Go: package main

import (
        "fmt"
)

const MAX int = 1000000
var primes [MAX]int

func main() {
        Init()
        var n, cnt int
        for {
                _, err := fmt.Scanf("%d", &n)
                if err != nil {
                        break
                }
                cnt = 0
                for i := 2; i <= n; i++ {
                        cnt += primes[i]
                }
                fmt.Printf("%d\n", cnt)
        }
}

func Init() {
        for i := 0; i < MAX; i++ {
                primes[i] = 1
        }
        primes[0] = 0
        primes[1] = 0
        Soe()
}

func Soe() {
        for i := 2; i < MAX; i++ {
                if primes[i] > 0 {
                        for j := i*2; j < MAX; j+=i {
                                primes[j] = 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<cstdio>
#include<cstring>
#include<algorithm>
#include<map>
#include<vector>
#include<cmath>
#include<string>
#include<queue>
#define eps 1e-9
#define ll long long
#define INF 0x3f3f3f3f
using namespace std;

const int n=1000000+10;
const int maxn=1000000+10;
int prime[maxn],vis[maxn];
int init()
{
    int c=0,m;
    memset(vis,0,sizeof(vis));
    m=sqrt(n+0.5);
    for(int i=2;i<=m;i++)
    {
       if(!vis[i])
       {
           for(int j=i*i;j<=n;j+=i)
              vis[j]=1;
       }
    }
    for(int i=2;i<=n;i++)
    {
        if(!vis[i])
            prime[c++]=i;
    }
    return c;
}


int main()
{
    int N;
    int k=init();
    while(~scanf("%d",&N))
    {
        int ans=upper_bound(prime,prime+k,N)-prime;
        printf("%d\n",ans);
    }
    return 0;
}

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

int main(void){
	int n;
	while(scanf("%d",&n)!=EOF){
		int count=0;
		std::vector<int> num;
		int max=2;
		for(int m=2;m<=n;m++){
			bool check=true;
			for(int i=0;i<count;i++){
				if(m!=num[i] && m%num[i]==0){
					check=false;
					if(max<m){
						max=m;
					}
					break;
				}
			}
			if(check==true){
				if(count>0){
					if(max>num[count-1]){
						for(int i=max+1;i<m;i++){
							if(i!=m && m%i==0){
								check=false;
								if(max<m){
									max=m;
								}
								break;
							}
						}
					}
					else{
						for(int i=num[count-1]+1;i<m;i++){
							if(i!=m && m%i==0){
								check=false;
								if(max<m){
									max=m;
								}
								break;
							}
						}
					}
				}
				else{
					for(int i=max;i<m;i++){
						if(i!=m && m%i==0){
							check=false;
							if(max<m){
								max=m;
							}
							break;
						}
					}
				}
			}
			if(check==true){
				num.push_back(m);
				count++;
			}
		}
		printf("%d\n",count);
	}
	return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
	int a,n,i,j;
	cin>>a;
	n=0;
	for(i=2;i<=a;i++){
		for(j=2;j<i;j++){
			if(i%j==0)break;
		}
		if(i == j) n += 1;
	}
	cout<<n;
}

C++: #define _CRT_SECURE_NO_WARNINGS

#include <iostream>
#include <vector>
#include <list>
#include <algorithm>

using namespace std;

int main(int argc, char const* argv[])
{
    ios::sync_with_stdio(false);
    cin.tie(0);

    vector<int> nl;
    int mx = 0;
    do{
        int n;
        cin >> n;
        if (!cin.eof()){
            nl.push_back(n);
            mx = max(mx, n);
        }else{
            break;
        }
    }while(1);


    double r = sqrt(mx);

    list<int> sl;
    vector<int> primes;

    for (int i = 2; i <= mx; i++) {
        sl.push_back(i);
    }

    while (sl.front() <= r) {
        int p = sl.front();
        primes.push_back(p);
        sl.pop_front();

        auto i = sl.begin();
        while(i != sl.end()){
            if(*i % p == 0){
                i = sl.erase(i);
            }
            else {
                i++;
            }
        }
    }

    for (auto&& i : sl) {
        primes.push_back(i);
    }

    for (auto&& n : nl) {
        auto p = upper_bound(primes.begin(), primes.end(), n);
        cout << p - primes.begin() << endl;
    }

    return 0;
}

C++: #include <iostream>
using namespace std;
int main(){
	int n,k,kk;
	while(cin>>n){
		kk=0;
		for(int j=3;j<=n;j+=2){
			k=0;
			for(int i=3;i<=sqrt(j);i+=2){
				if(j%i==0)k++;
		        }
		     if(k==1)kk++;
		   }
		if(n==1)cout<<"0"<<endl;
		else if(n==2)cout<<"1"<<endl;
		else cout<<kk+1<<endl;
	   }
	return 0;
	}

C++: #include<iostream>
#define N 999999
int p[(N+1)/2];int m[N+1];void k();using namespace std;main(){int n;p[0]=2;m[1]=0;m[2]=1;k();while(cin>>n){cout<<m[n]<<endl;}}void k(){int i,j,e=1;for(i=3;i<N+1;i++){for(j=0;p[j]*p[j]<=i;j++){if(i%p[j]==0){goto tag;}}p[e]=i;e++;tag:m[i]=e;}}

C++: #include<bits/stdc++.h>
#include<cstdio>

using namespace std;

int pri[1000000];

int main()
{

    pri[0]=1;
    pri[1]=1;

    for(int i=4;i<1000000;i+=2)
    {
        pri[i]=1;
    }

    for(int i=2;i*i<1000000;i++)
    {
        for(int j=2;j*i<1000000;j++)
        {
            pri[i*j]=1;
        }
    }
    int n,c;
    while(scanf("%d",&n))
    {
        c=0;
        for(int i=2;i<=n;i++)
        {
            if(pri[i]==0)
            {
                c++;
            }
        }
       printf("%d\n",c);

    }

    return 0;
}

C++: //--------------------------------------------------template----------------------------------------//
//c
#include <stdio.h>
#include <time.h>
#include <math.h>
//c++
#include <cstdio>
#include <iostream>
#include <algorithm>
#include <functional>
#include <cstdlib>
#include <sstream>
#include <string>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <deque>
#include <vector>
#include <complex>
#include <cmath>
//c++11
#include <tuple>
#define all(c) ((c).begin(),(c).end())
#define rall(c) (c).rbegin(),(c).rend()
//#define sort(v,n) sort(v,v+n)
//#define vsort(v) sort(v.begin(),v.end())
#define vvsort(v) sort(v.rbegin(),v.rend())
#define ll long long
#define pb(a) push_back(a)
#define fi first
#define se second
#define inf 999999999
using namespace std;
const ll MOD = 1e9 + 7;
const double PI = acos(-1.0);
//---------------------------------------------------------------------------------------------//
int n;

bool is_prime(int a) {
	int now = 2;
	while (now*now <= a) {
		if (a%now == 0)return 0;
		else now++;
	}
	return 1;
}


int main() {
	while (cin >> n) {
		ll ans = 0;
		for (int i = 2; i <= n; i++) {
			if (is_prime(i))ans++;
		}
		cout << ans << endl;
	}
	return 0;
}

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

int primes[78498], num_primes = 1;
int calculated = 2;
int count_pos(int n) {
	for (int i = 0; i < num_primes; i++) {
		if (primes[i] == n) {
			return i + 1;
		}
		else if (primes[i] > n) {
			return i;
		}
	}
	return num_primes;
}
int main() {
	primes[0] = 2;
	while (true) {
		int n;
		if (!scanf_s("%d", &n)) break;
		if (calculated+1 < n) {
			for (int i = calculated + 1; i < n + 1; i += 2) {
				bool is_prime = true;
				for (int w = 3; w < (int)(sqrt(i)) + 1; w += 2) {
					if (i%w == 0) {
						is_prime = false;
						break;
					}
				}
				if (is_prime) {
					primes[num_primes++] = i;
				}
			}
			calculated = n - (n % 2);
		}
		printf("%d\n", count_pos(n));
	}
}