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] = i
        }

        Soe()

        for i := 0; i < MAX; i++ {
                primes[i] %= 2
        }
        primes[1] = 0
        primes[2] = 1
}

func Soe() {
        for i := 2; i < MAX; i++ {
                if primes[i] > 0 {
                        for j := 2; j < MAX/i; j++ {
                                if i * j < MAX {
                                        primes[i*j] = 0
                                }
                        }
                }
        }
}

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

int ary[40],ary2[1000010];
bool nopri[1000010];
int main(){
	int N;
	for(int i=1;i<=31;i++){
		cin>>ary[i];
		if(cin.eof()){
			N=i-1;
			break;
		}
	}

	int cnt=2;
	while(cnt<1000000){
		for(int i=2;i*cnt<1000000;i++){
			nopri[i*cnt]=true;
		}
		cnt++;
		while(nopri[cnt])cnt++;
	}
	cnt=0;
	for(int i=2;i<=1000000;i++){
		if(!nopri[i]) cnt++;
		ary2[i]=cnt;
	}
	for(int i=1;i<=N;i++){
		cout<<ary2[ary[i]]<<endl;
	}
	return 0;
}

C++: #include<stdio.h>

int prime[1000000];
bool flag[1000000];

void solve(int n)
{
	int ans=0;
	for(int i=2;i<=n;i++)
	{
		if(flag[i])
		{
			prime[ans++]=i;
			for(int j=2*i;j<=n;j+=i)
				flag[j]=false;
		}
	}
	printf("%d\n",ans);
}

int main()
{
	int data;
	while(scanf("%d",&data)!=EOF)
	{
		for(int i=0;i<=data;i++)
			flag[i]=true;
		flag[0]=flag[1]=false;
		solve(data);
	}
}

C++: #include <bits/stdc++.h>
using namespace std;

int main(){
  vector<bool> is_prime(1000000, true);
  is_prime[0] = false; is_prime[1] = false;

  for(int64_t i=2; i<1000000; i++){
    if(is_prime[i]){
      for(int64_t j=2; i*j<1000000; j++) is_prime[i*j] = false;
    }
  }

  int n;
  while(cin >> n){
    int ans = 0;
    for(int i=0; i<=n; i++){
      if(is_prime[i]) ans++;
    }
    cout << ans << endl;;
  }
}

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

int count(int n);

int main(void){
	while (true){
		int n; cin >> n;
		if (!n){
			break;
		}
		cout << count(n) << endl;
	}
	return 0;
}

int count(int n){
	int ct = 0;
	bool *prime = new bool[n];
	//bool prime[999999];
	for (int i = 0; i <= n;i++){
		prime[i] = 0;
	}
	prime[2] = 1;
	for (int i = 3; i <= n; i = i + 2){
		prime[i] = 1;
	}
	for (int i = 3; i <= sqrt(n); i = i + 2){
		if (prime[i] == 0) continue;
		for (int j = i * 2; j <= n; j = j + i){
			prime[j] = 0;
		}
	}
	for (int i = 3; i <= n; i = i + 2){
		if (prime[i] == 1) ct++;
	}
	ct++; //2を数えていない分
	delete[] prime;
	return ct;
}

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

int main(){
  bool is_prime[1000000];

  is_prime[0]=is_prime[1]=false;
  for(int i=2;i<1000000i++){
    is_prime[i] = true;
  }

  for(int i=2;i*i<1000000;i++){
    if(is_prime[i]){
      for(int j=i*i;j<1000000;j=j+i){
        is_prime[j] = false;
      }
    }
  }

  int n_prime[1000000];

  int cnt = 0;
  for(int i=0;i<1000000;i++){
    if(is_prime[i]){
      cnt++;
    }
    n_prime[i] = cnt;
  }

  while(true){
    int n;
    cin >> n;

    if(cin.eof())
        break;

    cout << n_prime[n] << endl;
  }

  return 0;
}

C++: #include <iostream>
#include <cstdio>
using namespace std;
int main(){
	int a,b,c=0;
	while(cin>>a){
		for (int A=2;A<=a;A++){
			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 <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <iostream>

#define N 999999L
bool num[N];

void furui(){
  long int i;
  int divide;
  int rt = (int) sqrt(N);
  for(i = 0L; i < N; i++){
    num[i] = true;
  }
  num[0] = false;
  num[1] = false;
  for(divide = 2; divide <= rt; divide++){
    if(num[divide] == false) continue;
    for( i = divide; i < N; i+= divide){
      num[i] = false;
    }
    num[divide] = true;
  }
}

int main(void){
  int c, i, j;
  long int k;
  char buf[7];
  long int n;
  long int count;

  furui();

  for(j = 0; j < 30; j++){
    for(i = 0; ;i++){
      c = getchar();
      if(c == EOF || c == '\n'){
        buf[i] = '\0';
        break;
      }
      buf[i] = c;
    }
    n = atol(buf);
    count = 0L;
    for(k = 1L; k <= n; k++){
      if(num[k] == 1) count++;
    }

    std::cout << count << std::endl;
    if(c == EOF) break;
  }
  return 0;
}

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

int main(){
	int cnt=0, x, n, i, j;
	bool flg;
	double rootx;
	int rootintx;
	while(cin>>n){
		cnt=0;
		if(n>=2)cnt++;
		for(x=3;x<=n;x++){
			flg = true;
			if(x==2){
				cnt++;
				continue;
			}
			if(x<2 || !(x%2)){
				continue;
			}
			rootx = sqrt(x);
			rootx = ceil(rootx);
			rootintx = int(rootx);
			for(j=2;j<=rootintx;j++){
				if(!(x%j)){
					flg=false;
					break;
				}
			}
			if(flg)cnt++;
		}
		cout<<cnt<<endl;;
	}
	return 0;
}

C++: #include <vector>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <stack>
#include <bitset>
#include <algorithm>
#include <sstream>
#include <iostream>
#include <time.h>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cstring>
#include <iomanip>

#define out freopen("output.txt", "w", stdout);
#define in freopen("input.txt", "r", stdin);
#define clr(arr, key) memset(arr, key, sizeof arr)
#define pub push_back
#define pob pop_back
#define infinity 2147483647
#define LL long long
#define Pi acos(-1)
#define SZ 1000010

///int col[8] = {0, 1, 1, 1, 0, -1, -1, -1};
///int row[8] = {1, 1, 0, -1, -1, -1, 0, 1};
///int col[4] = {1, 0, -1, 0};
///int row[4] = {0, 1, 0, -1};

using namespace std;
bool primes[1100000];
int sieve(int n)
{
    int i, j;
    primes[1] = 1;
    for(i = 4; i <= 1100000; i += 2)
        primes[i] = 1;
    for(i = 3; i < 1100000 / i; i += 2)
    {
        if(primes[i] != 1)
        {
            for(j = 2; j * i < 1100000; j++)
                primes[i * j] = 1;
        }
    }
    int cnt = 0;
    for(i = 2; i <= n; i++)
        cnt += !primes[i];
    return cnt;
}

int main()
{
    int n;
    cin >> n;
    cout << sieve(n) << endl;
    return 0;
}

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

int prime(int num){

	if(num<2){
		return 0;
	}else if(num==2){
		return 1;
	}else if(num%2==0){
		return 0;
	}
	
	for(int s=3; pow(s,2)<=num; s+=2){
		if(num%s==0){
			return 0;
		}
	}
	
	return 1;
}

int main(){
	int num, count=0;
	
	cin >> num;
	
	for(int i=0; i<=num; i++){
		if(prime(i)==1){
			count++;
		}
	}
	
	cout << count << endl;
	
	return 0;
}

C++: #include<cstdio>
using namespace std;
const int MAX = 1200000;
bool prime[MAX];
int solve(int n){
  int cnt = 1;
  if(n == 1)
    return 0;
  else if(n == 2)
    return 1;
  else{
    for(int i = 3 ; i <= n ; i++){
      if(prime[i])
        cnt++;
    }
  }
  return cnt;
}
int main(){
  for(int i = 0 ; i < MAX ; i++)
    prime[i] = true;
  prime[0] = prime[1] = false;
  for(int i = 4 ; i < MAX ; i += 2)
    prime[i] = false;
  for(int i = 3 ; i < MAX ; i += 2){
    if(prime[i]){
      for(int j = i * 2 ; j < MAX ; j += i)
        prime[j] = false;
    }
  }
  int n;
  while(scanf("%d",&n) != EOF){
    printf("%d\n",solve(n));
  }
}