
# number by sequentially adding the prime numbers
# Function to store prime numbers using prime sieve
def prime_sieve(MAX, isprime, prime):
# iterate for all the numbers
i = 2
while (i * i <= MAX):
# If prime[p] is not changed,
# then it is a prime
if (isprime[i] == 1):
# append the prime to the list
prime.append(i)
# Update all multiples of p
for j in range(i * 2, MAX, i):
isprime[j] = 0
i += 1
# Function to print the nearest prime
def printNearest(N):
MAX = 10**6
# store all the index with 1
isprime = [1] * MAX
# 0 and 1 are not prime
isprime[0] = isprime[1] = 0
# list to store prime numbers
prime = []
# variable to add primes
i = 0
# call the sieve function
prime_sieve(MAX, isprime, prime)
# Keep on adding prime numbers
# till the nearest prime number
# is achieved
while not isprime[N]:
N += prime[i]
i += 1
# return the nearest prime
return N
# Driver Code
N = 8
print(printNearest(N))C#
// C# program to print the
// nearest prime number by
// sequentially adding the
// prime numbers
using System;
using System.Collections.Generic;
class GFG
{
// Function to store prime
// numbers using prime sieve
static void prime_sieve(int MAX, int []isprime,
List prime)
{
// iterate for all the numbers
int i = 2;
while (i * i <= MAX)
{
// If prime[p] is not changed,
// then it is a prime
if (isprime[i] == 1)
{
// append the prime to the list
prime.Add(i);
// Update all multiples of p
for (int j = i * 2;
j < MAX; j += i)
{
isprime[j] = 0;
}
}
i += 1;
}
}
// Function to print
// the nearest prime
static int printNearest(int N)
{
int MAX = (int) 1e6;
int i = 0;
// store all the
// index with 1 except 0,1 index
int [] isprime = new int[MAX];
for(i = 2; i < MAX; i++)
isprime[i] = 1;
// list to store
// prime numbers
List prime = new List();
// variable to add primes
i = 0;
// call the sieve function
prime_sieve(MAX, isprime, prime);
// Keep on adding prime
// numbers till the nearest
// prime number is achieved
while (isprime[N] == 0)
{
N += prime[i];
i += 1;
}
// return the
// nearest prime
return N;
}
// Driver Code
public static void Main(String[] args)
{
int N = 8;
Console.Write("{0}", printNearest(N));
}
}
// This code is contributed by Princi Singh 输出：
13