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>
using namespace std;

#define Max 1000000

int isPrime[Max];

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

    //配列の初期化
    isPrime[0] = isPrime[1] = 0;
    for(i = 2; i < Max; ++i){
        isPrime[i] = 1;
    }

    for(i = 2; i < Max; i++){
        if(isPrime[i] == 1){
            if(i % 2 == 0){
                isPrime[i] = 0;
            }else if(i % 3 == 0){
                isPrime[i] = 0;
            }else if(i % 5 == 0){
                isPrime[i] = 0;
            }else if(i % 7 == 0){
                isPrime[i] = 0;
            }
        }
    }

    isPrime[2] = 1;isPrime[3] = 1;isPrime[5] = 1;isPrime[7] = 1;

    while(cin >> num){
        int count = 0;
        for(i = 2; i <= num; ++i){
            if(isPrime[i] == 1){
                count++;
            }
        }
        cout << count << endl;
    }
}

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

int main(){
bool unti[1000000];
fill(unti,unti+1000000,0);
for(int n = 2;n < 1000000;n++)
{
if(unti[n] == 0)
{
for(int a = 2;(a * n) < 1000000;a++)
{
	unti[a * n]++;
}
}
}
int b;
while(cin >> b )
{
int cnt = (count(unti,unti+b+1,0) - 2);
cout << cnt ;
	}
return 0;
}

C++: #include<cstdio>
#include<iostream>
using namespace std;
int main(){ 
    int n;
    int prime[1000000];
    while(cin>>n){
    for(int i=0;i<1000000;i++)prime[i]=0;
    for(int i=2;i<=n;i++){ 
        for(int j=2;i*j<=n;j++){ 
            prime[i*j]=1;
        }
    }
    int ans=0;
    for(int i=2;i<=n;i++){
        if(prime[i]==0)ans++;
    }
    printf("%d\n",ans);
    }
}

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

#define pb push_back

using namespace std;

const int MAX=100000;
int main(void)
{
	int x;
	bool p[MAX];
	int cnt;
	int i1,i2;

	fill(p,p+MAX,false);

	for(i1=2;i1<MAX;i1++){
		for(i2=2;i2*i2<=i1;i2++){
			if(i1%i2==0) break;
		}
		if(i2*i2>i1) p[i1]=true;
	}

	while(1){
		cin>>x;
		if(cin.eof()) break;

		cnt=0;
		for(i1=0;i1<=x;i1++){
			if(p[i1]) cnt++;
		}
		cout<<cnt<<endl;
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
int main(void)
{
    int n;
    while (cin >> n)
    {
        int c = 0;
        for (int i = 2; i <= n; i++)
        {
            int t = 1;
            for (int j = 2; j < i; j++)
            {
                if (i % j == 0)
                {
                    t++;
                }
            }
            if (t == 1)
            {
                c++;
            }
        }
        cout << c << endl;
    }
}

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

int main() {
    long in, out;
    int p, count;
    
    do{
         cin >> in;
         out = 0; 
         
         for(int i = 1; i <= in; i++)
         {
                 count = 0;
                 for(p = 1; p <= i; p++)
                 {
                       if(i % p == 0){
                            count++;
                            }
                 }
                 
                 if(count == 2){
                          out++;
                 }
         }
         
         cout << out << endl;
         
    }while(in);  
    

    return 0;
}

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

using namespace std;

#define MAX_N 99999
#define MAX_D 30

bool prime_check(int n, int k = 2)
{
	if (n < k * k) { return true; }

	if (n % k == 0) { return false; }

	return prime_check(n, k + 1);
}

int main()
{
	int n;

	bool isprime[MAX_N + 1];

	for (int i = 2; i < MAX_N + 1; i++)
	{
		isprime[i] = prime_check(i);
	}

	while (cin >> n)
	{
		cout << count(isprime + 2, isprime + n + 1, true) << endl;
	}

	return 0;
}

C++: #include<stdio.h>

int main(void){
  int n;
  while(scanf("%d",&n)!=EOF){
    if(n==1){
      printf("%d\n",0);
      break;
    }
    int p[1000000];
    p[0]=2;
    int pi=1;
    int i=2;
    while(i<=n){
      int t=1;
      int j;
      for(j=0;j<pi;j++){
	if(i%p[j]==0){
	  t=0;
	  break;
	}
      }
      if(t==1){
	p[pi]=i;
	pi++;
      }
      i++;
    }
    printf("%d\n",pi);
  }
  return 0;
}

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

int main(){
  int prime[100000];
  int isprime[1000000];
  memset(isprime,1,sizeof(isprime));
  isprime[0]=0;
  isprime[1]=0;
  
  for(int i=2;i<1000000;i++){
    if(isprime[i]){
      for(int j=i*2;j<1000000;j+=i){
	isprime[j]=0;
      }
    }
  }
  int pi;
  for(int i;i<1000000;i++){
    if(isprime[i]==1){
      prime[pi]=i;
      pi++;
    }
  }

  int n;
  while(cin>>n){
    int ans;
    while(prime[ans]<=n){
      ans++;
    }
    cout<<ans<<'\n';
  }
  return 0;
}

C++: #include<cstdio>
using namespace std;
const int N = 1000000;

int main(){

  int IsPrime[N];
  IsPrime[0] = -1;
  IsPrime[1] = -1;
  for(int i=2; i*i<=N;i++){
    if(IsPrime[i]==0){
      IsPrime[i] = 1;
      for(int j=2*i; j<=N; j+=i)
        IsPrime[j] = -1;
      }
  }
  int max;
  while(~scanf("%d",&max)){
    int count=0;
    for(int i=1; i<max+1; i++)
      if(IsPrime[i]==1) count++;
    printf("%d\n",count);
  }
}

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

using namespace std;
int sieve[1000005]= {0}, n, i, j, cnt=0;;
int main()
{
    while(cin>>n)
    {
        cnt=0;
        for(i=2; i<=n+2; i++)
        {
            if(!sieve[i])
            {
                for(j=2*i; j<=n+1; j+=i)
                {
                    sieve[j]=1;
                }
            }
        }

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