Go: package main
import (
	"fmt"
	"os"
	"strconv"
	"bufio"
	//"math"
)

var sc = bufio.NewScanner(os.Stdin)

func main(){
	for sc.Scan(){
		n := nextInt()
		fmt.Println(calc(n))
	}
}

func calc(n int)int{
	// エラトステネスで数え上げる
	// 最初から999999まで用意しておいて入力のたびO(1)で出すのとどっちが速いのかはわからん
	eratos := make([]bool, n+1)
	c := 0
	for i:=2; i<len(eratos); i++{
		if eratos[i] == false{
			c++
			for j:=i*i; j<len(eratos); j+=i{
				eratos[j]=true
			}
		}
	}
	return c
}

func next()string{
	return sc.Text()
}

func nextInt()int{
	a, _ := strconv.Atoi(next())
	return a
}

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

using namespace std;

const int N = 1000010;
bool isPrime[N];

void sieve(){
    for(int i=2;i<N;i++) isPrime[i] = true;
    for(int i=2;i*i<N;i++){
        if(not isPrime[i]) continue;
        for(int j=i*i;j<N;j+=i){
            isPrime[j] = false;
        }
    }
}

int main(){
    vector<int> C(N);
    sieve();
    C[0] = 0;
    for(int i=1;i<N;i++){
        if(isPrime[i]){
            C[i] = C[i-1] + 1;
        }else{
            C[i] = C[i-1];
        }
    }
    int n;
    while(cin >> n){
        cout << C[n] << endl;
    }
    return 0;
}

C++: #include <stdio.h>

#define MAX (100000)

int main(void)
{
	int prime[MAX + 1] = {1};
	int n;
	int ans;
	int i;
	char str[8];
	
	for (int i = 0; i <= MAX; i++){
		prime[i] = 1;
	}
	
	prime[0] = prime[1] = 0;
	i = 0;
	while (i * i <= MAX){
		while (prime[++i] != 1 && i * i <= MAX);
		//printf("%d ", i);
		for (int j = 2; j * i <= MAX; j++){
			//printf("%d\n", i * j);
			prime[i * j] = 0;
		}
	}
	
	while (fgets(str, 7, stdin)){
		sscanf(str, "%d", &n);
		
		ans = 0;
		for (int i = 2; i <= n; i++){
			if (prime[i] == 1){
				ans++;
			}
		}
	
		printf("%d\n", ans);
	}
	
	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++: int main(void)
{
	int data,i,j,k=0,l=0,kai[100]={0},s[100000]={0};

	while(scanf("%d",&data) != EOF)
	{
		if(data<=1)
		   kai[k]=0;
		else
		{
			for(i=2;i<=data;i++)s[i]=i;

		    i=2;
			while(i*2<data)
			{
			  if(s[i] != 0)
			  {  
				j=2;
				while(j*i<=data)
				{
					s[j*i]=0;
					j+=1;
				}
			  }
			  i+=1;
			}

			
			for(i=2;i<=data;i++)
			{	
				if(s[i]!=0)
				{
					kai[k]+=1;
				}
			}
		}
		k+=1;
	}

  for(j=0;j<k;j++)
    printf("%d\n",kai[j]);
  return 0;
}

C++: #include<stdio.h>


int main(){
	int n;
	bool data[10000000];

	int i,j;
	data[0]=data[1]=0;
	for(i=2;i<10000000;i++){
		data[i]=1;
	}

	for(i=0;i<10000000;i++){
		if(data[i]==1){
			for(j=i+i;j<1000000;j=j+i){
				data[j]=0;
			}
		}
	}


	while(scanf("%d",&n)!=EOF){
		j=0;
		for(i=0;i<n;i++){
			if(data[i]==1)j++;
		}
		printf("%d\n",j);
	
	}
	return 0;
}

C++: #include <iostream>
#include <cstring>
#include <cmath>
using namespace std;
bool isPr(int n);
int main(){
    int a[1000000],n;
    memset(a,0,sizeof(a));
    a[2]=1;
    for(int i=3;i<1000001;i++)
        a[i]=a[i-1]+isPr(i);
    while(cin>>n){
        cout<<a[n]<<endl;
    }
    return 0;
}
bool isPr(int n){
    for(int i=sqrt(n)+1;i>1;i--){
        if(!(n%i)){
            return 0;
        }
    }
    return 1;
}

C++: #include <stdio.h>

const int MAX_V=1000000;
int prime[MAX_V+1];//1?´???°,0?´???°??§??????
int sum[MAX_V+1];
int main(){
    int i,k,v;
    for(i=2;i<=MAX_V;i++){
        prime[i]=1;//2??\???????´???°??¨??????
    }
    for(i=2;i*i<=MAX_V;i++){
        if(prime[i]){
            for(k=2*i;k<=MAX_V;k+=i){
                prime[k]=0;//?´???°???2?????\???????????°????´???°??§??????
            }
        }
    }
    for(i=0;i<MAX_V+1;i++){
        sum[i+1]=sum[i]+prime[i];
    }
    while(scanf("%d",&v)!=EOF){
        printf("%d\n",sum[v+1]);
    }
    return 0;
}

C++: #include <iostream>
using namespace std;
int main(){
	int x = 1000000;
	int p[x],q[x];
	for(int i=2;i<x;i++) p[i] = 0;
	for(int i=2;i<x;i++){
		if(q[i]==0) for(int j=2;j<x/i;j++) q[i*j] = 1;
	}
	int c = -1;
	for(int i=2;i<x;i++){
		if(q[i]==0){
			c++;
			p[c] = i;
		}
	}
	int n;
	while(cin >> n){
		int a = 0;
		for(int i=0;i<c;i++){
			if(p[i]>n) break;
			a++;
		}
		cout << a << endl;
	}
}

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

#define LIM 1000000
using namespace std;

void prime();

vector<int> prime_map;

int main(){
  int num, i=0,j=0;
  vector<int> count;
  prime();
  cout<<"plese"<<endl;
  while(cin){
    cin>>num;
    i=0;
    while(prime_map[i]<=num){
      i=i+1;
    }
    count.push_back(i);
    j++;
  }

  i=0;
  while(i<j){
    cout<< count[i]<<endl;
    i++;
  }
}

void prime(){
  bool judge;
  for(int i = 2; i <= LIM;i++){
    judge=1;
    for(int j= 2; j < i/2; j++){
      if(i%j == 0){judge = 0;
 break;
}
    }
    if(judge == 1)prime_map.push_back(i);
  }
}

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

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

int main(){
int n,a;
int pri[99999];
pri[0]=2;
int ans=1;
while(cin>>n){
ans=1;
for(int i=3;i<n+1;i++){
pri[ans]=i;
for(int j=0;j<ans&&pri[j]*pri[j]<i+1;j++){
a=i%pri[j];
if(a==0){
ans--;
break;}
}
ans++;
}
if(n==1){ans=0;}
cout<<ans<<endl;}}

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);
		result.push_back(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 <cstdio>
using namespace std;


#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define repback(i,n) for(int i=(int)n-1;i>=0;i--)
#define repa(i,a,n) for(int i=a;i<(int)(n);i++)
typedef pair<int, int> P;
typedef long long ll;
const ll INF = 1e7, MAX_N = 1000000, MAX_M = 1000, MAX_W = 20, MAX_H = 20;
int asc(const void* a, const void* b) {
    return *(int*)a - *(int*)b;
}

int is_prime[MAX_N]; //i番目の素数
bool prime[MAX_N]; //iが素数であるかどうか

int seive(int n) {
    rep(i, n+1) prime[i] = true;
    prime[0] = prime[1] = false;
    int p = 0;
    repa(i,2,n+1) {
        if (prime[i]) {
            is_prime[p++] = i;
            for (int j = i * 2; j <= n; j += i) prime[j] = false;
        }
    }
    return p;
}
int main() {
    int n;
    while (cin >> n) {
        int ans = seive(n);
        cout << ans << endl;
    }

    return 0;
}