Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	var primeCount int
	for sc.Scan() {
		input := nextInt()
		for i := 0; i <= input; i++ {
			if IsPrime(i) {
				primeCount++
			}
		}
		fmt.Printf("%d\n", primeCount)
		primeCount = 0
	}
}

func IsPrime(num int) bool {
	switch {
	case num == 2:
		return true
	case num < 2 || num%2 == 0:
		return false
	}
	sqrt := math.Sqrt(float64(num))
	for i := 3; i <= int(sqrt); i += 2 {
		if num%i == 0 {
			return false
		}
	}
	return true
}

func next() string {
	return sc.Text()
}

func nextInt() int {
	a, _ := strconv.Atoi(next())
	return a
}

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

int count(int n);

int main(void){
	while (true){
		int n; cin >> n;
		if (!n){
			break;
		}
		cout << count(n) << endl;
	}
	return 0;
}

int count(int n){
	int ct = 0;
	bool *prime = new bool[n];
	//bool prime[999999];
	for (int i = 0; i <= n;i++){
		prime[i] = 0;
	}
	prime[2] = 1;
	for (int i = 3; i <= n; i = i + 2){
		prime[i] = 1;
	}
	for (int i = 3; i <= sqrt(n * 1.0); i = i + 2){
		if (prime[i] == 0) continue;
		for (int j = i * 2; j <= n; j = j + i){
			prime[j] = 0;
		}
	}
	for (int i = 3; i <= n; i = i + 2){
		if (prime[i] == 1) ct++;
	}
	ct++; //2を数えていない分
	return ct;
	delete[] prime;
}

C++: #include <bits/stdc++.h>
#define MAX_N 1000000
using namespace std;
bool prime[MAX_N];
int table[MAX_N];

void initialize() {
    int p = 0;
    memset(prime, true, sizeof(prime));
    prime[0] = prime[1] = false;
    for(int r = 2; r * r <= MAX_N; r++) {
        if(prime[r]) {
            table[p++] = r;
            for(int c = r * 2; c <= MAX_N; c+= r) {
                prime[c] = false;
            }
        }
    }
}


int main(void) {
    initialize();
    int num;
    while(cin >> num) {
        int cnt = 0;
        for(int r = 0; table[r] <= num; r++) cnt++;
        cout << cnt << endl;
    }
    return(0);
}

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

int main(){
	const int max_num=1000000;
	int prime[max_num];
	int prime2[max_num];
	int min_prime[168]={2 ,3 ,5 ,7 ,11 ,13 ,17 ,19 ,23 ,29 ,31 ,37 ,41 ,43 ,47 ,53 ,59 ,61 ,67 ,71 ,73 ,79 ,83 ,89 ,97 ,101 ,103 ,107 ,109 ,113 ,127 ,131 ,137 ,139 ,149 ,151 ,157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233 ,239 ,241 ,251 ,257 ,263 ,269 ,271 ,277 ,281 ,283 ,293 ,307 ,311 ,313 ,317 ,331 ,337 ,347 ,349 ,353 ,359 ,367 ,373 ,379 ,383 ,389 ,397 ,401 ,409 ,419 ,421 ,431 ,433 ,439 ,443 ,449 ,457 ,461 ,463 ,467 ,479 ,487 ,491 ,499 ,503 ,509 ,521 ,523 ,541 ,547 ,557 ,563 ,569 ,571 ,577 ,587 ,593 ,599 ,601 ,607 ,613 ,617 ,619 ,631 ,641 ,643 ,647 ,653 ,659 ,661 ,673 ,677 ,683 ,691 ,701 ,709 ,719 ,727 ,733 ,739 ,743 ,751 ,757 ,761 ,769 ,773 ,787 ,797 ,809 ,811 ,821 ,823 ,827 ,829 ,839 ,853 ,857 ,859 ,863 ,877 ,881 ,883 ,887 ,907 ,911 ,919 ,929 ,937 ,941 ,947 ,953 ,967 ,971 ,977 ,983 ,991 ,997 };
	int sosuu=0;
	int k=168;

	for(int i=0;i<168;i++){
		prime[i]=min_prime[i];
	}
	for(int i=1001;i<max_num;i+=2){
		sosuu=0;
		for(int j=0;j<168;j++){
			if(i%min_prime[j]==0){
				sosuu++;
			}
		}
		if(sosuu==0){
			prime[k]=i;
			k++;
		}
	}
	int l=0;
	for(int i=0;i<max_num;i++){
		if(i>=prime[l]){
			l++;
		}
		prime2[i]=l;
	}

	int n;

	while(cin>>n){
		cout<<prime2[n]<<endl;
	}

	return 0;
}

C++: #include <cstdio>
#include<cmath>
using namespace std;
int n,sum,c,flag;
int a[1000000] = {0};
int main()
{
	c = 1000000;
	for(;c > 0;c--){
		flag = 0;
		for(int i = 2;i <= sqrt(c);i++){
			if(c % i == 0)
				flag = 1;
		}
		if(flag == 0)
			a[c] = c;
	}
	while(scanf("%d",&n) != EOF){
		sum = 0;
		for(int j = n;j > 1;j--){
			if(a[j] != 0)
				sum++;
		}
		printf("%d\n",sum);
	}
	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <vector>
#include <stack>
#include <queue>
#include <map>
#include <cmath>
#include <string>
#include <sstream>
#include <iomanip>
#include <complex>
using namespace std;

#define ll long long
#define vvi vector< vector<int> >
#define All(X) X.begin(),X.end()
#define FOR(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define REP(i,n) for(int i=0;i<(int)(n);i++)
#define pb push_back 
ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
ll lcm(ll a,ll b){return a/gcd(a,b)*b;}

#define MAXINT 1000000
bool isprime[MAXINT];
ll int sum[MAXINT];
void makeprime(){
	REP(i,MAXINT) isprime[i] = true;
	FOR(i,2,sqrt(MAXINT)+1){
		FOR(j,i+1,MAXINT-1){
			if(j%i==0) isprime[j] = false;
		}
	}
}

int main(){
	makeprime();
	sum[0] = 0;
	sum[1] = 0;
	sum[2] = 1;
	/*FOR(i,3,MAXINT){
		if(isprime[i]) sum[i] = sum[i-1]+1;
		else sum[i] = sum[i-1];
	}*/
	ll int n;
	while(cin >> n){
		ll int ans = 0;
		REP(i,n) if(isprime[i]) ans++;
		cout << ans << endl;
	}
}