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 <iostream>
#include <cstdio>
using namespace std;

#define MAX_SIZE (1000000)

// int prime[MAX_SIZE];

int main()
{
	int n;
	static int prime[MAX_SIZE];

	prime[0] = prime[1] = 0;

	for (int i = 2; i < MAX_SIZE; i++){
		prime[i] = 1;
	}

	// Sieve of Eratosthenes
	for (int i = 2; i * i < MAX_SIZE; i++){
		if (prime[i]){
			for (int j = i * i; j < MAX_SIZE; j += i){
				prime[j] = 0;
			}
		}
	}

	for (int i = 1; i < MAX_SIZE; i++){
		prime[i] += prime[i - 1];
	}

	while (scanf("%d", &n) != EOF){
		printf("%d\n", prime[n]);
	}

	return (0);
}

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

void count_Prime(int n);
bool IsPrime(int num);


int main()
{
	int Data = 0;
	while (cin >> Data)
	{
		count_Prime(Data);
	}

	return 0;
}

void count_Prime(int n)
{
	int counter = 0,  i = 0;
	for (i = 2; i <= n; ++i)
	{
		if (IsPrime(i))
			counter++;
	}

	cout << counter << endl;

}

bool IsPrime(int num)
{
	if (num < 2) return false;
	else if (num == 2) return true;
	else if (num % 2 == 0) return false; // ??¶??°????????????????????????

	double sqrtNum = sqrt(num);
	for (int i = 3; i <= sqrtNum; i += 2)
	{
		if (num % i == 0)
		{
			// ?´???°??§?????????
			return false;
		}
	}

	// ?´???°??§??????
	return true;
}

C++: #include <cctype>
#include <cmath>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)
#define FORR(i, m, n) for(int i = m;i >= n;i--)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define LL long long
#define pb(a) push_back(a)
#define INF 999999999
using namespace std;
typedef pair<int, int> P;
typedef pair<LL, LL> LP;
typedef pair<int, P> PP;
typedef pair<LL, LP> LPP;

int dy[]={0, 0, 1, -1, 0};
int dx[]={1, -1, 0, 0, 0};

int sosuu(int n){
	int ans = 0;
	
	FOR(i, 2, n){
		if(i == 2){
			ans++;
		}else if(i % 2 != 0){
			bool flag = true;
			int j = 3;
			while(j * j <= i){
				if(i % j == 0){
					flag = false;
					break;
				}
				j += 2;
			}
			if(flag) ans++;
		}
	}
	return ans;
}
 
int main(){
    int n;
    while(cin >> n){
        cout << sosuu(n) << endl;   
    }	
}

C++: #include<bits/stdc++.h>
#include<vector>
#include<list>
#include<stack>
#include<queue>
#include<algorithm>
using namespace std;

void pr(int n){
    int ans=0;
    vector<bool> prime;
    for(int i=0;i<=n;i++){
        prime.push_back(true);
    }
    prime[0]=prime[1]=false;

    for(int i=2;i<=n;i++){
        if(prime[i]){
            for(int j=2;j*i<=n;j++){
                prime[i*j]=false;
            }
        }
    }
    for(int i=2;i<=n;i++){
        if(prime[i]){
            ans++;
        }
    }
    printf("%d\n",ans);
}


int main(){
    int n;
    while(scanf("%d",&n)!=EOF){
        pr(n);
    }
    return 0;
}

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

using namespace std;

vector<int> ps;

int isprime(int n){
	vector<int>::iterator it;
	for(it=ps.begin();it!=ps.end();it++){
		if(n%(*it) == 0) return 0;
		if(n <= (*it)*(*it)) break;
	}
	ps.push_back(n);
	return 1;
}

int main(){
	int n;
	ps.push_back(2);
	while(scanf("%d", &n)!=EOF){

		int c=0;

		for(int i=3;i<=n;i++){
			if(i&1 && isprime(i)) c++;
		}

		printf("%d\n", c);
	}
}

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,3,10000){
	bool flg = true;
	REP(j,primes.size()) if(i % primes[j] == 0 ) flg = false;
	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<vector>
using namespace std;
#define MAX_N 100001
int main(){
	int n;
	while(cin >> n){
		vector<int> vec;
		vector<int>::iterator it =vec.begin();
		for(int i=2;i<=n;i++){
			int flag=0;
			it =vec.begin();
			while(it!= vec.end()){
				if(i% *it==0){
					flag=1;
					break;
				}
				it++;
			}
			if(!flag)vec.push_back(i);
		}
		it =vec.begin();
		//while(it!= vec.end()){
		//	cout<<*it<<endl;
		//	it++;
		//}
		cout << vec.size()<<endl;
	}
	return 0;
}

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

int main(){
 int i,j,n,count,flg;
 scanf("%d",&n);
 for(i=2,count=0;i<=n;i++){
  for(j=2,flg=1;j<=i/2;j++)
   if(i%j==0){ flg=0; break; }
  count+=flg;
 }
 printf("%d\n",count);
 return 0;
}

C++: #include <bits/stdc++.h>

#define MAX 1000000
#define NIL -1

int primes[MAX];

bool isPrime(int n)
{
    if (n <= 1) {
        primes[n] = 0;
        return false;
    }
    if (n <= 3) {
        primes[n] = 1;
        return true;
    }
    if (n % 2 == 0 || n % 3 == 0) {
        primes[n] = 0;
        return false;
    }
    for (int i = 5; i * i <= n; i = i + 6)
        if (n % i == 0 || n % (i + 2) == 0) {
            primes[n] = 0;
            return false;
    }
    primes[n] = 1;
    return true;
}

int main()
{
    for (int i = 1; i <= MAX; i++) primes[i] = NIL;
    int N, sum;
    while(std::scanf("%d", &N) != EOF) {
        sum = 0;
        for (int i = 2; i <= N; i++) {
            if (primes[i] != NIL) {
                if (primes[i] == 1) sum++;
            }
            else {
                if (isPrime(i)) sum++;
            }
        }
        std::printf("%d\n", sum);
    }
    return 0;
}

C++: #define _USE_MATH_DEFINES
#define llint long long
#include <iostream>
#include <algorithm>
#include <cmath>
using namespace std;

int pn = 0;
int p[1000000];
void getsosu(int Max) {
    for(int i = 2; i <= Max; i++) {
        bool flg = true;
        for(int j = 0; j < pn; j++) {
            if(p[j] * p[j] > i) break;
            if(i % p[j] == 0) {
                flg = false;
                break;
            }
        }
        if(flg) p[pn++] = i;
    }
}

int main() {
    getsosu(999999);

    int n;
    while(cin >> n) {
        int i = 0;
        while(p[i] <= n) i++;
        cout << i << endl;
    }
    return 0;
}

C++: <?php
$max=1000000;
$maxR=floor(sqrt($max));

$ch=[0,0,1,1];

for($i=4;$i<$max;$i+=2){$ch[]=0;$ch[]=1;}

for($i=3;$i<$maxR;$i+=2) if($ch[$i]) for($j=$i+$i;$j<$max;$j+=$i) $ch[$j]=0;

for($i=3;$i<$max;$i++) $ch[$i]+=$ch[$i-1];

while(fscanf(STDIN,"%d",$n)) printf("%d\n",$ch[$n]);