Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		a, _ := strconv.Atoi(sc.Text())
		fmt.Println(countPrime(a))
	}
}

func countPrime(a int) int {
	primes := make([]bool, a+1)
	n := 0
	for i := 2; i <= a; i++ {
		if primes[i] == false {
			n++
			for j := i * i; j <= a; j += i {
				primes[j] = true
			}
		}
	}
	return n
}

C++: #include <iostream>
#include <cmath>
using namespace std;
#define MAX 1000000

int main(){
	bool p[MAX+1];
	int n, ans;

	for(int i=0 ; i<=MAX ; i++){
		p[i] = (i<=1)? false : true ;
	}

	for(int i=2 ; i<=sqrt(MAX)+1 ; i++){
		if(p[i]){
			for(int j=i*2 ; j<=MAX ; j+=i){
				p[j] = false;
			}
		}
	}

	while( cin >> n ){		
		ans = 0;
		for(int i=0 ; i<=n ; i++){
			if(p[i]){
				ans++;
			}
		}
		cout << ans << endl;
	}
}

C++: #include<cstdio>
using namespace std;
#define MAX 1000000
int prime[MAX];
bool is_prime[MAX];
int sieve(int n){   //埃氏?
   int p=0;
   for(int i=0;i<=n;i++)
    is_prime[i]=true;
    is_prime[0]=is_prime[1]=false;
   for(int i=2;i<=n;i++){
    if(is_prime[i]){
        prime[p++]=i;    //??素数的个数和素数的?
        for(int j=2*i;j<=n;j+=i)
            is_prime[j]=false;
    }
   }
   return p;
}
int main()
{
  int num;
  while(scanf("%d",&num)!=EOF){
   printf("%d\n",sieve(num));
  }
  return 0;
}

C++: #include<stdio.h>

int main(void)
{
	int data=1,i,j,s[1000000],x;

	while((scanf("%d\n",&data)) != EOF)
	{
		for(i=0;i<=data;i++){
			s[i]=1;}
		x=0;
		if(data<=1){
			printf("%d\n",0);}
		else
		{
			for(i=2;i<=data/2;i++)
				for(j=i*2;j<data;j+=i)
				    s[j]=0;		
		for(i=0;i<data;i++)
			x+=s[i];
		printf("%d\n",x);
		}
	}
  return 0;
}

C++: #include <cstdio>

using namespace std;

int main() {
    long NUM = 1000000;
    long n;
	long counter = 0;
	bool primeNum[NUM + 10];

	long i, ii, j, len = NUM + 10;
	
	primeNum[0] = primeNum[1] = false;
	for (i = 2; i < len; i++) primeNum[i] = true;
	
	ii = 4;
    for(i = 2; ii < NUM;) {
        if (primeNum[i]) {
            for (j = ii; j < NUM; j += i) {
                primeNum[j] = false;
            }
        }
        
        i++;
        ii = i * i;
    }
    
    while (scanf("%ld", &n) != EOF) {
        counter = 0;
        
        for (i = 0; i <= n; i++) {
            if (primeNum[i]) counter++;
        }
        
        printf("%ld\n", counter);
    }

    return 0;
}

C++: #include <iostream>
using namespace std;
bool primes[1000000];
void Sieve(){
    for(int i = 2; i < 1000000; i++) {
      primes[i] = true;
    }//全部素数とする
    primes[0] = primes[1] = false;//0,1は素数ではない
    
    for(int i = 2; i < 1000000; i++){
            for(int j = 2 * i; j < 1000000; j += i){
                primes[j]=false;
            }
        
            
        }
    
}
int main(void){
    int n, count;
    Sieve();
    while(cin>>n){
        count = 0;
        for(int k = 2; k <= n; k++){
            if(primes[k]) count++;
        }
        cout<<count<<endl;
    }
    return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
	int n, cnt;
	bool prime[1000000] = {false};
	prime[1] = true; //1???????????\
	
	while(cin >> n){
		// ??¨??????????????????????????????
		for(int j = 2; j*j <= n; j++){
			if(prime[j] == false){
				for(int k = j; k*j <= n; k++){
					prime[j*k] =true;
				}
			}
		}
		
		for(int i = 1; i <= n; i++){
			if(prime[i] == false) cnt++;
		}
		
		cout << cnt << endl;
	}
}

C++: #include<iostream>
#include<cmath>
#define REP(i,n) for(int i=0;i<n;i++)
using namespace std;

bool isPrime[1000000];

void setIsPrime(int x) {
    isPrime[1] = true;
	for(int i = 1; i < x; i++) {
		int n = (int)sqrt(i);
		int yakusu = 0;
		for(int j = 1; j<=n; j++) {
			if(i%j==0) yakusu++;
			if(yakusu > 2) break;
		}
		if (yakusu==1) {
			isPrime[i] = true;
		}
	}
}

int numPrimes(int x) {
    int cnt = 0;
    for (int n=1; n <= x; n++) {
        if (isPrime[n]) cnt++;
    }
    return cnt - 1;
}

int main(){
    int x;
    setIsPrime(1000000);
	while(cin >> x) {
		cout << numPrimes(x) << endl;
	}
    return 0;
}

C++: #include<stdio.h>
#include<stdlib.h>
#include<iostream>
#include<string>
#include<algorithm>
#include<vector> 
#include<limits>
#include<numeric>
#include<math.h>
using namespace std;

#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define in(x) cin >> x
#define out(str) cout << str << endl

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

int n,ans;

int main(){
    int N=1000000;
    int prime[N];
    rep(i,N) prime[i] = 1;
    prime[0] = prime[1] = 0;
    rep(i,N){
        if(prime[i]){
            for(int j = i + i; j < N; j+=i){
                prime[j] = 0;
            }
        }
    }
    while(cin>>n){
        ans=0;
        rep(i,n+1)ans+=prime[i];
        out(ans);
    }
}

C++: #include<iostream>
#include<string>
#include<algorithm>
#include<vector>
#include<cmath>
using namespace std;
int main(){
	int n,cnt=0,ans,sosu[1000003]={0};
	
	for(int i=2;i<1000002;i++){
		bool flag=true;
		for(int j=2;j<=sqrt(i);j++){
			if(i%j==0){
			flag=false;
			break;//素数でない時点で打ち切り
			}
		}
		/*if(flag){
		cnt++;
		sosu[i]=cnt;
		}*/
		if(flag)cnt++;
		sosu[i]=cnt;		
	}
	//for(int i=0;i<31;i++)cout<<sosu[i]<<" ";
	while(cin>>n){
	cout<<sosu[n]<<endl;
	}
    return 0;
}

C++: #include <iostream>

using namespace std;

int main()
{
    int x,prime=0,count=0;
    
    cin >> x;
    
    for (int j=1; j<=x; j++)
    {
        for (int i=1; i<=x; i++)
        {
            if(j%i==0)
              count+=1;
        }
        if (count==2)
           prime+=1;
        
        count=0;
    }
    
     cout << prime;  
        
    return 0;    
}

C++: #pragma  warning (disable:4996)
#include<iostream>
#include<cstdio>
#include<queue>
#include<cstring>
#include<string>
#include<cmath>
#include<algorithm>
#define MAX 1000000
using namespace std;
int amount[MAX];
int vis[MAX];
int prime[MAX];
int main(){
	memset(vis, 0, sizeof(vis));
	memset(prime, 0, sizeof(prime));
	prime[0] = prime[1] = 0;
	for (int i = 2; i*i <=MAX; i++){
		if (!vis[i]){
			prime[i] = 1;
			for (int j = i+i; j*j <=MAX; j += i){
				vis[j] = 1;
				prime[j] = 0;
			}
		}
	}
	int sum = 0;
	for (int i = 0; i <= MAX; i++){
		if (prime[i]){
			sum++;
		}
		amount[i] = sum;
	}
	int n;
	while (cin >> n){
		cout << amount[n] << endl;
	}
	return 0;
}

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

int main(){
	bool prime[1000001];
	for(int i = 0; i < 1000001; i++) prime[i] = true;
	prime[0] = false;
	prime[1] = false;
	for(int i = 2; i < 1000; i++){
		for(int j = i*2; j < 1000; j += i){
			prime[j] = false;
		}
	}

	int n;
	while(cin >> n){
		int count = 0;
		for(int i = 0; i <= n; i ++){
			if(prime[i]) count++;
		}
		cout << count << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <set>
#include <cmath>
std::vector<int> PRIMES;

void init_primes(int max)
{
  if (max < 2) return;
  std::set<int> candidate;
  PRIMES.push_back(2);
  for (int i = 3; i < max; i+=2) {
    candidate.insert(i);
  }
  for (int i = 3; i < sqrt(max); i+=2) {
    std::set<int>::iterator it = candidate.find(i);
    if (it != candidate.end()) {
      PRIMES.push_back(i);
      int del_num = i;
      while (del_num < max) {
        it = candidate.find(del_num);
        if (it != candidate.end()) candidate.erase(it);
        del_num += i;
      }
    }
  }
  for (std::set<int>::iterator it = candidate.begin();
       it != candidate.end(); ++it) {
    PRIMES.push_back(*it);
  }
}

int main()
{
  const int max = 1000000;
  init_primes(max);
  int i;
  while (std::cin >> i) {
    std::vector<int>::iterator it = PRIMES.begin();
    int n = 0;
    while (*it <= i) {
      n++;
      it++;
    }
    std::cout << n << std::endl;
  }
  return 0;
}