Go: package main

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

func main() {
	scanner := bufio.NewScanner(os.Stdin)

	for scanner.Scan() {
		num, _ := strconv.Atoi(scanner.Text())
		flag := false
		count := 0

		for i := 2; i <= num; i++ { //1を除く入力値以下の数字
			for j := i - 1; j >= 1; j-- {
				if i%j == 0 && j != 1 {
					flag = true
					break
				}
			}
			if !flag {
				count++
			} else {
				flag = false
			}
		}
		fmt.Println(count)
	}
}

C++: #include<stdio.h>
#include<iostream>
#include<math.h>
#include<chrono>
#include<string>
#include<fstream>
#include<vector>

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


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

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

class Mod{
		const int n;
		public:
			Mod(int const _n):n(_n){}
			bool operator()(const int _src)const{
				return ! (_src % n);
			}
	};

const std::vector<int> Eratosthenes(const std::vector<int> src){
	std::vector<int> result,tmp(src);
	while(!tmp.empty()){
		int n(tmp[0]);
		result.push_back(n);
		Mod mod(n);
		std::vector<int>::iterator it= std::remove_if( tmp.begin(), tmp.end(), mod );
		tmp.erase(it,tmp.end());
	}
	return result;
}

static int const MAX(1000000);

int main(int,char**){
	std::vector<int> vec;
	vec.reserve(MAX);
	for(int i(2);i<=MAX;++i) vec.push_back(i);
	std::vector<int> const primes= Eratosthenes(vec);
	int n;
	while(std::cin >>n){
		std::vector<int>::const_iterator it =  std::upper_bound(primes.begin(),primes.end(),n);
		std::cout << it-primes.begin() <<std::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*(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>

using namespace std;

int main(void)
{
	int n;
	int pnum;
	
	while (cin >> n){
		int ct  = 0;
		for (int i = 2; i <= n; i++){
			for (pnum = 2; pnum <= n; pnum++){
				if (i % pnum == 0){
					break;
				}
			}
			if (i == pnum){
				ct++;
			}
		}
		cout << ct << endl;
	}
	
	return (0);
}

C++: #include<iostream>

#define loop(i,a,b) for(int i=a;i<b;i++)
#define rep(i,a) for(int i=0;i<a;i++)


using namespace std;



int main(){
	bool prime[1000001];
	rep(i, 1000001){
		prime[i] = true;
	}
	prime[0] = prime[1] = false;
	
	loop(i, 2, 1001){
		if (prime[i]){
			for (int j = i * 2; j < 1000001; j += i){
				prime[j] = false;
			}
		}
	}



	int ord;
	while (cin >> ord){
		int cnt=0;
		rep(i, ord + 1){
			if (prime[i]){
				cnt++;
			}
		}
		cout << cnt << endl;
		cnt = 0;
	}


}

C++: #include <stdio.h>

int main(){
	int sosuu[] = { 2, 3, 5 }, i, x, y = 3,n=0,z,d=0;
	while (scanf("%d", &n) != EOF){
		z = 0;
		if(d < n)
		{for (i = 3; i < n; i += 2){
			for (x = 0; x < y; x++){
				if (i % sosuu[x] == 0){ break; }
				if (x + 1 == y){ sosuu[y] = i, y++; d = i; }
			}
		}
	}
		for (i = 0; i < n; i++){ if (sosuu[i] > n + 1)break; z++; }printf("%d\n", z);
	}
	return 0;
}

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

int main(){
	bool *map;
	string str;
	int n, cnt;

	while(1){
		cnt = 0;
		getline( cin, str );
		if( str.size()==0 )	break;
		sscanf(str.c_str(), "%d", &n);
		map = new bool[n+1];
		for( int i=2; i<=n; i++ ){
			map[i] = true;
		}

		for( int i=2; i<=n/2; i++ ){
			for( int j=2*i; j<=n; j+=i ){
				if( map[j] ){
					map[j] = false;
				}
			}
		}

		for( int i=2; i<=n; i++ ){
			if( map[i] ){
				cnt++;
			}
		}
		cout << cnt << endl;
		delete[] map;
		map = 0;
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <cctype>
#include <cmath>
#include <stdio.h>
#include <algorithm>
#include <vector>
#include <map> // pair
#include <set>
#include <string>

#define MAXSIZE 200000
#define PI 3.141592653589
using namespace std;

template<typename T>
T add(T a,T b){
  return a+b;
}

namespace A
{
  int f(void){
    return 1;
  }
}

int main(){
  int n;
  while (std::cin >> n) {
    int count=0;//素数の数
    for(int i=2;i<=n;i++){
      if(i==2){
        count++;
        continue;
      }
      int flag=0;
      for(int j=2;j<=sqrt(i);j++){
        if(i%j==0){
          flag=1;
        }
      }
      if(flag==0){
        count++;
      }
    }
    std::cout << count << '\n';
  }

}

C++: #include<iostream>
#include<string>
#include<cstdio>
#include<algorithm>
#include<stack>
#include<queue>
#include<vector>
#include<cmath>
#define ll long long int
using namespace std;
typedef pair<int, int> P;

string str;

int main()
{
  int n;
  int i,j;

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

    if(n==1){
      cout<<"0"<<endl;
      continue;
    }else if(n==2){
      cout<<"1"<<endl;
      continue;
    }
    int sum=1;
    int prime[1000000]={};
    for(i=2;i<n;i++){
      if(prime[i]==0)
	for(j=2;j*i<n;j++){
	  if(prime[i*j]==0){
	    prime[i*j]=1;
	    sum++;
	  }
	}
    }
    cout<<n-sum<<endl;
  }

}

C++: #include<iostream>
#include<string>
#include<algorithm>
#include<map>
#include<set>
#include<utility>
#include<vector>
#include<cmath>
#include<cstring>
#include<cstdio>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define rp(a) while(a--)
#define pb push_back
#define mp make_pair
#define all(in) in.begin(),in.end()
const double PI=acos(-1);
const double EPS=1e-10;
using namespace std;
typedef long long ll;
typedef vector<int> vi;
bool sosu[1000010]={1,1,0};
int main(){
	for(int i=4;i<1000010;i+=2)sosu[i]=true;
	for(int i=3;i<1000010;i+=2)
		for(int j=i*3;j<1000010;j+=2*i)sosu[j]=true;
	
	int n;
	while(cin>>n){
		int co=0;
		loop(i,2,n+1)if(!sosu[i])co++;
		cout<<co<<endl;
	}
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <cstdio>
using namespace std;
int main()
{
	int tmp;
	while (scanf("%d", &tmp) != EOF)
	{
		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;
	}

	return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <string.h>

int prime(int n) {
	int k;
	if(n == 2) return 1;
	if(n % 2 == 0) return 0;
	for(k = 3; k * k <= n; k += 2) {
		if(n % k == 0) return 0;
	}
	return 1;
}

int cntPrime(int n) {
	int cnt = 0, i, k;
	for(i = 2; i <= n; i++) {
		cnt += prime(i);
	}
	return cnt;
}

int main() {
	int a;
	int arg[1024] = { 0 };
	int cnt = 0;
	while(scanf("%d", &a) != EOF) {
		arg[cnt++]=a;
	}
	int i;
	for(i = 0; i < cnt; i++) {
printf("%d\n", cntPrime(arg[i]));
	}
}

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

using namespace std;

bool p[10000000];
vector<int> prime;

void init(){
    memset(p,1,sizeof(p));
    p[0] = p[1] = false;
    for(int i = 2; i <= 1000000; i++){
        if(!p[i]) continue;
        for(long long j = 1LL*i*i; j <= 1000000; j += i)
            p[j] = false;
        prime.push_back(i);
    }
}

int CntPrime(int n){
    vector<int>::iterator it = lower_bound(prime.begin(), prime.end(), n);
    if( *it > n) it--;
    return (it - prime.begin() + 1);
}

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