Go: package main

// https://ja.wikipedia.org/wiki/%E3%82%A8%E3%83%A9%E3%83%88%E3%82%B9%E3%83%86%E3%83%8D%E3%82%B9%E3%81%AE%E7%AF%A9

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() (int, error) {
	return strconv.Atoi(nextString())
}

func main() {
	scanner.Split(bufio.ScanWords)

	for {
		n, err := nextInt()
		if err != nil {
			break
		}
		maybePrime := make([]bool, n+1)
		for i := 0; i <= n; i++ {
			maybePrime[i] = true
		}

		maybePrime[0] = false
		maybePrime[1] = false
		for i := 2; i <= int(math.Sqrt(float64(n)))+1; i++ {
			j := i * 2
			for j <= n {
				maybePrime[j] = false
				j += i
			}
		}

		nPrimes := 0
		for _, p := range maybePrime {
			if p {
				nPrimes++
			}
		}
		// fmt.Println(maybePrime)
		fmt.Println(nPrimes)
	}
}

C++: #include <cstdio>
#include <cstdlib>
#include <iostream>
#include <math.h>

using namespace std; 

int list[1000000]={2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73,
79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163,
 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251,
 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349,
 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443,
 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541, 547, 557,
 563, 569, 571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643, 647,
 653, 659, 661, 673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751, 757,
 761, 769, 773, 787, 797, 809, 811, 821, 823, 827, 829, 839, 853, 857, 859, 863,
 877, 881, 883, 887, 907, 911, 919, 929, 937, 941, 947, 953, 967, 971, 977, 983,
 991, 997, 1009, 1013, 1019, 1021, 1031, 1033, 1039, 1049, 1051, 1061, 1063, 1069};

long int a,b,loop,hasil,input,mid;
long int lo;
long int hi;

 bool Prima(unsigned long n) 
{
    for (unsigned short i = 0; list[i]*list[i] <= n; i++) 
	{
        if (n % list[i] == 0) 
		{
            return false;
        }
    }
 
    return true;
}

int main()
{
	lo=1087;
	a=180;
	while(lo<=1000003)
	{
		if(Prima(lo))
		{
			list[a]=lo;
			a++;
		}
		lo+=2;
	}
	printf("%d",list[78498]);
	while(scanf("%ld",&input)!= EOF)
	{	
	lo=0;
	hi=78497;
	/*while(lo<=hi)
        {
            mid=(lo+hi)/2;
            if (list[mid]<input)
            {
                lo=mid+1; 
            } 
            else if(list[mid]>input)
            {
                hi=mid-1; 
            }
            else 
            {
                break; 
            } 
        }*/
	while(input>=list[lo])
	{
		lo++;
	}
	printf("%d\n",lo);
	}
	return 0; 
}

C++: #include<iostream>
#include<vector>
#include<string>
#include<cstdlib>
#include<cstring>
#include<algorithm>
#include<cstdio>
using namespace std;
#define MAX 1000000
bool prime[MAX];
int main(void){ 
  
  prime[0] = prime[1] = false;
  //memeset(prime,sizeof(prime),true);
  fill(prime,prime+MAX,true);
  long long int ans[MAX];
  
  ans[0] = ans[1] = 0;
  ans[2] = 1;
  int now=0;
  for(long long int i = 2 ; MAX > i ; i ++){
    ans[i] = now + prime[i];
    now += prime[i];
    //cout<<now<<" "<<ans[i]<<endl;
    if(prime[i]){
      for(long long int j = i + i ; MAX > j * j ; j += i){
	prime[j] = false;
      }
    }
  }
  
  int n;
  while(cin>>n){
    cout<<ans[n]<<endl;
  }
}

C++: #include <stdio.h>

int main(void)
{
	int n;
	int i;
	int flag;
	int ans;
	
	scanf("%d", &n);
	
	ans = 0;
	while (n >= 2){
		flag = 0;
		for (i = n - 1; i >= 2; i--){
			if (n % i == 0){
				flag = 1;
			}
		}
		
		if (!flag){
			ans++;
		}
		
		n--;
	}
	
	printf("%d\n", ans);
	
	return (0);
}

C++: #include<iostream>
#include<vector>
using namespace std;

class Sieve_of_Eratosthenes {
public:
	vector<bool> PN;
	
	Sieve_of_Eratosthenes(int x){
		init(x);
	}
	
	void huga(int t, int x){
		if(PN[t]) return;
		for(int i = t*t; i <= x; i+=t)
			PN[i] = true;
	}
	
	void init(int x){
		PN.resize(x+5);
		PN[0]=PN[1]=PN[4]=true;
		for(int i = 6; (i-1)*(i-1) <= x; i+=6)
		huga(i-1,x),huga(i+1,x);
		for(int i = 6; i <= x; i+=6)
			PN[i]=PN[i+2]=PN[i+3]=PN[i+4]=true;
	}
};

int main(){
	const int MAX = 1000000;
	int n;
	
	Sieve_of_Eratosthenes S(MAX);
	vector<int> sum(MAX);
	
	for(int i = 1; i < MAX; i++){
		sum[i] += sum[i-1] + !S.PN[i];
	}
	
	while(cin>>n){
		cout<<sum[n]<<endl;
	}
	return 0;
}

C++: #include<iostream>
#include<cmath>
using namespace std;

int main(){
	int num, cou, i, j, flag;
	
	while(cin >>num){
		cou = 0;
		for(i = 2; i <= num; i++, flag = 0){
			for(j = 2; j < i; j++){
				if(i % j == 0){
					flag = 1;
					break;
				}
			}
			if(flag == 0){
				cou++;
			}
		}
		if(num % 2 == 0) cou++;
		cout <<cou<<endl;
	}
	
	return 0;
}

C++: #include <iostream>
using namespace std;

int main()
{
	unsigned int  n;
	cin >> n;
	unsigned  int num,count = 0;
	for(int i = 0; i < n; i++)
	{
		cin >> num;
		for(int j = 2; j <=num; j++)
		{
			if(num == j)count++;
			if((num % j) == 0)
			{
				break;
			}
		}
	}
	cout << count << endl;
	return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
typedef long long LL;
using namespace std;
#define REPF( i , a , b ) for ( int i = a ; i <= b ; ++ i )
#define REP( i , n ) for ( int i = 0 ; i < n ; ++ i )
#define CLEAR( a , x ) memset ( a , x , sizeof a )
const int N=1e7;
int sum[N],vis[N];
void initPrime()
{
    int num = 0, m = sqrt (N + 0.5);
    for (int i = 2; i <= m; ++i)
        if (vis[i] == 0)
            for (int j = i * i; j <= N; j += i) vis[j] = 1;
    sum[1]=0;
    for(int i=2;i<=N;i++)
    {
        if(!vis[i])  sum[i]=sum[i-1]+1;
        else   sum[i]=sum[i-1];
    }
//    for(int i=2;i<=10;i++)
//        cout<<"233  "<<sum[i]<<endl;
}
int main()
{
    int n;
    initPrime();
    while(~scanf("%d",&n))
        printf("%d\n",sum[n]);
    return 0;
}

C++: #include <iostream>

using namespace std;

int main()
{
	unsigned int  n;
	cin >> n;
	unsigned int num, count = 0;
	for(int i = 0; i < n; i++)
	{
		cin >> num;
		for(int j = 2; j <=num; j++)
		{
			if(num == j)count++;
			if(num % j == 0)
			{
				break;
			}
		}
	}
	cout << count << endl;
	return 0;
}

C++: #include <iostream>
#include <math.h>
#include <vector>
using namespace std;
int main(){
	int n;
	while(cin>>n){
		bool prime[1000000];
		for(int l=2;l<=n;l++){
			prime[l] =true;
		}
		/* prime[0]=false;
		   prime[1]=false; */
		for(int i=2;i<=n;i++){
			if(prime[i]==true){
				for(int j=i+1;j<=n;j++){
					if(j%i==0) prime[j]=false;
				}
			}
		}
		int count=0;
		for(int k=2;k<=n;k++){
			if(prime[k]==true) count++;
		}
		cout<<count<<endl;
	}
	return 0;
}