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<sstream>
#include<cstdio>
#include<fstream>
#include<vector>
#include<list>
#include<stack>
#include<queue>
#include<deque>
#include<map>
#include<set>
#include<algorithm>
#include<cmath>
#include<cstdlib>
#include<string>
using namespace std;

#define ull unsigned long long
ull power(ull b,ull e,ull p){
	ull ret = 1;
	while(e>0){
		if((e&1)!=0){
			ret = (ret*b)%p;
		}
		b = (b*b)%p;
		e>>=1;
	}
	return ret%p;
}

bool fermat(ull p){
	if(p!=2 && p%2==0)return false;
	if(p!=3 && p%3==0)return false;
	if(p!=5 && p%5==0)return false;
	if(p!=7 && p%7==0)return false;
	for(int x=0;x<100;++x){
		ull a = rand()%(p-1)+1;
		if(power(a,p-1,p)!=1)return false;
	}	
	return true;
}

int main(){
	int n;
	while(scanf("%d",&n)==1){
		int tot = 0;
		for(int x=2;x<=n;++x){
			if(fermat(x))++tot;
		}
		cout<<tot<<endl;
	}
	return 0;
}

C++: #include<bits//stdc++.h>
using namespace std;
int main(){
	int a[1000000];
	a[0]=2;
	int z=1;
	for(int i=2;i<100000;){
		for(int j=0;j<z;j++){
			if(i%a[j]==0)goto stop;
		}
		a[z]=i;
		z++;
		stop:;
		if(i==2) i++;
		else i+=2;
	}
	int b;
	while(cin>>b){
		int c=0;
		for(int i=0;i<z;i++){
			if(a[i]>b)break;
			else c++;
		}
		cout<<c<<endl;
	}
}

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

bool isPrime(int num) {
    if(num < 2) return(false);
    for(int r = 2; r * r <= num; r++) {
        if(num % r == 0) return(false);
    }
    return(true);
}

int main(void) {
    int num;
   
    while(cin >> num) {
        int prime[num + 1];
        memset(prime, -1, sizeof(prime));
        prime[0] = prime[1] = 0;
        int sum = 1;
        
        for(int r = 3; r <= num; r += 2) {
            if(prime[r] != -1) continue;
            if(isPrime(r)) {
                prime[r] = 1;
                sum++;
                for(int c = 2 * r; c <= num; c += r) prime[c] = 0;
            }
        }
        
        cout << sum << endl;
    }
}

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

int main()
{
	const int N = 1000000;
	vector<bool> vec(N, true);
	double limit = sqrt(N);
	for (int i = 2; i < limit; i++){
		for (int j = 2; i * j < N; j++){
			vec[i * j] = false;
		}
	}
	int n, cnt = 0;
	while (cin >> n){
		for (int i = 2; i <= n; i++){
			if (vec[i])cnt++;
		}
		cout << cnt << endl;
		cnt = 0;
	}
}

C++: #include<iostream>
int fact(int x){
	int sx=0,sy=0;
	for(int i=2;i<x+1;i++){
		if(x%i==0)sx++,sy+=i;
	}
	if(sx==1)return x;
	else return sy+1;
}
int main(){
	int a;
	while(std::cin>>a){
		int n=0;
		for(int i=2;i<a+1;i++){
			if(i==fact(i))n++;
		}
		std::cout<<n<<std::endl;
	}
	
	return 0;
}

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

bool prime(int n) {
	for(int i=2;i<=sqrt(n);++i) {
		if(n%i==0) { return false; }
	}
	return true;
}

int main() {
	int n;
	while(scanf("%d",&n) != EOF) {
		int cnt=0;
		for(int i=2;i<=n;++i) {
			if(prime(i)) { ++cnt; } 
		}
		printf("%d\n",cnt);
	}
	return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>

int main(void) {
  int n, *ar, *sum, i, j;
  if((ar = (int*)malloc(sizeof(int) * 1000000)) == NULL) return 1;
  if((sum = (int*)malloc(sizeof(int) * 1000000)) == NULL) return 1;
  for(i = 1; i < 1000000; ++i) ar[i] = 1, sum[i] = 1;
  for(i = 2; i < 1000; ++i) {
    if(ar[i]) for(j = 2; i * j < 1000000; ++j) ar[i * j] = 0;
  }
  for(i = 2; i < 1000000; ++i) sum[i] = sum[i - 1] + ar[i];
  while(scanf("%d", &n) != EOF) {
    printf("%d\n", sum[n]);
  }
  free(ar);
  free(sum);
  return 0;
}

C++: //
//  main.cpp
//  TestCpp
//
//  Created by Shahab Uddin on 05/21/13.
//  Copyright (c) 2013 ___Shahab__. All rights reserved.
//

#include <cstdio>
#include <cmath>

bool isPrime (int p)
{
    if ( p < 2 ) return false;

    if ( p == 2 ) return true;

    int squareRoot = (int) sqrt (p);

    for ( int i = 2; i <= squareRoot; i++ ) {
        if ( p % i == 0 )
            return false;
    }

    return true;
}

int main(int argc, const char * argv[])
{
    int n;

    while ( scanf ("%d", &n) != EOF ) {
        int ans = 0;

        for ( int i = 2; i < n; i++ ) {
            if (isPrime(i)) ans++;
        }

        printf ("%d\n", ans);
    }

    return 0;
}

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

int main(){
	int n;
	int a=0;
	cin >> n;
		for(int i = 0; i < n; ++i){
			for(int j = 2; j <= i; ++j){

				if(i%j == 0){
				if(i==j){
					++a;
				}else{
					break;
				}
				}
		}
	
	}
		cout << a << '\n';
}

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

int count_prime( unsigned int const& n )
{
    if ( n <= 0 || 1000000 <= n ) return -1;
    std::vector<bool> memo_prime( n+1, true );
    for ( unsigned int i = 2; i < n; ++i )
    {
        for ( unsigned int j = 2; i*j <= n; ++j )
        {
            memo_prime.at(i*j) = false;
        }
    }
    memo_prime.at(0) = memo_prime.at(1) = false;
    memo_prime.at(2) = true;
    return std::count( memo_prime.begin(), memo_prime.end(), true );
}

int main()
{
    int n;
    while( std::cin >> n)
        std::cout << count_prime(n) << std::endl;
    return 0;
}

C++: #include<stdio.h>
const int MAX_V=1000000;
int prime[MAX_V+1];
int sum[MAX_V+2];
int main(){
  int i,k,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;
}

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,3,1000000){
	bool flg = true;
	for(int j=0;primes[j]*primes[j]<=i;j++){
	    if(i % primes[j] == 0 ) {
	    flg = false;
	    break;
	    }
	}
	if(flg) primes.PB(i);
	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;
}

C++: #include <iostream>
#include <stdio.h>
#include <algorithm>
#include <functional>

using namespace std;

#define MAXN 1000016

int isprime[MAXN];

void sieve(int n)
{
	int i, j, num, p = 0;
	fill(isprime, isprime + MAXN + 1, 1);
	for (i = 2, num = 1; i <= n; i++)
	{
		if (isprime[i])
		{
			isprime[i] = num++;
			for (j = 2 * i; j <= n; j += i)
				isprime[j] = 0;
		}
	}
}

int main()
{
	int n;
	sieve(MAXN);
	while (scanf("%d", &n) != EOF)
	{
		if (n == 1)
		{
			printf("0\n");
			continue;
		}
		while (!isprime[n])
		{
			n--;
		}
		printf("%d\n", isprime[n]);
	}
	return 0;
}