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 <math.h>

using namespace std;

int main() {

	int n,count;

	while (cin >> n) {
		count = 0;
		
		//?´???°????????????????¨???¶?????????????????????????¢????
		bool *is_prime;
		is_prime = (bool *)malloc(n+1);
		if (is_prime == NULL) {
			cout << "malloc error" << endl;
			break;
		}

		//????????¨????´???°??§????????¨??????
		for (int i = 0; i <= n; i++) {
			is_prime[i] = true;
		}

		for (int i = 2; i <= n; i++) {
			//?´???°???????????§????????£???????????????????????§??????
			if (is_prime[i] == true) {
				//????????°???????????§????????°????????£?????????
				for (int j = i+1; j <= n; j++) {
					//cout << "i=" << i << " j=" << j << " is_prime(i)=" << is_prime[i] << " is_prime(j)=" << is_prime[j] << endl;
					if (is_prime[j] == true) {
						if (j%i == 0) {
							//????????????????????????????´???°??§?????????
							is_prime[j] = false;
						}
					}
				}
			}
		}

		//?´???°??¨?¢????????????°????????°????????????
		for (int i = 2; i <= n; i++) {
			if (is_prime[i] == true) {
				count++;
				//cout << " " << i;
			}
		}

		//??¨???
		cout << count << endl;
	}
	return 0;
}

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];

	//while (cnt < 3){
		//std::cin >> n;
	while (std::cin >> n){
		result[cnt] = 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++: #define _USE_MATH_DEFINES
#include <iostream> 
#include <math.h>
#include <vector>
#include <algorithm>
#include <cmath>
using namespace std; 
int main(){
	int N;
	while (cin >> N){
		int p=log2(N);
		cout << N*p << endl;
	}
	return 0;
}

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

const int ARR_MAX = 1000000;
int fl[ARR_MAX];
int sm[ARR_MAX+1] = {0};

void init_arr();
void isfl(int);

int main(void) {
  int tag;
  init_arr();

  while(1){
    cin >> tag;
    if(tag == 0 || cin.eof()) break;
    isfl(tag);
    cout << sm[tag] << endl;
  }
  return 0;
}

void init_arr(){
  for(int i=0;i<ARR_MAX;i++) fl[i] = 1;
}

void isfl(int tag){
  int i,lim,idx = 2;
  fl[0] = 0;
  fl[1] = 0;

  lim = sqrt(tag);

  while(idx <= lim){
    if(fl[idx] == 1) for(i = idx*2;i<=tag;i+=idx) fl[i] = 0;
    idx++;

  }
  for(int i = 2;i<=tag;i++) sm[i] = sm[i-1] + fl[i];
  return;
}

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

#define rep(i,n) for(int i=2; i<=n; i++)
bool isprime(long long int n){
	rep(i,sqrt(n)){
		if(n%i==0) return false;
	}
	return true;
}
int main(){
	long long int n, ans;
	while(cin >> n){
		ans=0;
		rep(i,n) if(isprime(i)) ans++;
		cout << ans << endl;
	}
    return 0;
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <set>
#include <vector>
#include <cstring>
#include <climits>
#include <queue>
using namespace std;

#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define EPS 1e-9


bool primes[1000000];

void make_prime(){
	primes[0] = primes[1] = true;
	for(int i=2; i*i<=1000000; i++)if( !primes[i] )
		for(int j=i*2; j<1000000; j+=i)primes[j]=true;
}

int main(){
	int n,ret;
	make_prime();
	while(~scanf("%d",&n)){
		ret = 0;
		for(int i=2; i<=n; i++)if( !primes[i] )ret++;
		printf("%d\n",ret);
	}
	return 0;
}

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


int main()
{
	int n;
	
	while(cin >> n)
	{
		if (n>999999) continue;
		int i, j;
		int k = 0;
		for (i = 2; i <= n; i++)
		{
			int c = 0;
			for(j = 1; j<=i /2; j++)
			{
				if(i % j == 0)
				{
					c++;
					if (c > 1) break;
				}
			}
			if (c == 1)
			{
				k++;
			}
		}
		cout << k << endl;
	}
}

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

bool prime(int n) {
	if (n < 2) return false;
	for (int i = 2; i < sqrt(n) + 1; i++) {
		if (n % i == 0) return false;
	}
	return true;
}

int main() {
	vector<int> v = vector<int>(1, 0);
	for (int i = 1; i < 1000000; i++) {
		if (prime(i)) {
			v.push_back(v.back() + 1);
		} else {
			v.push_back(v.back());
		}
	}
	int n;
	while (cin >> n) {
		cout << v[n] + 1 << endl;
	}
}

C++: #include <iostream>
#include <queue>
#include <stack>
#include <vector>
#include <stdio.h>
#include <algorithm>
#include <string.h>
#include <string>
#include <cmath>
#include <complex>
#include <map>
using namespace std;

#define rep(i,n) for(int i=0;i<int(n);++i)
#define ALL(v) (v).begin(),(v).end()
#define PB push_back
#define EPS 1e-8

static const double PI=6*asin(0.5);
typedef complex<double> P;
typedef long long ll;

int t;

int main(){
	int *prime;
	prime = (int *)malloc(1000010*sizeof(t));
	if(prime==NULL) cout<<"no mem\n";
	rep(i,1000010){
		prime[i]=true;
	}
	prime[0]=prime[1]=false;
	for(int i=2;i<1000010;i++){
		if(prime[i]){
			for(int j=i*2;j<1000010;j+=i){
				prime[j]=false;
			}
		}
	}
	//rep(i,30) cout<<i<<" "<<prime[i]<<endl;
	int *sum;
	int num=0;
	sum=(int *)malloc(1000010*sizeof(t));
	if(sum==NULL) cout<<"no mem\n";
	rep(i,1000010){
		if(prime[i]){
			num++;
		}
		sum[i]=num;
	}
	int n;
	while(cin>>n){
		cout<<sum[n]<<endl;
	}
	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
int n;
const int MAX=1e7+5;
int pri[MAX];
bool py[MAX];
int c0=0;
void isprime()
{
	memset(py,true,sizeof(py));
	int t=sqrt(MAX);
	pri[0]=pri[1]=false;
	for(int i=2;i<=t;i++)
	{
		if(py[i])
		{
			for(int j=2*i;j<=MAX;j+=i)
			py[j]=false;
			pri[c0++]=i;
		}
	}
}
int main()
{
	c0=0;
	isprime();
	while(cin>>n)
	{
		int c=0;
		for(int i=0;i<c0;i++)
		{
			if(pri[i]<=n) c++;
		}
		cout<<c<<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <limits>
#include <map>
#include <string>
using namespace std;

bool is_prime(int n){
for(int i=2;i*i<=n;i++){
if(n % i == 0) return false;
}
return n!=1;
}

int main(){
int n;
while(~scanf("%d",&n)){
int* table =  new int[n+1];
fill((int*)table,(int*)table+n+1,1);
table[0]=0;
table[1]=0;

for(int i=2;i<=n;i++){
if(table[i] && !is_prime(i)) {
for(int j=i;j<=n;j+=i){
table[j] = 0;
}
}
}
printf("%d\n",count(table,table+n+1,1));
}
}