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 <cstring>
using namespace std;

int prime[1000000];

int main()
{
  int n;
  for (int i = 0; i < 1e6; i++) prime[i] = 1;
  prime[0] = prime[1] = 0;
  for (int i = 2; i < 1000000; i++){
    if (prime[i] == 1){
      for (int j = 2 * i; j < 1000000; j += i){
        prime[j] = 0;
      }
    }
  }
  for (int i = 0; i < 1000000; i++){
    prime[i + 1] += prime[i];
  }
  while (cin >> n){
    cout << prime[n] << endl;
  }

  return 0;
}

C++: #include<iostream>
#include<cmath>
using namespace std;
bool isprime(int n) {
	if (n == 2)return true;
	if (n % 2 == 0)return false;
	for (int i = 3; i <= sqrt(n); i += 2) if (n%i == 0)return false;
	return true;
}
int main() {
	long prime[80000];
	prime[0] = 2;
	int num = 1;
	for (int i = 3; i < sqrt(1000000); i += 2) {
		if (isprime(i)) {
			prime[num] = i;
			num++;
		}
	}
	long n; while (cin >> n) {
		int j = 0;
		for (; prime[j] <= n; j++) {}
		cout << j << endl;
	}
	return 0;
}

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

using namespace std;

bool table[1000000];
vector<int> primes;

int main() {
  table[2] = true;
  for (int i = 3; i <= 999999; i += 2)
    table[i] = true;
  for (int i = 3; i <= 1000; i += 2)
    if (table[i])
      for (int j = 3 * i; j <= 999999; j += 2 * i)
        table[j] = false;
  primes.push_back(2);
  for (int i = 3; i <= 999999; i += 2)
    if (table[i])
      primes.push_back(i);
  int n;
  while (scanf("%d", &n) == 1) {
    printf("%d\n", upper_bound(primes.begin(), primes.end(), n)-primes.begin());
  }
  return 0;
}

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

int main()
{
	unsigned int  n;
	cin >> n;
	unsigned  int num,count = 0;
	for(int i = 0; i < n; i++)
	{
		cin >> num;
		for(int j = 2; j <=num; j++)
		{
			if(num == j)count++;
			if((num % j) == 0)
			{
				break;
			}
		}
	}
	cout << count << endl;
	return 0;
}

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

bool fermat_test(int n) {
    if (n < 2) {
        return false;
    } else if (n == 2) {
        return true;
    } else {
        return (static_cast<int>(std::pow(2, n - 1)) % n) == 1;
    }
}

bool prime_test(int n) {
    if (n < 2) {
        return false;
    } else if (n == 2) {
        return true;
    } else {
        for (int i = 3; i < static_cast<int>(std::sqrt(n)); i += 2) {
            if ((n %= 2) == 0) {
                return false;
            }
        }
    }
    return true;
}

int main() {
    std::map<int, int> m;
    int n;
    while (~scanf("%d", &n)) {
        for (int i = 2; i <= n; ++i) {
            if (m.count(i) > 0)
                continue;

            m[i] = m[i - 1] + (fermat_test(i) && prime_test(i));
        }
        printf("%d\n", m[n]);
    }
}

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

const int MAXN = 1e6;

bool is_prime[MAXN];
int prime_count[MAXN];

void init() {
	fill(is_prime, is_prime + MAXN, true);
	int cnt = 0;
	for (int i = 2; i < MAXN; ++i) {
		if (is_prime[i]) {
			++cnt;
			for (int j = i + i; j < MAXN; j += i) {
				is_prime[j] = false;
			}
		}
		prime_count[i] = cnt;
	}
}

int main() {
	//freopen("0009.txt", "r", stdin);
	init();
	int n;
	while (cin >> n) {
		cout << prime_count[n] << "\n";
	}
}

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

int main()
{
	vector<int> prime,count;
	int j, k, x, num;
	while (cin >> num){
		prime.push_back(2);
		x = 1;
		k = 1;
		while (prime[k-1] <= num)
		{
			x += 2;
			j = 0;
			while (j < k && x % prime[j] != 0)j++;
			if (j == k)
			{
				prime.push_back(x);
				k++;
			}
		}
		count.push_back(prime.size() - 1);
		prime.clear();
	}
	for (int i = 0; i < count.size(); i++)
	{
		cout << count[i] << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<math.h>
using namespace std;
int isPrime(int n) {
	if (n == 2) {
		return 1;
	}
	if ((n <2) || (n % 2 == 0)) {
		return 0;
	}
	int i = 3;
	while (i <= sqrt(n)) {
		if (n % i == 0) {
			return 0;
			
		}
		i += 2;
	}
	return 1;
}
int main()
{
	int x[30];
	int i = 0;
	while (cin >> x[i]) {
		int k = 0;
		for (int j = 0; j < x[i]; j+=2) {
			
			k += isPrime(j+1);
		}
		cout << k+1 << endl;

		i++;
	}

	return 0;
}

C++: #include <cstdio>

using namespace std;

int main() {
    long NUM = 1000000;
	long n = NUM;
	long m = 2;
	bool flag;
	long counter = 0;
	bool primeNum[NUM + 10];

	long i, len = NUM + 10;
	
	for (i = 0; i < len; i++) primeNum[i] = false;
	
    while (m <= n) {
        flag = true;
        len = m / 2;

        for(i = 2; i <= len; i++) {
            if(m % i == 0) {
                flag = false;
                
                break;
            }
        }

        if (flag) primeNum[m] = true;

        m++;
    }
    
    while (scanf("%ld", &n) != EOF) {
        counter = 0;
        
        for (i = 0; i <= n; i++) {
            if (primeNum[i]) counter++;
        }
        
        printf("%ld\n", counter);
    }

    return 0;
}

C++: #include<iostream>

using namespace std;

int main(){
  int n;
  int *prime;
  int i,j;
  while(cin >> n){
    //init
    prime = new int[n+1];
    for(i=0; i<=n; i++) prime[i] = 0;

    //素数判定
    for(i=2; i*i<=n; i++){
      if(prime[i] == 0){
	prime[i] = 1; //iは素数
	for(j=2; i*j<=n; j++){//倍数ふるい落とし
	  prime[i*j] = -1;//i*jは素数じゃない
	}
      }
    }
    for(i=i; i<=n; i++){
      if(prime[i] == 0) prime[i] = 1;
    }
    
    //output
    int ans=0;
    for(i=2; i<=n; i++){
      if(prime[i] == 1) {
	ans ++;
	//cout << "prime:" << i << endl;;
      }
    }
    cout << ans << endl;

    delete[] prime;
  }
  return 0;
}

C++: #include <iostream>
#include <sstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <climits>
#include <cfloat>
using namespace std;

void makePrimeNumber(int N, vector<bool>& prime) // 0ツ暗按湘」Nツ暗按可コツづ個青ョツ青板つゥツづァツ素ツ青板づーツ探ツつキ
{
    prime.assign(N+1, true);
    prime [0] = prime[1] = false;
    for(int i=2; i*i<=N; i++){
        if(prime[i]){
            for(int j=i; i*j<=N; j++){
                prime[i*j] = false;
            }
        }
    }
}

int main()
{
    for(;;){
        int n;
        if(!(cin >> n))
            return 0;
        vector<bool> prime;
        makePrimeNumber(n, prime);
        int ret = 0;
        for(int i=0; i<=n; ++i)
            if(prime[i])
                ++ ret;
        cout << ret << endl;
    }
}

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

#define REP(i, s, n) for (int i = s; i < n; ++i)
#define rep(i, n) REP(i, 0, n)
#define SORT(c) sort((c).begin(), (c).end())
#define IINF INT_MAX
#define LLINF LLONG_MAX
#define DEBUG false

int prime[100000] = {0};

void makeprime(long long int n)
{
    int k = 1;
    prime[0] = 2;
    for (long long int i = 3; i < n; i++)
    {
        double temp = sqrt(i);
        for (int j = 0; i % prime[j] != 0; j++)
        {
            if (temp < prime[j])
            {
                prime[k++] = i;
                break;
            }
        }
    }
    // rep(i,1000){
    //     cout<<prime[i]<<" ";
    // }
}

int main()
{
    makeprime(999999);
    int n;
    int i;
    while (cin >> n)
    {
        i = 0;
        while (true)
        {
            if (n < prime[i])
            {
                break;
            }
            i++;
        }
        cout << i << endl;
    }
    return 0;
}