Go: package main

// https://ja.wikipedia.org/wiki/%E3%82%A8%E3%83%A9%E3%83%88%E3%82%B9%E3%83%86%E3%83%8D%E3%82%B9%E3%81%AE%E7%AF%A9

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() (int, error) {
	return strconv.Atoi(nextString())
}

func main() {
	scanner.Split(bufio.ScanWords)

	for {
		n, err := nextInt()
		if err != nil {
			break
		}
		maybePrime := make([]bool, n+1)
		for i := 0; i <= n; i++ {
			maybePrime[i] = true
		}

		maybePrime[0] = false
		maybePrime[1] = false
		for i := 2; i <= int(math.Sqrt(float64(n)))+1; i++ {
			j := i * 2
			for j <= n {
				maybePrime[j] = false
				j += i
			}
		}

		nPrimes := 0
		for _, p := range maybePrime {
			if p {
				nPrimes++
			}
		}
		// fmt.Println(maybePrime)
		fmt.Println(nPrimes)
	}
}

C++: #include <bits/stdc++.h>
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define RFOR(i,a,b) for(int i=(b) - 1;i>=(a);i--)
#define REP(i,n) for(int i=0;i<(n);i++)
#define RREP(i,n) for(int i=n-1;i>=0;i--)

#define PB push_back
#define INF INT_MAX/3
#define ALL(a) (a).begin(),(a).end()
#define CLR(a) memset(a,0,sizeof(a))

typedef long long int ll;

using namespace std;

int main(){
    vector<int> primes;
    primes.PB(2);
    FOR(i,3,100000){
	bool flg = true;
	REP(j,primes.size()) if(i % primes[j] == 0 ) flg = false;
	if(flg) primes.PB(i);
    }

    int n;
    while(scanf("%d",&n)){
	int res = upper_bound(ALL(primes),n) - primes.begin();
	cout << res << endl;
    }

    return 0;
}

C++: #include <bits/stdc++.h>
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define RFOR(i,a,b) for(int i=(b) - 1;i>=(a);i--)
#define REP(i,n) for(int i=0;i<(n);i++)
#define RREP(i,n) for(int i=n-1;i>=0;i--)

#define PB push_back
#define INF INT_MAX/3
#define ALL(a) (a).begin(),(a).end()
#define CLR(a) memset(a,0,sizeof(a))

typedef long long int ll;

using namespace std;

int main(){
    vector<int> primes;
    primes.PB(2);
    FOR(i,3,300000){
	bool flg = true;
	REP(j,primes.size()) if(i % primes[j] == 0 ) {
	    flg = false;
	    break;
	}
	if(flg) primes.PB(i);
    }

    int n;
    while(true){
	cin >> n;
	if(cin.eof())break;
	int res = upper_bound(ALL(primes),n) - primes.begin();
	cout << res << endl;
    }

    return 0;
}

C++: #include <iostream>
#include <math.h>
#define MAX 1000000
using namespace std;

int main(){
	int n;
	int *prime,i,j;
	prime = new int[80000];
	int *answer = new int[MAX];
	int count;
	bool flag;

	count = 0;
	answer[1] = 0;
	answer[2] = 1;
	for(i = 3; i <= 1000000; i+=2){
		flag = true;
		for(j = 0; j < count; j++){
			if(i % prime[j] == 0){
				flag = false;
				break;
			}
			if(prime[j] * prime[j] > i) break;
		}
		if(flag == true){
			prime[count++] = i;
		}
		answer[i] = count;
		answer[i+1] = count;
	}
	while((cin >> n)!= NULL){
		cout << answer[n] << endl;
	}
	return 0;
}

C++: #include<iostream>
using namespace std;
int main()
{
	bool p[1000000] = { false };
	int n;

	while (cin >> n){
		for (int i = 2; i <n ; i++){
			for (int i2 = 2; i*i2 <= n; i2++){
				p[i*i2] = true;
			}
		}
		p[2] = false;
		p[3] = false;
		int c = 0;
		for (int i = 2; i <= n; i++){
			if (!p[i])c++;
		}
		cout << c << endl;
	}
}

C++: #include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <ctime>
#include <cassert>
#include <iostream>
#include <cctype>
#include <sstream>
#include <string>
#include <list>
#include <vector>
#include <queue>
#include <set>
#include <stack>
#include <map>
#include <utility>
#include <numeric>
#include <algorithm>
#include <iterator>
#include <bitset>
#include <complex>
#include <fstream>
using namespace std;
typedef long long ll;
const double EPS = 1e-9;
typedef vector<int> vint;
typedef pair<int, int> pint;
#define rep(i, n) REP(i, 0, n)
#define ALL(v) v.begin(), v.end()
#define MSG(a) cout << #a << " " << a << endl;
#define REP(i, x, n) for(int i = x; i < n; i++)
template<class T> T RoundOff(T a){ return int(a+.5-(a<0)); }
template<class T, class C> void chmax(T& a, C b){ if(a < b) a = b; }
template<class T, class C> void chmin(T& a, C b){ if(b < a) a = b; }
template<class T, class C> pair<T, C> mp(T a, C b){ return make_pair(a, b); }

// stl;
const int MAX = 10000;
vector<bool> count()
{
	vector<bool> res(MAX + 1, false);
	
	for(int i = 2; i <= MAX; i++)
	{
		res[i] = true;
		for(int j = 2; j * j <= i; j++)
		{
			if(i % j == 0)
			{
				res[i] = false;
				break;
			}
		}
	}	
	
	return res;
}

int main()
{
	vector<bool> table(count());
	
	int n;
	while(cin >> n)
	{
		cout << count(table.begin(), table.begin() + n + 1, true) << endl;
	}
}

C++: #include <stdio.h>
const int MAX_V = 1000000;
int prime[MAX_V + 1];
int sum[MAX_V + 1] = {0};
int main() {
  int i, k, v, n, max = 0;
  for(i = 2; i <= MAX_V; i++) {
    prime[i] = 1;
  }
  for(i = 2; i*i <= MAX_V; i++) {
    if(prime[i]){
      for(k = 2 * i; k <= MAX_V; k += i) {
        prime[k] = 0;
      }
    }
    sum[i] = prime[i - 1] + sum[i - 1];
  }
  while(scanf("%d", &n) != EOF){
    printf("%d\n", sum[i + 1]);
  }
  return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <iomanip>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <bitset>
#include <stack>
#include <utility>
#include <numeric>
#include <algorithm>
#include <functional>
#include <cctype>
#include <complex>
#include <string>
#include <sstream>

using namespace std;

#define all(c) c.begin(),c.end()
#define rall(c) c.rbegin(),c.rend()
#define rep(i,n) for(int i=0;i<(n);i++)
#define tr(it,container) for(typeof(container.begin()) it = container.begin(); \
                                                  it != container.end(); ++it)
#define mp(a,b) make_pair((a),(b))
#define eq ==


const int N = 1000010;
bool isPrime[N];

void sieve(){
    for(int i=2;i<N;i++) isPrime[i] = true;
    for(int i=2;i*i<N;i++){
        if(not isPrime[i]) continue;
        for(int j=i*i;j<N;j+=i){
            isPrime[j] = false;
        }
    }
}

int main(){
    vector<int> C(N);
    sieve();
    C[0] = 0;
    for(int i=1;i<N;i++){
        if(isPrime[i]){
            C[i] = C[i-1] + 1;
        }else{
            C[i] = C[i-1];
        }
    }
    int n;
    while(cin >> n){
        cout << C[n] << endl;
    }
    return 0;
}

C++: #include<iostream>

using namespace std;

int limit(int n) {
	int i = 1;
	while (i*i <= n) {
		i += 1;
	}
	return i - 1;
}

int main() {
	int n;
	while (std::cin >> n) {
		int *array = new int[n + 1];

		for (int i = 1; i <= n; i++) {
			array[i] = 0;
		}

		int num_max,count;
		num_max = limit(n);
		count = 0;
		for (int i = 2; i <= num_max; i++) {
			if (array[i] == 0) {
				for (int j = 2 * i; j <= n; j += i) {
					if (array[j] == 0) {
						array[j] = 1;
						count += 1;
					}
				}
			}
		}
		std::cout << n - (count+1) << std::endl;
	}
}

C++: import sys

def prime_calc(n):
    if n < 2:
        return False
    else:
        i = 2
        while n > i:
            if n % i == 0:
                return False
            else:
                i += 1
    return True

def prime(n):
    cnt = 0
    for i in range(0, n+1):
        ans = prime_calc(i)
        if ans is True:
            cnt = cnt + 1
    return cnt

def main():
    a = []

    for line in sys.stdin:
        a.append(int(line))

    for line in l:
        print(prime(line))

if __name__ == "__main__":
    main()

C++: #include <stdio.h>

inline int IsPrime(int n);

int main(void){
    int n, cnt;
    int list[100000];

    cnt=0;
    for (long i=2; i<1000000; i++){
        if (i%10==0) list[i/10] = cnt;
        cnt += IsPrime(i);
    }
	printf("input : ");
    while (scanf("%d", &n) && n!=EOF){
        cnt = list[n/10];
        for (long i=1; i<n%10; i++){
            cnt += IsPrime(i);
        }
        printf("%d\n", cnt);
    }
    return 0;
}

inline int IsPrime(int n){
    int i;

    for (i=2; i*i<=n; i++){
        if (n%i==0) return 0;
    }
    return 1;
}