Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

func nextInt() int {
	i, e := strconv.Atoi(scanner.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func check(n int) int {
	eratos := make([]bool, n+1)
	cnt := 0
	for i := 2; i < len(eratos); i++ {
		if !eratos[i] {
			cnt++
			for j := i*i; j < len(eratos); j += i {
				eratos[j] = true
			}
		}
	}
	return cnt
}

func main() {
	for scanner.Scan() {
		n := nextInt()
		fmt.Println(check(n))
	}
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <cmath>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <stdio.h>

using namespace std;

int main()
{
	vector<int> p;
	int flag,n;
	
	p.push_back(2);	p.push_back(3);	p.push_back(5);

	for(int i=7;i*i<999999;i++){
		flag=1;
		for(int j=0;j<p.size();j++){
			if(i%p[j]==0){flag=0;break;}
		}
		if(flag) p.push_back(i);
	}

	while(scanf("%d",&n)!=EOF){
		int ans=0;
		for(int i=0;p[i]<=n;i++) ans++;
		cout<<ans<<endl;
	}


	return 0;
}

C++: #include <stdio.h>

int main(void){
	bool a[1000000];
	int n = 0;
	int c = 0;
	while(scanf("%d", &n) != EOF){
		switch(n){
		case 1:
			c = 0;
			break;
		case 2:
			c = 1;
			break;
		default:
			c = 2;
			for(int i=3; i<=n/2; i+=2){
				a[i] = true;
			}
			for(int i=3; i<=n/2; i+=2){
				if(a[i] == false){ continue; }
				c++;
				for(int j=i; j<=n; j+=i){
					a[j] = false;
				}
			}
			break;
		}
		printf("%d\n", c);
	}
	return 0;
}

C++: #include<iostream>

using namespace std;

int main(){
        int n;
        int d; //素数を入れるための添字
        bool count; //割りきれるかどうかの判定
        int number[400000]; //素数を入れておく
        number[0] = 2;
        
        
        while(cin >>n){
                d = 0;
                for(int i = 3; i <= n; i++){
                        count = true;
                        for(int j = 0; j <= d; j++)
                                if(i % number[j] == 0)
                                        count = false;
                        if(count == true){
                                d++;
                                number[d] = i;
                        }
                }
        cout << d+1 << endl;
        }
}

C++: #include<iostream>
#include<math.h>
#include<string.h>
#define N 1000000
#define rep(i,a,b,c) for(int i=a;i<=b;i+=c)
using namespace std;
int a[1000000];
void seive (){
    int n=sqrt(N);
    int i,j;
    memset(a,0,sizeof(a));
    a[0]=a[1]=1;
    rep(i,2,n,1){
        if(!a[i]){
            rep(j,i+i,N,i){
                a[j]=1;
            }
        }
    }
}

int main(){
    seive();
    int m;
    while(cin>>m){
        int cnt = 0;
        rep(i,0,m,1){
            if(!a[i])cnt++;
        }
        cout << cnt << endl;
    }
    return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <cmath>
#include <utility>

#define ll (long long)
#define REP(i,n) for(int i = 0;i<(int)n;i++)
#define FOR(i,a,b) for(int i = (a);i < (b);i++)
#define RFOR(i,a,b) for(int i = (a);i>(b);i--)

using namespace std;

int main (void){
    
    int number[40] = {0};
    int answer[40] = {0};
    int i = 0,j = 0,k = 0;
    while(cin >> number[i])
    {
        for(k = number[i];k>1;k--)
            for (int l = 2; l<k; l++)
            {
                if( (k % l ) == 0)
                    break;
                if((k % l ) != 0 && (l == k -1) )
                    answer[i]++;
            }
        i++;
        
        
    
    }
    REP(j,i)
    cout << answer[j]+1 << endl;
    return 0;
    
}

C++: #include <stdio.h>

int main(){
	long 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 <vector>
#include <algorithm>
#include <cmath>
#include <string>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <iomanip>
#include <utility>
#include <tuple>
#include <functional>
#include <bitset>
#include <cassert>
#include <complex>
#include <stdio.h>
#include <time.h>
#include <numeric>
#include <unordered_map>
#include <unordered_set>
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
# define ll int64_t
# define str string
# define rep(i,n) for(ll i=0;i<n;i++)
# define rrep(i,n) for(ll i=1;i<=n;i++)
# define ALL(x) (x).begin(), (x).end()
# define SZ(x) ((int)(x).size())
# define pb push_back
# define mod 1000000007
# define PI 3.141592653589793
# define vec vector
#define dump(x) cerr<<#x<<"="<<x<<endl
using namespace std;



#define INF 2000000000
#define MAX_V 10



bool compare_by_b(pair<string,ll> a,pair<string,ll> b){
    if(a.second != b.second) return a.second<b.second;
    else return a.first<b.first;
}

bool my_compare(pair<ll,ll> a,pair<ll,ll> b){
    if(a.first != b.first) return a.first<b.first;
    if(a.second != b.second) return a.second>b.second;
    else return true;
}

ll modpow(ll a,ll n,ll mod1) {
    ll res=1;
    while(n>0){
        if(n&1) res=res*a%mod1;
        a = a*a%mod1;
        n >>= 1;
    }
    return res;
}

ll factorial(ll n){
    ll x=1;
    rrep(i,n) x*=i;
    return x;
}

ll gcd(ll a,ll b) { return b ? gcd(b, a%b) : a; }

ll func(ll a,ll b,ll& ans_x,ll& ans_y){
    if(b == 0){
        ans_x = 1;
        ans_y = 0;
        return a;
    }else{
        ll ret = func(b,(a%b),ans_y,ans_x);
        ans_y -= (a/b)*ans_x;
        return ret;
    }
}

vector<bool> IsPrime;

ll sieve(ll max){
    ll sum=0;
    if(max+1 > IsPrime.size()){     // resizeで要素数が減らないように
        IsPrime.resize(max+1,true); // IsPrimeに必要な要素数を確保
    }
    IsPrime[0] = false; // 0は素数ではない
    IsPrime[1] = false; // 1は素数ではない

    for(ll i=2; i<=max; ++i) // 0からsqrt(max)まで調べる
        if(IsPrime[i]) {
            sum++;
            for(size_t j=2; i*j<=max; ++j) IsPrime[i*j] = false;
        }
    return sum;
}



ll lcm(ll a,ll b){
   return a/gcd(a,b)*b;
}
const ll MAX = 5100000;
const ll MOD = 1000000007;
 
ll fac[MAX], finv[MAX], inv[MAX];
 
// テーブルを作る前処理
void COMinit() {
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (ll i = 2; i < MAX; i++){
        fac[i] = fac[i - 1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
        finv[i] = finv[i - 1] * inv[i] % MOD;
    }
}

// 二項係数計算
ll COM(ll n, ll k){
    if (n < k) return 0;
    if (n < 0 || k < 0) return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

using graph = vector<vector<ll>>;
pair<pair<ll,ll>,ll> p[100010];

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n;
    while(scanf("%d",&n)!=EOF){
        cout<<sieve(n)<<endl;
    }
    return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main()
{
    int i,j,flag,n,c=0;
    while(scanf("%d",&n)==1)
    {
        for(i=2;i<=n;i++)
        {
            flag=0;
            for(j=2;j<=sqrt(i);j++)
            {
                if(i%j==0)
                {
                    flag=1;
                }
            }
            if(flag==0)
            {
                c++;
            }
        }
        printf("%d\n",c);
        c=0;
    }
    return 0;
}

C++: #include <stdio.h>
const int MAX_V = 999999;
int main(){
int prime[MAX_V+1]={0};
int sum[MAX_V+2]={0};
int i, k, v ,n;
  for(i = 2; i <= MAX_V; i++) {
    prime[i] = 1;
  }
  for(i = 2; i*i <= MAX_V; i++) {
    if(prime[i]) {
      for(k = 2 * i; k <= MAX_V; k += i) {
        prime[k] = 0;
      }
    }
  }
  for(i=0 ;i<=MAX_V+1 ;i++){
    sum[i+1] += sum[i] + prime[i];
  }

  while(scanf("%d" ,&n)!=EOF){
     printf("%d\n" ,sum[n+1]);
  }
  return 0;
}