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>
using namespace std;

int main(){

	bool pn[1000010];
	int sum[1000010] = {0};
	int n;
	
	for(int i = 0; i <= 1000000; i++) {
		pn[i] = true;
	}
	
	pn[1] = false;
	for(int i = 1; i <= 1000000; i++) {
		if(!pn[i]) continue;
		for(int j = 2; i*j <= 1000000; j++) {
			if(pn[i*j]) pn[i*j] = false;
		}
	}
	
	for(int i = 1; i <= 1000000; i++) {
		sum[i] = sum[i-1] + pn[i];
	}
	
	while(cin >> n) {
		cout << sum[n] << endl;
	}

    return 0;
}

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

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

C++: #include <iostream>
#include <vector>
using namespace std;
const int N = 1000000;
int main(){
	int n;
	vector<bool> v(N,true);
	v[0] = false;
	v[1] = false;
	for (int i=2; i<N; ++i){
		if (v[i] == true){
			for (int j=2*i; j<N; j+=i){
				v[j] = false;
			}
		}
	}
	vector<int> cnt(N);
	for (int i=2; i<N; ++i){
		if (v[i]) cnt[i] = cnt[i-1]+1;
	   else cnt[i] = cnt[i-1];
	}
	while (cin >> n){
		cout << cnt[n] << '\n';
	}
	return 0;
}

C++: #include<iostream>

using namespace std;

int main()
{
    int n,i,k,j,b=0;

    cin >> n;

     for(i=2; i<=n; i++)
    {
        k = 1;
        for(j=2; j<=i/2; j++)
        {
            if(i%j==0)
            {
                k = 0;
                break;
            }
        }

        if(k==1)
        {
            b++;
        }
    }

    cout << b << endl;

    return 0;
}

C++: #include<iostream>
int main() {

  int n,i,j,c;
  while(std::cin>>n){
    c=0;
    int list[n-1];
    for(i=2;i<n+1;i++)list[i-2]=i;
    for(j=2;j*j<n+2;j++)
      if(list[j-2]!=0)
	for(i=1;j-2+list[j-2]*i<n;i++)
	  list[j-2+list[j-2]*i]=0;
    for(i=0; i<n-1; i++)
      if(list[i]!=0)c++;

    std::cout<<c<<"\n";
  }
}

C++: #include <stdio.h>

int main(){
	long sosuu[] = { 2, 3, 5 }, i, x, y = 3,n=0,z,d=0;
	while (scanf("%d", &n) != EOF){
		z = 0;
		if(d < n)
		{for (i = 3; i < n; i += 2){
			for (x = 0; x < y; x++){
				if (i % sosuu[x] == 0){ break; }
				if (x + 1 == y){ sosuu[y] = i, y++; d = i; }
			}
		}
	}
		for (i = 0; i < n; i++){ if (sosuu[i] > n + 1)break; z++; }printf("%d\n", z);
	}
	return 0;
}

C++: #include <iostream>

using namespace std;

int main()
{
	int i,n,k;
	
	while(cin >> n){
		int flug[n+1];
		int count = 0;
		for(i=0;i<n+1;i++) flug[i] = 1;
		// 0, 1は除外
		flug[0] = flug[1] = 0;
		for(i=2;i*i<n;i++){
			for(k=n;k>=2;k--){
				if((k % i == 0) && (k != i)){
					flug[k] = 0;
				}
			}
		}
		
		// 個数を走査
		for(i=2;i<n+1;i++){
			if(flug[i] == 1){
				count++;
			}
		}
		
		cout << count << endl;
	}
	return 0;
}

C++: while True:
	try:
		n=int(input())
		a=[True]*(n+1)
		a[0]=a[1]=False
		for i in range(2,int(n**0.5)+1):
			if a[i]:
				for j in range(i**2,n+1,i):
					a[j]=False
		print(a.count(True))
	except:
		break

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

using namespace std;

int MAX = 9;

vector <int> v;

int main() {
	int n;
	
	while (cin>>n) {
		v.clear();
		
		for (int i=0; i<=MAX; i++) v.push_back(n-i);
		sort(v.begin(), v.end());
		
		int ans = 0;
		
		for (int i=0; i<=MAX; i++) {
			for (int j=0; j<=MAX; j++) {
				for (int k=0; k<=MAX; k++) {
					if (binary_search(v.begin(), v.end(), i+j+k)) ans++;
				}
			}
		}
		cout << ans << endl;
	}
	
	return 0;
}

C++: #include<iostream>
#include<string>
#include<string.h>
#include<algorithm>
#include<stdio.h>
#include<cmath>
#include<vector>
#include<utility>
#include<stack>
#include<queue>
#include<list>
#include<bitset>

#define FOR(i, a, b) for(int i=(a);i<=(b);i++)
#define RFOR(i, a, b) for(int i=(a);i>=(b);i--)
#define MOD 1000000007
#define INF 1000000000

using namespace std;

int main(void) {
	int n;
	int count = 1;
	if (n == 1) {
		cout << 0 << endl;
	}
	else if (n == 2) {
		cout << 1 << endl;
	}
	else {
		for (int t = 3; t <= n - 1; t += 2) {
			FOR(i, 1, t - 1) {
				if (t%i == 0) {
					break;
				}
				if (i == t - 1) {
					count++;
					break;
				}
			}
		}
		cout << count << endl;
	}
	
	return 0;
}

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

bool isPrime(int x) {
  if(x == 2) {
	return true;
  }
  else if(x < 2 || x % 2 == 0) {
	return false;
  }
  else {
	for(int i = 3; i <= (int)sqrt(x); i += 2) {
	  if(x % i == 0) {
		return false;
	  }
	}
  }
  return true;
}

int main() {
  int n;
  bool a[1000000];

  for(int i = 1; i < 1000000; ++i) {
	if(isPrime(i)) {
	  a[i] = true;
	}
  }
  
  while(cin >> n) {
	cout << accumulate(&a[1], &a[n+1], 0) << endl;
  }
  return 0;
}

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

C++: #include<iostream>
#include<stdio.h>
using namespace std;
typedef long long ll;
const ll MAX=1e6;
ll prime[MAX+2],mark[MAX+2],ans[MAX+2];
void Prime(){
    memset(mark,0,sizeof(mark));
    ll index=0;
    for(ll i=2;i<=MAX;i++){
        if(mark[i]) continue;
        else{
            prime[++index]=i;
            for(ll j=i*i;j<=MAX;j+=i){
                mark[j]=1;
            }
        }
    }
}
int main(){
    ll a;
    memset(ans,0,sizeof(ans));
    Prime();
    for(int i=2;i<=MAX-2;i++){
        if(mark[i]==0) ans[i]=ans[i-1]+1;
        else ans[i]=ans[i-1];
    }
    while(scanf("%I64d",&a)!=EOF){
        cout<<ans[a]<<endl;;
    }
    return 0;
}

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

int main(){
    bool is_prime[1000000];

    is_prime[0] = is_prime[1] = false; 
    for(int i=2; i<1000000; i++){
        is_prime[i] = true;
    }
    for(int i=2; i*i<1000000; i++){
        if(is_prime[i]){
            for(int j=i*i; j<1000000; 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>
#include <cstring>
using namespace std; 
int main()
{
	    int n;
//		for(int i=2;i<=1000;++i)
//		{
//			for(int j=2;j*i<=1000000;++j)
//			{
//				a[i*j]=1;
//			}
//	}
//for(int i=2;i*i<=n;i++)
//		{
//			if(a[i]==false)
//				for(int j=i*2;j<=n;j+=i)
//					a[j]=true;
//		}
	while(cin>>n)
	{ 
	    bool a[1000001]; 
	   // memset(a,0,sizeof(a));
		int count=0;
		for(int i=2;i*i<=n;i++)
		{
			if(a[i]==false)
				for(int j=i*2;j<=n;j+=i)
					a[j]=true;
		}
		for(int k=2;k<=n;k++)
		{
			if(!a[k])
			count++;
		}
		cout<<count<<endl;
	
	
		
	}
	return 0;
}

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

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