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>

using namespace std;

int main(){
  int n, N;
  int *prime;
  int i,j;
  while(cin >> N){
    n = N + 1; 
    //init
    prime = new int[n];
    for(i=0; i<=n; i++) prime[i] = 0;

    //素数判定
    for(i=2; i*i<=n; i++){
      if(prime[i] == 0){
	prime[i] = 1; //iは素数
	for(j=2; i*j<=n; j++){//倍数ふるい落とし
	  prime[i*j] = -1;//i*jは素数じゃない
	}
      }
    }
    for(i=i; i<=n; i++){
      if(prime[i] == 0) prime[i] = 1;
    }
    
    //output
    int ans=0;
    for(i=2; i<=n; i++){
      if(prime[i] == 1) {
	ans ++;
	//cout << "prime:" << i << endl;;
      }
    }
    cout << ans << endl;

    delete[] prime;
  }
  return 0;
}

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


int main() {
	int n;
	bool a;
	std::vector<int> prime;
	prime.emplace_back(2);
	for (int i = 3; i < 1000000; i += 2) {
		a = false;
		for (int j = 3; j <= std::sqrt(i); j += 2) {
			if (i%j == 0) {
				a=true;
			}
		}
		if (a == false) {
			prime.emplace_back(i);
		}
	}
	while (std::cin >> n) {
		int ans = 0;
		for (int i = 0; i < prime.size(); i++) {
			if (prime[i] <= n) {
				ans++;
			}
		}
		std::cout << ans << std::endl;
	}
}

C++: #include<iostream>
#include<vector>
#define max 1000010
using namespace std;

bool isp[max];
vector<int> P;

void sieve(){
	for(int i=0;i<max;i++)isp[i]=true;
	for(int i=2;i<max;i++){
		if(isp[i]){
			P.push_back(i);
			for(int j=2*i;j<max;j+=i)isp[j]=false;
		}
	}
	return;
}

int main()
{
	int n;

	sieve();
	while(cin>>n){
		int c=0;
		for(int i=0;i<P.size();i++)if(P[i]<=n)c++;
		cout<<c<<endl;
	}

	return 0;
}

C++: #include <iostream>
#include <bitset>
using namespace std;
bitset<1000000> p;
int main(){
  p.flip();
  p[0] = 0;
  p[1] = 0;
  for(int i = 2; i < 1000; ++i){
    if(p[i]){
      for(int j = 2*i; j < 1000000; j += i) p[j] = 0;
    }
  }
  int n;
  while(cin >> n){
    int ret = 0;
    for(int i = 2; i <= n; ++i) ret += p[i];
    cout << ret << '\n';
  }
  return 0;
}

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

void solve(int x){
	bool C[x];
	
	C[0]=false; C[1]=false;
	for(int i=2; i<=x; i++){
		C[i]=true;
	}
	
	for(int i=2;i<(int)sqrt(x)+1; i++){
		if(C[i]){
			for(int y=i*2; y<=x; y+=i) C[y]=false;
		}
	}
	
	int count=0;
	for(int i=0; i<=x; i++){
		if(C[i]){
			count++;
		}
	}
	
	cout << count << endl;  
}

int main(){
	int n;
	
	while(cin >> n){
		solve(n);
	}
	
	return 0;
}

C++: #include<stdio.h>
#include<math.h>
int prime(int n);

int main()
{
	int k,j,n;
	int store,sum=0;

	while(scanf("%d",&n)!=EOF)
{

			for(j=1;j<=n;j++)
			{
				store=prime(j);
				if(store==1)
					{
						sum++;
					}

			}
			printf("%d\n",sum);
			sum=0;



}


return 0;

}
int prime(int n)
{


	int i;
	float x,k;
	int l;


	x=float(n);
	k=sqrt(x);

	l=int(k);

		if(n==1 || n==0)
			{
				return 0;
			}


		if(n==2)
			{
				return 1;
			}

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

		}

	return 1;


}

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

int n,mem[1000000];
int count;
int now=2;

//?´???°??????0
int main(){
	mem[1]=1;
	for(int i=2;i<=(int)sqrt(1000000.0)+1;i++){
		if(mem[i]==0){
			for(int j=2*i;j<=1000000;j+=i){
				mem[j]=1;
			}
		}
	}
	while(cin>>n){
		count=0;
		for(int i=1;i<n+1;i++){
			if(mem[i]==0) count++;	
		}
		cout<<count<<endl;
	}
	return 0;
}

C++: #include <stdio.h>
#include <math.h>
#define MAX 999999

int main(void) {

  int flag[MAX+1];
  int n,count;

  int i,j;

  for(i=0;i<=MAX;i++) {
    flag[i]= (i<=1)? 0: 1;
  }

  for(i=2;i<=(int)sqrt(MAX)+1;i++) {
    if(flag[i]==1) {
      for(j=i*2;j<=MAX;j+=i) flag[j]=0;
    }
  }
  //for(i=0;i<=MAX;i++) printf("%d\n",flag[i]);
  while(scanf("%d",&n)!=EOF) {

    count=0;

    for(i=2;i<=n;i++) {

      if(flag[i]==1) count++;
    }

    printf("%d\n",count);

  }

  return 0;

}

C++: #include<iostream>
int main(){int m=1,n=999999,o,c=n/2+1,d;bool*v=new bool[c],*p=v+c;for(;*--p=1,p!=v;);for(;m<499;m++)for(o=m*2+1,d=o*3;v[m]&&d<n+1;d+=o*2)v[d/2]=0;for(;!(std::cin>>n).eof();printf("%d\n",c))for(c=n-->1,m=1;m<n/2+1;)c+=v[m++];delete[]v;}

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

int prime(int n);
int main (void){
	int n;
	while(~scanf("%d",&n)){
		std::cout << prime(n) << std::endl;
	}

	return 0;
}

int prime(int n){
	bool *flag = new bool[n];
	int res=0,i=0;
	for(i=0;i<=n;flag[i++]=0);
	for(i=2;i<=n;){
		for(int j=2;i*j<=n;j++)
			flag[i*j]=1;
		for(i++;flag[i]==1 ;i++);

	}
	for(i=0;i<=n;i++)
		res += flag[i]?0:1;
	delete [] flag;
	return res-2;
}

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

int prime[1000000];


bool isPrime(int n) {
  int x = 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<iostream>

#define LIM 1000000
using namespace std;

int main(){
  bool num_map[LIM];
  int in_num,prime_count;
  for(int i=0;i<LIM;i++){
    num_map[i]=1;
  }
  
  for(int i=2;i<LIM;i++){
    if(num_map[i]==1)
      for(int j=i*2;j<LIM;j+=i){
	num_map[j]=cout<<in_num<<endl<<endl;
	for(int i=2;i<=in_num;i++){
	  if(num_map[i]==1)
	    prime_count++;
	}
	cout<<prime_count<<endl;  
      }
    
  }
}

C++: #include<iostream>
#include<conio.h>
#include<cmath>    

using namespace std;

int main()
{
     int n;
     
     cin>>n;

     for (int i=2; i<n; i++)
        for (int j=2; j*j<=i; j++)
        {
            if (i % j == 0)
                break;
            else if (j+1 > sqrt(i)) {
                cout << i << endl;
            }
        }

     return 0;
 }

C++: #include <iostream>
#include <vector>
 
using namespace std;
 
const int N = 1000000;
bool flag[N];
int num[N];
 
int main(){
	for(int i=2;i<N;i++) flag[i] = true;
	for(int i=2;i*i<N;i++){
		if(not flag[i]) continue;
		for(int j=i*i;j<N;j+=i){
			flag[j] = false;
		}
    }
    num[0] = 0;
    for(int i=1;i<N;i++){
        if(flag[i]){
            num[i] = num[i-1] + 1;
        }else{
            num[i] = num[i-1];
        }
    }
    int n;
    while(cin >> n){
        cout << num[n] << endl;
    }
    return 0;
}

C++: #include<stdio.h>
int main(void)
{
    int n,i,j,z,x;
	
    
	while(scanf("%d",&n)!=EOF){
		for(i=2;i<=n;i++){
			z=0;
			for(j=2;j*j<=i;j++){
				if(i%j==0){
					z++;
					break;
				}
			}
			if(z==0){
				x++;
			}
		}
		printf("%d\n",x);
		x=0;
	}
	return 0;
}

C++: #include<iostream>
using namespace std;
bool prime(int n){
	for(int i=2;i<n;i++){
		if(n%i==0)return false;
	}
	return true;
}
int main(){
	int a,sum=0;
	while(cin>>a){
	for(int j=2;j<a;j++){
		if(prime(j))sum++;
	}
	cout<<sum<<endl;
	sum=0;
	}
}