Go: package main
import (
	"fmt"
	"os"
	"strconv"
	"bufio"
	//"math"
)

var sc = bufio.NewScanner(os.Stdin)

func main(){
	for sc.Scan(){
		n := nextInt()
		fmt.Println(calc(n))
	}
}

func calc(n int)int{
	// エラトステネスで数え上げる
	// 最初から999999まで用意しておいて入力のたびO(1)で出すのとどっちが速いのかはわからん
	eratos := make([]bool, n+1)
	c := 0
	for i:=2; i<len(eratos); i++{
		if eratos[i] == false{
			c++
			for j:=i*i; j<len(eratos); j+=i{
				eratos[j]=true
			}
		}
	}
	return c
}

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

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

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

int main(void)
{
	int prime[168] = {0};
	unsigned long n;
	prime[0] = 2, prime[1] = 3;

	for(int i = 5; i < 1000; i += 2) {
		bool p_flag = true;
		int j;
		for(j = 0; prime[j] != 0; j++) {
			if(i % prime[j] == 0) {
				p_flag = false;
				break;
			}
		}
		if(p_flag == true) {
			prime[j] = i;
		}
	}

	while(cin >> n){
		unsigned long cnt = 2;

		if(n == 1) {
			cout << 0 << endl;
			continue;
		} else if(n == 2) {
			cout << 1 << endl;
			continue;
		} else if(n == 3 || n == 4) {
			cout << 2 << endl;
			continue;
		}

		for(unsigned long i = 5; i <= n; i += 2) {
			bool p_flag = true;
			for(int j = 1; prime[j] <= (int)sqrt((double)i) + 1; j++) {
				if(i % prime[j] == 0) {
					p_flag = false;
					break;
				}
			}
			if(p_flag == true) {
				cnt++;
			}
		}

		cout << cnt << endl;
	}
	
	return 0;
}

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

int main(){
    int n;
    const int N = 1e6;

    vector<bool> is_prime(N,true);
    is_prime[0] = false;
    is_prime[1] = false;

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

    vector<int> cnt_prime(N,0);
    for(int i=1;i<N;i++){
        if(is_prime[i]) cnt_prime[i]++;
        cnt_prime[i] += cnt_prime[i-1];
    }

    while(cin >> n){
        cout << cnt_prime[n] << endl;
    }

}

C++: #include <iostream>

#define N 1000000

using namespace std;

int main() {
	int n;
	bool p[N];
	for(int i = 0; i <= N; i++) p[i] = 0;
	for(int i = 3; i <= N; i += 2) p[i] = 1;
	p[2] = 1;
	for(int i = 0; i < N; i++){
		if(!p[i]) continue;
		for(int j = 2 * i; j < N; j += i) p[j] = 0;
	}

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

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

int main(){
	int  n,i,j,sqrtn,sum;
	while(scanf("%d",&n)!=EOF){
		sum=(n-1)/2;
		for(i=3;i<=n;i+=2){
			sqrtn=(int)sqrt(i);
			for(j=1;j*5-1<sqrtn;j++){
				if(i%(j*5+1)==0 || i%(j*5-1)==0){
					sum--;
					break;
				}
			}
		}
		printf("%d\n",sum);
		
	}
	return 0;
}

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

using namespace std;

int main(){
  int k, n;
  vector<int> prime;
  vector<int>::iterator i, j;

  cin >> n;

  for(k=2; k<=n; ++k)
    prime.push_back(k);

  i = prime.begin();
  while(i != prime.end()){
    j = i + 1;
    while(j != prime.end()){
      if((*j)%(*i) == 0)
        j = prime.erase(j);
      else
        ++j;
    }
    ++i;
  }

  cout << prime.size() << endl;
  return 0;
}

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

int PrimeCount(int n){
	int A[n+5];
	memset(A,0,sizeof(A));//0は素数
	for(int i=2;i*i<=n;i++){
		if(A[i]==0){
			for(int j=i*2;j<=n;j+=i){
				A[j]=1;
			}
		}
	}
	int count=0;
	for(int i=2;i<=n;i++){
		if(A[i]==0) count++;
	}
	return count;
}

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

C++: 
int main()
{
    bool num[1000000]={false};
    for(int i=2;i<1000;i++){
        if(!num[i]){
            for(int j=i*i;j<1000000;j+=i){
                num[j]=true;
            }
        }
    }
    int n;
    while(cin>>n){
        int ans=0;
        for(int i=2;i<=n;i++){
            if(!num[i])
                ans++;
        }
        cout<<ans<<endl;
    }
    return 0;
}

C++: #include <iostream>
#include <vector>
using namespace std;
int main( void )
{
	while ( !cin.eof() )
	{
		int n;
		cin >> n;
		if ( cin.eof() )
			break;

		if ( n <= 1 )
			cout << "0" << endl;
		
		vector <bool> p(n+1, true);

		p[0] = p[1] = false;
		for ( int i = 4; i <= n; i += 2 )
		{
			p[i] = false;
		}

		for ( int i = 3; i <= n; i += 2 )
		{
			if ( p[i] )
			{
				for ( int j = i+i; j <= n; j += i )
				{
					p[j] = false;
				}
			}
		}

		int kotae = 1;
		for ( int i = 3; i <= n; i += 2 )
		{
			if ( p[i] )
				kotae++;
		}

		cout << kotae << endl;
	}

	return 0;
}

C++:  #include<iostream>
 using namespace std;
 
 int main()
 {
 	int N;
 	while(cin>>N)
 	{
 		int count=0;
 		for(int i=2;i<=N;i++)
 		{
 			bool isPrime=true;
 			for(int j=2;j*j<=i;j++)
 			{
 				if(i%j==0)
 				{
 					isPrime=false;
 					break;
 				}
 			}
 			if(isPrime) 
 			{
 				count++;
 			}
 		}
 		cout<<count<<endl;
 	}
 	return 0;
 }

C++: #include<iostream>
#include<math.h>
main(){int i,j,n,p[1000000];std::cin>>n;for(i=2;i<=n;i++)p[i]=1;for(i=2;i<n;i++)if(p[i])for(j=2;i*j<=n;j++)p[i*j]=0;j=0;for(i=2;i<=n;i++)j+=p[i];std::cout<<j<<lend;}

C++: #include <cstdio>
#include <vector>
#include <algorithm>

using namespace std;

vector<int> sieves_nonprime(int n) {
    /* sieve of Eratosthenes */
    // returns is_prime([0, n))
    if (n <= 2)
        return vector<int>(n, false);

    vector<int> is_prime(n, true); is_prime[0]=is_prime[1]=false;

    for (long long i=2; i*i<n; ++i)
        if (is_prime[i])
            for (long long j=i*i; j<n; j+=i)
                is_prime[j] = false;

    return is_prime;
}

int main() {
    size_t n=1e6;
    vector<int> is_prime=sieves_nonprime(n);
    for (size_t i=1; i<n; ++i)
        is_prime[i] += is_prime[i-1];

    while (true) {
        int n;
        if (scanf("%d", &n) == EOF)
            break;

        printf("%d\n", is_prime[n]);
    }

    return 0;
}

C++: #include<stdio.h>



int main(){
	int n,i,j;
	bool c[1000000];
	int count;


	while(scanf("%d",&n)!=EOF){
		count=n;
		c[0]=c[1]=0;
		for(i=2;i<=n;i++){
			c[i]=1;
		}
		
		for(i=0;i*i<=n;i++){
			if(c[i]==1){
				for(j=i+1;j<=n;j++){
					if(j%i==0){
						c[j]==0;
						count--;
					}
				}
			}
		}

		printf("%d",count);


	}





	return 0;
}

C++: #include <cstdio>
#include <cstring>
#include <algorithm>

using namespace std;
typedef long long lint;

int idx;
int prime[5761455];
bool sieve[10002];
bool sieveBig[1000002];

void segmentSieve(lint a, lint b)
{
	memset(sieveBig, 0, sizeof(sieveBig));
	if (a == 1) sieveBig[0] = true;
	for (int i = 2; (lint)i * i < b; i++){
		if (sieve[i]){
			for (int j = max(2ll, (a + i - 1) / i) * i; (lint)j < b; j += i) sieveBig[j - a] = true;
		}
	}
	
	for (int i = 0; i < b - a; i++){
		if (!sieveBig[i]){
			prime[idx++] = (lint)i + a;
		}
	}
}

int main()
{
	for (int i = 3; i <= 10000; i += 2) sieve[i] = true;
	sieve[2] = true;
	
	for (int i = 3; i <= 10000; i += 2){
		if (sieve[i]){
			for (int j = i * 3; j <= 10000; j += 2 * i) sieve[j] = false;
		}
	}
	
	for (int i = 0; i < 100; i++) segmentSieve(1000000 * i + 1, 1000000 * (i + 1) + 1);
	
	
	//int N;
	//scanf("%d", &N);
	
	//for (int i = 0; i < N; i++){
		int a, b;
		while (scanf("%d", &b) == 1){
		    int *low = upper_bound(prime, prime + 5761455, 0) - 1, *up = upper_bound(prime, prime + 5761455, b) - 1;
		    printf("%d\n", up - low);
                }
	//}
	
	
	return (0);
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
using namespace std;
const int MAX_N = int(1e6);

int main(){

    int n, i, j, maxin = 2;
    bool jud;
    vector <int> vec;
    for (i = 2; i <= MAX_N; i++){
        jud = 0;
        for (j = 0; j < vec.size(); j++){
            if (vec[j] * vec[j] > i) break;
            if (!(i % vec[j])){
                jud = 1;
                break;
            }
        }
        if (!jud)
            vec.push_back(i);
    }
    while (cin >> n){
        cout << upper_bound(vec.begin(), vec.end(), n) - vec.begin() << endl;
    }
}