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<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;
}

C++: #include <iostream>
#define _USE_MATH_DEFINES
#include <math.h>
#define MAX_N 100000

void q9() {
	bool IsPrime[MAX_N];
	memset(IsPrime, 1, sizeof(IsPrime));
	IsPrime[0] = false;
	IsPrime[1] = false;
	for (int i = 0; i < sqrt(MAX_N); i++) {
		if (!IsPrime[i]) continue;
		for (int j = i * 2; j < MAX_N; j += i) {
			IsPrime[j] = false;
		}
	}

	int NofP[MAX_N];
	NofP[0] = 0;
	for (int i = 1; i < MAX_N; i++) {
		NofP[i] = NofP[i - 1] + IsPrime[i];
	}

	int n;
	for (; cin >> n;) {
		cout << NofP[n] << endl;
	}
}

int main() {
	q9();
	return 0;
}

C++: #include <stdio.h>
int main(){
    bool prime[1000000];
    int n,ans;
    prime[2]=true;
    for(int i=3;i<1000000;i+=2)prime[i]=true;
    for(int i=3;i<=1000;i+=2){
        if(prime[i]){
            for(int j=i*i;j<1000000;j+=i)
                prime[j]=false;
        }
    }
    while(scanf("%d",&n)!=EOF){
        ans=1;
        if(n<3){printf("%d\n",n-1);
            continue;}
    for(int i=3;i<=n;i+=2)if(prime[i])ans++;
        printf("%d\n",ans);
    }
}

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

int main(){
    int N;
    bool isPrime[1001000];
    for(int i=0; i<=1000010; i++)isPrime[i]=true;
    isPrime[0]=false;
    isPrime[1]=false;
    for(int i=2; i<=1000010; i++){
        if(isPrime[i]){
            for(int j=2*i; j<=1000010; j+=i)isPrime[j]=false;
        }
    }
    while(cin>>N){
        int cnt=0;
        for(int i=2; i<=N; i++){
            if(isPrime[i])cnt++;
        }
        cout<<cnt<<endl;
    }
}