Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	num := make([]bool, 1000000)

	for sc.Scan() {
		var nmpr int

		n, _ := strconv.Atoi(sc.Text())

		lmt := int(math.Sqrt(float64(n)))

		for i := 2; i <= lmt; i++ {
			for j := 2; i*j <= n; j++ {
				if !num[i*j] {
					num[i*j] = true
				}
			}
		}

		for t := 2; t <= n; t++ {
			if !num[t] {
				nmpr++
			}
		}

		fmt.Println(nmpr)
	}

}

C++: #include<iostream>
using namespace std;
int main()
{
	bool p[1000000] = { false };
	int n;
	for (int i = 2; i <1000000; i++){
			for (int i2 = 2; i*i2 <= n; i2++){
				p[i*i2] = true;
			}
		}
		p[2] = false;
		p[3] = false;
	while (cin >> n){
		
		
		int c = 0;
		for (int i = 2; i <= n; i++){
			if (!p[i])c++;
		}
		cout << c << endl;
	}
}

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){
      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]==0) count++;
    printf("%d\n",count);
  }
}

C++: // http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=0009
// for ref

#include <iostream>
#include <cstring>
#define Rep(i, b, e)\
    for (int i = (int)b; i < (int)e; i++)

using namespace std;

const int MAXN = 999999;
bool isPrime[MAXN];

int countPrime(int n) {
    memset(isPrime, true, sizeof isPrime);
    Rep(i, 0, 1000)
        isPrime[i] = true;

    isPrime[0] = false;
    isPrime[1] = false;
    // isPrime[2] = true;
    // isPrime[3] = true;

    Rep(i, 2, n) {
        Rep (k, 2, 100) {
            isPrime[i*k] = false;
        }
    }

    int primeCount = 0;
    int j = 0;
    // while (primeCount < n) {     // GS q
    while (j <= n) {
        if (isPrime[j]) {
            // cout << primeCount << " th prime\t";
            // cout << j << endl;
            primeCount++;
        }
        j++;
    }

    return primeCount;
}

int num;

int main()
{
    while (cin >> num) {
        cout << countPrime(num) << endl;
    }
 //   cout << "Hello world!" << endl;
    return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
	int n, cnt;	

	while(cin >> n){
		bool prime[n+1] = {false};
		prime[1] = true;
		
		// ??¨??????????????????????????????
		for(int j = 2; j*j <= n; j++){
			if(prime[j] == false){
				for(int k = j; k*j <= n; k++){
					prime[j*k] = true;
				}
			}
		}
		cnt = 0;
		for(int i = 1; i <= n; i++){
			if(prime[i] == false) cnt++;
		}
		
		cout << cnt << endl;
	}
}

C++: #include<iostream>
#include<cmath>
#define MAX 1000000

using namespace std;

bool prime[MAX+1];
int T[MAX];

void eratos( int n ){
    for ( int i = 0; i < n; i++ ) prime[i] = false;
    for ( int i = 3; i <= n; i += 2 ) prime[i] = true;
    prime[2] = true;
    for ( int i = 3, limit = (int)sqrt(n); i <= limit; i += 2 ) {
	if ( !prime[i] ) continue;
	for ( int j = i * i, k = i * 2; j <= n; j += k ) prime[j] = false;
    }
}

void initialize(){
    eratos( MAX );
    for ( int i = 0; i < MAX; i++ ){
	T[i] = T[i - 1];
	if ( prime[i] ) T[i]++;
    }
}

main(){
    initialize();
    int n;
    while ( cin >> n ) cout << T[n] << endl;
}

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;
int* table =  new int[1000000];
while(~scanf("%d",&n)){

fill((int*)table,(int*)table+n+1,1);
table[0]=0;
table[1]=0;

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

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

int main(){
	
	char p[1000000];
	for(int i=2;i<1000000;i++)p[i]=1;
	for(int i=2;i<1000;i++){
		if(p[i]==0)continue;
		for(int j=i+1;j<1000000;j++){
			if(p[j]==0)continue;
			if(j%i==0)p[j]=0;
		}
	}
	
	int n,c;
	while(scanf("%d",&n)!=EOF){
		c=0;
		for(int i=0;i<=n;i++)if(p[i]==1)c++;
		cout<<c<<endl;
	}
}

C++: #include <iostream>
#include <cmath>
using namespace std;
// have you mooed today?
bool isprime(int& num)
{
    static int c;
    static int max;
    max = sqrt(num) + 2;
    if(num == 2) return true;

    for(c=2; c < max; c++)
        if(num % c ==0) return false;
    return true;
}

int main()
{
    int ans = 0;
    int buff;
    while(cin >> buff);
    {
        for(int d =2; d <= buff; d++)
        {
            if(isprime(d)) ans++;
        }
        cout << ans << endl;
        ans  = 0;
    }
    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
int a[1000005] = {0,};
vector<int> primeVec;
void genPrime(){
	for (int i=2;i<=1000000;i++){
		if (a[i]) continue;
		else{
			a[i] = 2;
			primeVec.push_back(i);
			if (i >= 1000) continue;
			for (int j=i*i;j<=1000000;j+=i) a[j] = 1;
		}
	}
}
int main(){
	genPrime();
	int n;
	while(cin >> n)
		cout << upper_bound(primeVec.begin(), primeVec.end(), n) - primeVec.begin() << endl;
	return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <string>
#include <queue>
#include <cmath>

#define int64 long long int

using namespace std;

int main()
{
	int n;
	bool p[1000000] = {0};

	for(int i=2; i<=1000; i++){
		if(!p[i])
			for(int j=2*i; j<1000000; j+=i) p[j] = true;
	}

	while(cin >> n){
		int count = 0;
		for(int i=2; i<=n; i++)
			if(!p[i]) count++;
		cout << count << '\n';
	}

	return 0;
}

C++: #include<iostream>

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

int main(){
	int n;
	int count = 0;
	while (std::cin >> n){
		count = 0;
		for (int i = 2; i <= n; ++i){
			if (i == 2)++count;
			else if (isPrime(i))++count;
		}
		std::cout << count << std::endl;
	}
	return 0;
}

C++: #include<stdio.h>
int main(void)
{
	int a,x,i;
	while((scanf("%d",&a))!=EOF){
		x=0;
		for(i=2;i<a+1;i++){
			if(i%2!=0&&i%3!=0&&i%5!=0&&i%7!=0){
				x++;
			}
			else if(i==2||i==3||i==5||i==7){
				x++;
			}
		}
		printf("%d\n",x);
	}
	return 0;
}

C++: #include <stdio.h>

int main(void){
int n;
while(scanf("%d",&n)!=EOF){
int count=0;
for(int m=2;m<=n;m++){
bool check=true;
for(int i=2;i<m;i++){
if(i!=m && m%i==0){
check=false;
}
}
if(check==true){
count++;
}
}
printf("%d\n",count);
}
return 0;
}

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


int main() {

	int judge[1000000];
	for(int k=0;k<1000000;k++)judge[k]=1;
	int n;
	
	while(cin>>n){
		int sum = 1;
		for(int i=3;i<n+1;i+=2){
			if (judge[i]==1){
				sum++;
				for(int j=2;j<=n/i;j++)
					judge[i*j] = 0;
			}	
		}
		
		cout << sum <<endl;
	}

	return 0;
}

C++: #include<iostream>
#include<cmath>
using namespace std;
bool prime(int x){
  for(int i=2;i<x;i++){
    if(x%i==0)return false;
  }
  return true;
}

int main(){
  int n;
  while(cin>>n){
    int sum=0;
    for(int i=2;i<=n;i++){
      if(prime(i))sum++;
    }
    cout<<sum<<endl;
  }
}

C++: #include<iostream>
#include<string>
#include<algorithm>
#include<vector>
#include<cmath>
using namespace std;
int main(){
	int n,cnt=0,ans,sosu[1000005]={0};
	for(int i=2;i<1000000;i++){
		bool flag=true;
		for(int j=2;j<=sqrt(i);j++){
			if(i%j==0)flag=false;
		}
		if(flag){
		sosu[i]=1;
		}
	}
	while(cin>>n){
	ans=0,cnt=0;
	for(int i=2;i<=n;i++){
		if(sosu[i]==1){
		ans++;
		}
	}
	cout<<ans<<endl;
	}
    return 0;
}