Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		input := nextInt()
		fmt.Println(IsPrime(input))
	}
}

func IsPrime(num int) int {
	eratos := make([]bool, num+1)
	c := 0
	for i := 2; i < len(eratos); i++ {
		if eratos[i] == false {
			c++
			eratosCalc(i, eratos)
		}
	}
	return c
}

func eratosCalc(primeNum int, eratos []bool) {
	for i := primeNum * primeNum; i < len(eratos); i += primeNum {
		eratos[i] = true
	}
}

func next() string {
	return sc.Text()
}

func nextInt() int {
	a, _ := strconv.Atoi(next())
	return a
}

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

using namespace std ;

int n, cnt[1000008]; 
bool prime[1000008];

int main() {
	prime[1] = true;
	for (int i = 2; i <= 999999; i ++) prime[i] = false;
	for (int i = 2; i <= 999999; i ++) {
		if (!prime[i]) {
			for (int j = 2*i; j <= 999999; j += i) {
				prime[j] = true;
			}
		}
	}
	cnt[1] = 0;
	for (int i = 2; i <= 999999; i ++) {
		cnt[i] = cnt[i - 1] + (int)(!prime[i]);
	}
	while (cin >> n) {
		cout << cnt[n] << endl ;
	}
	return 0 ;
}

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

int main(){

  int n,i,j,ans;


  while(cin >> n){
    ans = 0;

    for(;n > 1 ; n--){
      if(n == 2 ) ans++;
      for(i = 2 ; i <= (int)sqrt((double)n) ; i++){
	if(n%i == 0){
	  break; 
	}
      }
      if(n%i != 0) ans++;
    }
    cout << ans << endl;

  }
  return 0;
}

C++: #include <iostream>

int prime_numbers[999999];
int cur_index = 4;

bool is_prime(int p) 
{
    for (int i = 0; i < cur_index; i++) {
        if (p % prime_numbers[i] == 0) {
            return false;
        }
    }
    return true;
}

int main(void)
{
    prime_numbers[0] = 2;
    prime_numbers[1] = 3;
    prime_numbers[2] = 5;
    prime_numbers[3] = 7;

    int n;
    std::cin >> n;

    for (int i = 8; i <= n; i++) {
        if (is_prime(i)) {
            prime_numbers[cur_index] = i;
            cur_index++;
        }
    }

    std::cout << cur_index << std::endl;
//    for (int i = 0; i < cur_index; i++) {
//        std::cout << prime_numbers[i] << std::endl;
//    }
}

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

int main(void){
  int n;
  int p[1000000];
  p[0] = p[1] = 0;
  p[2] = 1;
  int ans = 0;
  for(int i = 3; i <= 1000000; i += 2){
    bool flag = true;
    for(int j = 3; j <= sqrt(i); j += 2){
      if(i % j == 0){
	flag = false;
      }
    }
    if(flag)
      ans++;
    p[i] = ans;
  }
  while(cin >> n){
    cout << p[n] << endl;
  }
  return 0;
}

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

	bool is_prime[1000000];
	int n_prime[1000000];
int main()
{
	
	is_prime[0]=is_prime[1]=false;
	for(int i=2; i<1000000; i++){
		is_prime[i]=true;
	}
	
	for(int i=2; i*i<1000000; i++){
		if(is_prime[i]){
			for(int j=i*i; j<1000000; j+=i){
				is_prime[j]=false;
			}
		}
	}


	int cnt=0;
	for(int i=0; i<1000000; i++){
		if(is_prime[i]){
			cnt++;
		}
		n_prime[i]=cnt;
	}
	cout<<"finish"<<endl;
	while(true){
		int n;
		cin>>n;

		if(cin.eof())
			break;
	
		cout<<n_prime[n]<<endl;
	}
	
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main()
{
	while (cin)
	{
		int tmp;

		cin >> tmp;

		vector<int> prime;

		for (int i(2); i <= tmp; ++i)
		{
			for (vector<int>::iterator i2(prime.begin()), n2(prime.end()); i2 != n2; ++i2)
			{
				if ((i % *i2) == 0)
				{
					goto nx;
				}
			}

			prime.push_back(i);
		nx:;
		}

		cout << prime.size() << endl;

		//if (cin.eof()) { break; }
	}

	return 0;
}

C++: #include <cstdio>
#include <cstring>
#include <algorithm>
 
using namespace std;
 
int p[1000000];
int cnt[1000000];
 
int main()
{
  fill(p, p + 1000000, 1);
  p[0] = p[1] = 0;
  for(int i=2;i<1000000;i++)
    for(int j=i*2;j<1000000;j+=i)
      p[j] = 0;
 
  for(int i=1;i<1000000;i++)
    cnt[i] = cnt[i-1] + p[i];
 
  int n;
  while(scanf("%d", &n) != -1)
    printf("%d\n", cnt[n]);
 
  return 0;
}

C++: #include <stdio.h>
int main(){ 
	const int MAX_V=999999;
	int prime[MAX_V+1];
	int i,k,v,n,sum[MAX_V+1];
	prime[0]=0;
	prime[1]=0;
	for(i=2;i<=MAX_V;i++){
		prime[i]=1;
	}
	for(i=2;i*i<=MAX_V;i++){
		if(prime[i]){
			for(k=2*i;k<=MAX_V;k+=i){ 
				prime[k]=0;
			}
		}
	}
	sum[0]=0;
	for(i=0;i<=MAX_V;i++){ 
		sum[i+1]=sum[i]+prime[i]; 
	}
	while(scanf("%d",&n)!=EOF){
		printf("%d\n",sum[n+1]);
	}
	return 0; 
}

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

typedef unsigned int Int;

Int sqrt_prm(const Int& target, Int x = 0) {
	while (x*x < target) {
		x++;
	}
	return x;
}

int main() {
	while (not std::cin.eof()) {
		Int target;
		std::cin >> target;

		std::vector<Int> primes = {};
		Int sqrt = 1;
		for (Int n = 3; n <= target; n+=2) {
			bool flag = true;

			sqrt = sqrt_prm(n, sqrt);
			for (auto&& p = primes.begin(); not(*p > sqrt or p == primes.end()); p++) {
				if (n % *p == 0) {
					flag = false;
					break;
				}
			}

			if (flag) {
				primes.push_back(n);
			}
		}

		std::cout << primes.size() + (target >= 2 ? 1 : 0) << std::endl;
	}
}

C++: #include<iostream>
#include<string>
#include<algorithm>
#include<vector>
using namespace std;
int main(){
	int n,cnt=0,ans,sosu[100000]={0};
	for(int i=2;i<100000;i++){
		bool flag=true;
		for(int j=2;j<=sqrt(i);j++){
			if(i%j==0)flag=false;
		}
		if(flag){
		sosu[cnt]=i;
		cnt++;
		}
	}
	while(cin>>n){
	ans=0,cnt=0;
	for(int i=2;i<=n;i++){
		if(sosu[cnt]==i){
			cnt++;
			ans++;
		}
	}
	cout<<ans<<endl;
	}
    return 0;
}

C++: #include<iostream>
#include<cstdio>
#include <cstring>
using namespace std;
int main()
{
	    int n;
		bool a[1000001]; 
		memset(a,0,sizeof(a));
for(int i=2;i*i<=n;i++)
		{
			if(a[i]==false)
				for(int j=i*2;j<=n;j+=i)
					a[j]=true;
		}
	while(cin>>n)
	{
		int count=0;
	
		
		for(int k=2;k<=n;k++)
		{
			if(!a[k])
			count++;
		}
		cout<<count<<endl;
		
	}
	return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <numeric>
using namespace std;
#define OVER_SIX_DIGITS 1000000

int prime[OVER_SIX_DIGITS];

void make_prime(void) {
    for(int r = 0; r < OVER_SIX_DIGITS; r++) prime[r] = 1;
    prime[0] = prime[1] = 0;
    
    for(int r = 2; r < OVER_SIX_DIGITS; r++) {
        if(prime[r]) {
            for(int c = 2 * r; c < OVER_SIX_DIGITS; c += r) prime[c] = 0;
        }
    }
    //累積和配列を作成
    for(int r = 1; r < OVER_SIX_DIGITS; r++) prime[r] += prime[r - 1];
}

int main(void) {
    int n;
    make_prime();
    while(cin >> n) {
        cout << prime[n];
    }
    return(0);
}

C++: #include <bits/stdc++.h>
using namespace std;

int dp[1000000] = {};

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

int main() {
  int n;
  for (int i=2; i<= 1000000; i++) {
    if (is_prime(i)) {
      dp[i] = dp[i-1] + 1;
    }
    else {
      dp[i] = dp[i-1];
    }
  }
   
  while(cin>>n) {
    cout << dp[n] << endl;
  }
}

C++: #include<iostream>
#include<iomanip>
#include<cstdio>
#include<vector>
#include<algorithm>
#include<cmath>
#include<queue>
#include<stack>
#include<functional>
#include<string>
#include<cstring>
#include <sstream>
using namespace std;
int main(void){
    int n,ans,i;
    while(cin >> n){
        ans = 0;
        if(n < 2){
            cout << 0;
            continue;
        }else if(n == 2){
            cout << 1;
        }else{
            ans++;
        }
        for(i = 3; i <= n; i += 2){
            if(ceil(sqrti) == floor(sqrti)){
                continue;
            }
            ans++;
        }
        cout << ans << endl;
    }
    return 0;
}