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>
#include <queue>
#include <stack>
#include <vector>
#include <stdio.h>
#include <algorithm>
#include <string.h>
#include <string>
#include <cmath>
#include <complex>
#include <map>
using namespace std;

#define rep(i,n) for(int i=0;i<int(n);++i)
#define ALL(v) (v).begin(),(v).end()
#define PB push_back
#define EPS 1e-8

static const double PI=6*asin(0.5);
typedef complex<double> P;
typedef long long ll;

int t;

int main(){
	int *prime;
	prime = (int *)malloc(100010*sizeof(t));
	if(prime==NULL) cout<<"no mem\n";
	rep(i,100010){
		prime[i]=true;
	}
	prime[0]=prime[1]=false;
	for(int i=2;i*i<100010;i++){
		if(prime[i]){
			for(int j=i*i;j<100010;j+=i){
				prime[j]=false;
			}
		}
	}
	
	int *sum;
	int num=0;
	sum=(int *)malloc(100010*sizeof(t));
	if(sum==NULL) cout<<"no mem\n";
	rep(i,100010){
		if(prime[i]){
			num++;
		}
		sum[i]=num;
	}
	int n;
	while(cin>>n){
		cout<<sum[n]<<endl;
	}
	return 0;
}

C++: #include <bits/stdc++.h>
#include <vector>
#include <numeric>
#define PI 3.14159265358979323846
#define MAXINF (1e18L)
#define INF (1e9L)
#define EPS (1e-9)
#define MOD (1e9+7)
#define REP(i, n) for(int i=0;i<int(n);++i)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define RREP(i, n) for(int i=int(n)-1;i>=0;--i)
#define ALL(v) v.begin(),v.end()
#define FIND(v,x) (binary_search(ALL(v),(x)))
#define SORT(v) sort(ALL(v))
#define RSORT(v) sort(ALL(v));reverse(ALL(v))
#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl
#define Yes(n) cout<<((n)?"Yes":"No")<<endl
#define YES(n) cout<<((n)?"YES":"NO")<<endl
#define pb push_back
#define fi first
#define se second
using namespace std;
template<class A>void pr(A a){cout << (a) << endl;}
template<class A,class B>void pr(A a,B b){cout << a << " "  ;pr(b);}
template<class A,class B,class C>void pr(A a,B b,C c){cout << a << " " ;pr(b,c);}
template<class A,class B,class C,class D>void pr(A a,B b,C c,D d){cout << a << " " ;pr(b,c,d);}
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll,ll> pll;

int count_prime(int n){
    int p = 0;
    vector<int> prime(n+1, true);
    prime[0] = prime[1] = false; 
    for(int i = 2; i <= n; i++)
        if(prime[i]) {
            p++;
            for (int j = i*2; j <= n; j += i) prime[j] = false;
        }

    return p;
}

int main(void)
{
    int n;
    while(cin >> n)
        pr(count_prime(n));
}

C++: #include <iostream>
#include <math.h>
#define MAX 1000000
using namespace std;

int main(){
	int n;
	int *prime,i,j;
	prime = new int[80000];
	int *answer = new int[MAX];
	int count;
	bool flag;

	count = 1;
	answer[0] = 0;
	answer[1] = 0;
	answer[2] = 1;
	for(i = 3; i <= 1000000; i+=2){
		flag = true;
		for(j = 0; j < count; j++){
			if(i % prime[j] == 0){
				flag = false;
				break;
			}
			if(prime[j] * prime[j] > i) break;
		}
		if(flag == true){
			prime[count++] = i;
		}
		answer[i] = count;
		answer[i+1] = count;
	}
	while((cin >> n)!= NULL){
		cout << answer[n] << endl;
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
int main(){
	int a,b,c=0;
	int flg=0;
	while(cin>>a){
		for (int A=2;A<=a;A++){
		flg=0;
			for (int B=2;B<=A;B++){
				b=A%B;
				if (A==B){
					c++;
					break;
				}
				if (b==0) break;
			}
		}
		cout<<c<<endl;
		c=0;
	}
	return 0;
}

C++: #include <iostream>
#include <vector>
//#include <math.h>
//#include <string>
#include <cmath>

int main(void);
int count_prime_number(int n);

int main(){
	int n;
	int cnt = 0;
	int result[30];
	//std::vector<int> result;

	//while (cnt < 3){
		//std::cin >> n;
	while (std::cin >> n){
		result[cnt] = count_prime_number(n);
		cnt++;
	}

	for (int i = 0; i < cnt; i++){
		std::cout << result[i] << std::endl;
	}

	//while (1);
	return 0;
}


//0..nまでの素数の個数を出力する
//args:数値
//return:素数の個数
inline int count_prime_number(int n){
	int rtn = 0;

	//2だけ個別チェック
	if (n >= 2){
		rtn++;
	}

	for (int i = 3; i <= n; i+= 2)
	{
		bool is_prime_num = true;
		for (int j = 3; j <= sqrt(i); j+= 2)		//ルートi以上のiの約数はルートi以下のiの約数と対になっているので範囲はルートiまででいい
		{
			if (i % j == 0)
			{
				is_prime_num = false;
				break;
			}
		}

		if (is_prime_num){
			rtn++;

			//デバッグ
			//std::cout << i << std::endl;
		}
	}

	return rtn;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main()
{
	vector<int> input;

	while (1)
	{
		int tmp;

		cin >> tmp;

		if (!tmp)
		{
			return 0;
		}

		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>

int main(void){
	long i,j;
	long n;
	int cnt;
	int prime_cnt;

	while(scanf("%ld",&n) != EOF){
		prime_cnt = 0;
		for(i = 0;i < n+1;i++){
			cnt = 0;
			if(i < 2) continue;
			if(i == 2){
				prime_cnt++;
				continue;
			}
			for(j = 2;j <= i/2;j++){
				if(i % j == 0){
					cnt++;
					break;
				}
			}
			if(cnt == 0) prime_cnt++;
		}
		printf("%d\n",prime_cnt);		
	}
	return 0;
}

C++: #include<stdio.h>
int flg[1000000];
int main(void)
{
	int n,i,j,cnt;
	for(i=2;i<=999999;i++)	{
		flg[i]=1;
	}
	for(i=2;i*i<=999999;i++)	{
		if(flg[i]==1)	{
			j=2;
			while(i*j<=999999)	{
				flg[i*j]=0;
				j++;
			}
		}
	}
	while(scanf("%d",&n)!=EOF)	{
		cnt=0;
		for(i=2;i<=n;i++)	{
			if(flg[i]==1)	{
				cnt++;
			}
		}
		printf("%d\n",cnt);
	}
	return 0;
}

C++: #include <bits/stdc++.h>
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define RFOR(i,a,b) for(int i=(b) - 1;i>=(a);i--)
#define REP(i,n) for(int i=0;i<(n);i++)
#define RREP(i,n) for(int i=n-1;i>=0;i--)

#define PB push_back
#define INF INT_MAX/3
#define ALL(a) (a).begin(),(a).end()
#define CLR(a) memset(a,0,sizeof(a))

typedef long long int ll;

using namespace std;

int main(){
    vector<int> primes;
    primes.PB(2);
    FOR(i,3,100000){
	bool flg = true;
	REP(j,primes.size()) if(i % primes[j] == 0 ) flg = false;
	if(flg) primes.PB(i);
    }

    int n;
    while(true){
	cin >> n;
	if(cin.eof())break;
	int res = upper_bound(ALL(primes),n) - primes.begin();
	cout << res << endl;
    }

    return 0;
}