Go: package main

import (
        "fmt"
)

const MAX int = 1000000
var primes [MAX]int

func main() {
        Init()
        var n, cnt int
        for {
                _, err := fmt.Scanf("%d", &n)
                if err != nil {
                        break
                }
                cnt = 0
                for i := 2; i <= n; i++ {
                        cnt += primes[i]
                }
                fmt.Printf("%d\n", cnt)
        }
}

func Init() {
        for i := 0; i < MAX; i++ {
                primes[i] = i
        }

        Soe()

        for i := 0; i < MAX; i++ {
                primes[i] %= 2
        }
        primes[1] = 0
        primes[2] = 1
}

func Soe() {
        for i := 2; i < MAX; i++ {
                if primes[i] > 0 {
                        for j := 2; j < MAX/i; j++ {
                                if i * j < MAX {
                                        primes[i*j] = 0
                                }
                        }
                }
        }
}

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

int eratos(int n){
	vector<int> v;
	for(int i=0; i<=n; i++) v.push_back(i);
	v[1]=0;
	for(int i=2; i<=(int)sqrt((double)n); i++){
		if(v[i]!=0){
			for(int j=2; i*j<=n; j++){
			v[i*j]=0;
			}
		}
	}
	int count=0;
	for(int i=2; i<=n; i++)
		if(v[i]!=0) count++;
	return count;
	return 0;
}

int main(void){
	int n;
	while(cin >> n)
		cout << eratos(n) << endl;
	return 0;
}

C++: #include<stdio.h>
#include<iostream>
#include<math.h>


using namespace std;



bool isPrime(int num) {
	if (num < 2)return false;
	if (num == 2)return true;
	if (num % 2 == 0)return false;
	int root = (int)sqrt(num);
	for (int i = 3; i <= root; i += 2) {
		if (num % i == 0)return false;
	}
	return true;
}

int main() {

	int num;
	while (cin >> num) {
		int count = 0;
		if (num >= 2)count++;

		for (int i = 3; i <= num; i += 2) {
			if (isPrime(i))count++;
		}
		cout << count << endl;
	}


	
	//std::system("pause");
	return 0;
}

C++: #include <cstdio>
#include <iostream>

using namespace std;

const int MAX_N = 1000000;

int N;

int len;
int prime[MAX_N];				//save all prime number less than MAX_N
bool isPrime[MAX_N];

void sieve(){
	//init
	fill(isPrime, isPrime + MAX_N, true);
	isPrime[0] = isPrime[1] = false;
	
	for(int i = 2; i < MAX_N; i ++){
		if(isPrime[i]){
			prime[len ++] = i;
			for(int j = 2 * i; j < MAX_N; j += i)
				isPrime[j] = false;
		}
	}
}

void solve(){
	int ans = 0;
	for(int i = 0; i < len; i ++){
		if(N >= prime[i])
			ans ++;
	}
	printf("%d\n", ans);
}

int main(int argc, char const *argv[]){

	sieve();
	while(scanf("%d", &N) != EOF){
		solve();
	}

	return 0;
}

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

int count(int n);

int main(void){
	while (true){
		int n; cin >> n;
		if (!n){
			break;
		}
		cout << count(n) << endl;
	}
	return 0;
}

int count(int n){
	int ct = 0;
	bool *prime = new bool[n];
	//bool prime[999999];
	for (int i = 0; i <= n;i++){
		prime[i] = 0;
	}
	prime[2] = 1;
	for (int i = 3; i <= n; i = i + 2){
		prime[i] = 1;
	}
	for (int i = 3; i <= sqrt(n); i = i + 2){
		if (prime[i] == 0) continue;
		for (int j = i * 2; j <= n; j = j + i){
			prime[j] = 0;
		}
	}
	for (int i = 3; i <= n; i = i + 2){
		if (prime[i] == 1) ct++;
	}
	ct++; //2を数えていない分
	return ct;
	delete[] prime;
}

C++: #include<stdio.h>
int num[1000000]={};
int main ()
{
int p,n,m,cou;m=0;cou=0;p=3;


for(int i=2;i<=10000001;i++)
	num[i]=1;
for(int i=2;i<=1001;i++){
for(int k=2;k*k<=i;k++){
if(i%k==0){num[i]=0;}}
}
while(scanf("%d",&n)!=EOF)
{
for(int i=2;i<=n;i++)
	cou+=num[i];

if(n==2)cou=1;
if(n==1)cou=0;
//else cou++;
printf("%d\n",cou);
cou=0;
}
return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
  int n;
  while(cin >> n){
    int ans=0,math[1001000]={0};
    math[1]=1;
    for(int i=2;i<n+1;i++){
      int bell=i;
      if(math[i]==0){
	ans++;
	math[i]=1;
      }
      while(bell+i<=n){
	bell+=i;
	if(math[bell]==0){
	  math[bell]=1;
	}
      }
    }
    cout << ans << endl;
  }
}

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

int main()
{
	int n;
	while(cin >> n && n<1000000) {
		int s = 0;
			for (int j = 2;j <= n;j++){
				int yaku = 0;
				for (int m = 1;m <= j;m++){
					if (j % m == 0){yaku++;}
				}
				if (yaku == 2){s++;}
			}
			cout << s << endl;
	}
    return 0;
}

C++: #include<iostream>
#include<vector>
#define MAX 1000000
using namespace std;
int main(){
    vector<int> v(MAX);
    for(int i = 0;i<MAX;i++)
      v[i]=true;

      v[0]=false;
      v[1]=false;
    for(int i = 2;i<MAX;i++){
      if(v[i]==true){
        for(long long int j = (long long int)i;j<MAX;j=j*j){
         v[j]=false;
       }
      }
    }
    for(int i = 2;i<MAX;i++)v[i]+=v[i-1];
    while(true){
      int a;
      cin >> a;
      if(cin.eof())return 0;
      cout << v[a]<<endl;
    }
}

C++: #include <iostream>
#include <cstdio>
#define MAX 999999
using namespace std;

int main(void){
  int num,cnt;
  bool is_prime[MAX];

  for( int i = 0 ; i <= MAX ; i++ ) is_prime[i] = true;
  is_prime[0] = is_prime[1] = false;

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

  while(scanf("%d" ,&num) != EOF){
    cnt = 0;
    for( int i = 2 ; i <= num ; i++ ){
      if( is_prime[i] ) cnt++;
    }
    cout << cnt << endl;

  }
  return 0;
}

C++: #include <stdio.h>

int IsPrime(int n);

int main(void){
    int n, cnt, minID, min=1;
    int list[100000];

    cnt=0;
    for (int i=0; i<1000000; i++){
        if (i%10==0) list[i/10] = cnt;
        cnt += IsPrime(i);
    }
    while (scanf("%d", &n) && n!=EOF){
        cnt = list[n/10];
        for (int i=1; i<n%10; i++){
            cnt += IsPrime(i);
        }
        printf("%d\n", cnt);
    }
    return 0;
}

int IsPrime(int n){
    int i;

    for (i=2; i*i<=n; i++){
        if (n%i==0) return 0;
    }
    return 1;
}

C++: #include <iostream>
#include <cmath>

int prime[1000000];


bool isPrime(int n) {
  int x = sqrt(static_cast<double>(n));
  for (int i = 3; i <= x; i += 2) {
    if (n % i == 0) {
      return false;
    }
  }
  return true;
}


void get_prime(int max_num) {
  int primecount = 1;
  prime[2] = primecount;
  for (int i = 3; i <= max_num; i += 2) {
    if (isPrime(i))
      prime[i] = ++primecount;
    else
      prime[i] = primecount;
  }
}


int main () {
  int ns[31];
  int i = 0, max_num;
  while (std::cin >> ns[i]) {
    max_num = std::max(max_num, ns[i]);
    if (ns[i] % 2 == 0)
      ns[i] --;
    i ++;
  }
  get_prime(max_num);
  for (int j = 0; j < i; j ++) {
    std::cout << prime[ns[j]] << std::endl;
  }
  return 0;
}

C++: #include <stdio.h>
#include<math.h>
int main(){


   long long int i, j ,n,c,k,p;
   while(scanf("%lld",&n)==1){
        k=0;
    for(i=2;i<=n;i++){
            c=0;
        for(j=2;j<=sqrt(i);j++){
        if(i%j==0)c++;}
        if(c==0){


                k++;}
    }
         printf("%lld\n",k);
   }
return 0;
}

C++: #include <iostream>
#include <vector>

#define MAX_MAX 1000010

using namespace std;

bool isChecked[MAX_MAX];
vector <int> prime;

int CountPrimeNum(int max) {
	
	int count = 0;
	for (int i = 0; i < MAX_MAX; i++) {
		if (prime[i] > max) break;
		count++;
	}	
	return count;
}


int main() {
	for(int i = 0; i < MAX_MAX; i++) {
		isChecked[i] = false;
	}
	
	isChecked[0] = true;
	isChecked[1] = true;
	
	for (int i = 2; i < MAX_MAX; i++) {
		if(isChecked[i]) continue;
		
		prime.push_back(i);
		
		for (int j = 1;j * i < MAX_MAX;j++) {
			isChecked[i*j] = true;
		}
	}
	
	int n;
	while(cin >> n) {
		cout << CountPrimeNum(n) << endl;
	}
	
	return 0;
}