Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		a, _ := strconv.Atoi(sc.Text())
		fmt.Println(countPrime(a))
	}
}

func countPrime(a int) int {
	primes := make([]bool, a+1)
	n := 0
	for i := 2; i <= a; i++ {
		if primes[i] == false {
			n++
			for j := i * i; j <= a; j += i {
				primes[j] = true
			}
		}
	}
	return n
}

C++: #include<iostream>
#include<string>
#include<cstdio>
#include<algorithm>
#include<stack>
#include<queue>
#include<vector>
#include<cmath>
#define ll long long int
using namespace std;
typedef pair<int, int> P;

string str;

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

  while(scanf("%d", &n)!=EOF){

    if(n==1){
      cout<<"0"<<endl;
      continue;
    }else if(n==2){
      cout<<"1"<<endl;
      continue;
    }
    int sum=1;
    int prime[1000000]={};
    for(i=2;i<n;i++){
      if(prime[i]==0)
	for(j=2;j*i<n;j++){
	  if(prime[i*j]==0){
	    prime[i*j]=1;
	    sum++;
	  }
	}
    }
    cout<<n-sum<<endl;
  }

}

C++: #include<iostream>
#include<string>
#include<algorithm>
#include<map>
#include<set>
#include<utility>
#include<vector>
#include<cmath>
#include<cstring>
#include<cstdio>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define rp(a) while(a--)
#define pb push_back
#define mp make_pair
#define all(in) in.begin(),in.end()
const double PI=acos(-1);
const double EPS=1e-10;
using namespace std;
typedef long long ll;
typedef vector<int> vi;
bool sosu[1000010]={1,1,0};
int main(){
	for(int i=4;i<1000010;i+=2)sosu[i]=true;
	for(int i=3;i<1000010;i+=2)
		for(int j=i*3;j<1000010;j+=2*i)sosu[j]=true;
	
	int n;
	while(cin>>n){
		int co=0;
		loop(i,2,n+1)if(!sosu[i])co++;
		cout<<co<<endl;
	}
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <cstdio>
using namespace std;
int main()
{
	int tmp;
	while (scanf("%d", &tmp) != EOF)
	{
		vector<int> prime;

		for (int i(2); i <= tmp; ++i)
		{
			for (vector<int>::iterator i2(prime.begin()), n2(prime.end()); i2 != n2; ++i2)
			{
				if ((i % *i2) == 0)
				{
					goto nx;
				}
			}

			prime.push_back(i);
		nx:;
		}

		cout << prime.size() << endl;
	}

	return 0;
}

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

int prime(int n) {
	int k;
	if(n == 2) return 1;
	if(n % 2 == 0) return 0;
	for(k = 3; k * k <= n; k += 2) {
		if(n % k == 0) return 0;
	}
	return 1;
}

int cntPrime(int n) {
	int cnt = 0, i, k;
	for(i = 2; i <= n; i++) {
		cnt += prime(i);
	}
	return cnt;
}

int main() {
	int a;
	int arg[1024] = { 0 };
	int cnt = 0;
	while(scanf("%d", &a) != EOF) {
		arg[cnt++]=a;
	}
	int i;
	for(i = 0; i < cnt; i++) {
printf("%d\n", cntPrime(arg[i]));
	}
}

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

using namespace std;

bool p[10000000];
vector<int> prime;

void init(){
    memset(p,1,sizeof(p));
    p[0] = p[1] = false;
    for(int i = 2; i <= 1000000; i++){
        if(!p[i]) continue;
        for(long long j = 1LL*i*i; j <= 1000000; j += i)
            p[j] = false;
        prime.push_back(i);
    }
}

int CntPrime(int n){
    vector<int>::iterator it = lower_bound(prime.begin(), prime.end(), n);
    if( *it > n) it--;
    return (it - prime.begin() + 1);
}

int main(){
    init();
    int n;
    while(cin >> n){
        cout << CntPrime(n)<< endl;
    }
    return 0;
}

C++: #include <iostream>
using namespace std;
const int MAX_V = 1000000; // 100????????§?´???°??¨?????????
int prime[MAX_V+1]; // 1????´???°???0????´???°??§?????? 
int pcount[MAX_V+1];
int main() {
    int i, k, v,a,sum=0;
    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?????\????????°?´???°??§?????? 
            }
            
        } 
        
    }
    pcount[0]=0;
    pcount[1]=0;
    for(i=2;i<MAX_V;i++){
        if(prime[i]){
            sum++;
        }
        pcount[i]=sum;
    }
    while(1) {
        scanf("%d",&a);
        printf("%d\n",pcount[a]);
    } 

    return 0; 
    
}

C++: #include <iostream>
#include <cmath>
using namespace std;
int whether(int num);
int count(int num);

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

int whether(int num){
    int s=sqrt(num);
    int i;
    
    for(i=3; i<=s; i+=2){
        if(num%i==0){ return 0; }
    }
    
    return 1;
}

int count(int num){
    int a=1;
    if(num<2){ return 0; }
    if(num==2){ return 1; }
    for(int i=3; i<=num; i+=2){
        if(whether(i)){ a+=1; }
    }
    
    return a;
}

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

int main()
{
	int n;
	int max;
	while(cin>>n)
	{
		max = sqrt(1.0*n);
		vector<bool> prime(n+1, true);
		prime[0]=false;
		prime[1]=false;
		for(int i=2; i<=max; ++i)
		{
			if(prime[i])
			{
				for(int j=i*2; j<=prime.size()-1; j+=i)
				{
					prime[j]=false;
				}
			}
		}
		cout << accumulate(prime.begin(), prime.end(), 0) << '\n';
	}
}

C++: #ifndef VS
#include<iostream>
#include<vector>
#include<algorithm>
#include<stack>
#include<queue>
#include<map>
#include<set>
#include<iomanip>
#include<assert.h>
#include<math.h>
#endif

using namespace std;
typedef long long LL;

#ifdef BTK
#define DEBUG if(1)
#else
#define CIN_ONLY if(1)
struct cww {
	cww() { CIN_ONLY{ ios::sync_with_stdio(false); cin.tie(0); } }
}star;
#define DEBUG if(0)
#endif

#define FOR(i,bg,ed) for(int i=(bg);i<(ed);i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
#define REC(ret, ...) std::function<ret (__VA_ARGS__)>
template <typename T>inline bool chmin(T &l, T r) { bool a = l>r; if (a)l = r; return a; }
template <typename T>inline bool chmax(T &l, T r) { bool a = l<r; if (a)l = r; return a; }
namespace prime {
	const int size = 1123456;
	int p[size];
	struct prime {
		prime(){
			REP(i, size)p[i] = 1;
			p[0] = p[1] = 0;
			REP(i, size)if (p[i]) {
				for (int j = i + i; j < size; j += i)p[j] = 0;
			}
			FOR(i, 1, size) {
				p[i] += p[i - 1];
			}
		}
	}prime_init;
}
int main() {
	int n;
	while (cin >> n) {
		cout << prime::p[n] << endl;
	}
}

C++: #include<cstdio>
#include<cmath>

long long gcd(long long a, long long b)
{
    return b?gcd(b,a%b):a;
}
long long m,n;
int main()
{
    while(~scanf("%lld%lld",&m,&n))
    {
        long long temp = gcd(m,n);
        printf("%lld %lld\n",temp,m/temp*n);
    }
    return 0;
}

C++: #include <iostream>
int main(int argc, const char * argv[])
{

    // insert code here...
    
    while(true)
    {
        int n;
        std::cin >> n;
        int count = 0;
        
        for(int i=2;i<n;i++)
        {
            bool flag = true;
            for(int j=2;j<i/j+1;j++)
            {
                if(i%j==0)
                {
                    flag=false;
                    break;
                }
            }
            if(flag)
            {
                count++;
            }
        }
        
        std::cout << count <<"\n";
        
    }
    
    return 0;
}

C++: #include <iostream>

using namespace std;

void solve(int n) {
	int prime[n];
	int j=0;
	for (int i=2; i<=n; i++) {
		prime[j] = i;
		j++;
	}
	int p;
	for (int i=0; i<n; i++) {
		p = prime[i];
		if (p == -1) continue;
		for (int k=i+1; k<n; k++) {
			if (prime[k]%p == 0)
				prime[k] = -1;
		}
	}
	int count = 0;
	for (int i=0; i<n; i++)
		if (prime[i] != -1) count++;
	cout << count << endl;
}

int main() {
	int n;
	while (cin >> n) {
		if (n<=1) cout << 0 << endl;
		solve(n);
	}
	return 0;
}