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 <cmath>
#include <vector>
#include <time.h>     // for clock()
using namespace std;

/*
1???2 ?????? n ?????§?????´??°???????????????
2??????????????£???????????°????????§???????°?????????????????????? p ??¨?????????????????????????????????????????°?????? p ??¨?????????
p ??\?????? p ????????°?????¨????¶????????????????
3???2??????????????°?????????????????????p ??? ???n ????¶??????????????????????????????????????????£?????????????´???°???
*/
// ??¨????????????????????????
int SieveOfEratosthenes(int num) {
	
	if (num < 2) {
		return 0;
	}
	// num???????´???°?????°
	int count = 0;
	int i = 2;
	while (i <= num) {
		if (i == 2) {
			count++;
			i++;
			continue;
		}
		else if (i % 2 == 0) {
			//count++;
			i++;
			continue;
		}
		else {
			// Route Num?????´??°??¨??????????????????
			int routeN_int = (int)sqrt(i);
			int j = 3;
			while (1) {
				if (j > routeN_int) {
					count++;
					break;
				}
				if (i % j == 0) {
					break;
				}
				j += 2;
			}
		}
		i++;
	}


	return count;
}

int main() {

	int num = 0;
	while (cin >> num) {
		//clock_t start = clock();    // ??????????????????
		cout << SieveOfEratosthenes(num) << endl;
		//clock_t end = clock();     // ????????????
		//std::cout << "duration = " << (double)(end - start) / CLOCKS_PER_SEC << "sec.\n";
	}
	return 0;
}

C++: #include <iostream>
#include <math.h>
#define MAX 1000010
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[0] = 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 + 1;
		answer[i+1] = count + 1;
	}
	while((cin >> n)!= NULL){
		cout << answer[n] << endl;
	}
	return 0;
}

C++: #include <iostream>

using namespace std;

int main()
{
	int num[1000000];
	for (int i = 0; i < 1000000; i++) {
		num[i] = 1;
	}
	num[0] = 0;
	num[1] = 0;

	for (int i = 2; i < 1000000; i++) {
		for (int j = 2; i * j < 1000000; j++) {
			num[i * j] = 0;
		}
	}

	int n;
	while (cin >> n) {
		int ans = 0;
		for (int i = n; i >= 2; i--) {
			if (num[i]) {
				ans++;
			}
		}
		cout << ans << endl;
	}
	
	return 0;
}

C++: #include <iostream>
#include <iomanip>
#include <list>
#include <map>
#include <algorithm>
#include <stack>
#include <string>

using namespace std;


int main(){
	int n = 0;
	int count;
	bool flag = false;
	while(cin >> n){
		count = 0;
		for(int i = 1; i <= n; i++){
			if(i < 2){
			}
			else if(i == 2){
				count++;
			}
			else if(i % 2 != 0){
				for(int j = 3; j * j <= i; j += 2){
					if(i % j == 0){
						flag = true;
					}
				}
				if(flag != true){
					count++;
				}
			}
			flag = false;
		}

		cout << count << endl;

	}

	return 0;
}

C++: #include <iostream>
#include <cmath>
#include <time.h>     // for clock()
using namespace std;

/*
1???2 ?????? n ?????§?????´??°???????????????
2??????????????£???????????°????????§???????°?????????????????????? p ??¨?????????????????????????????????????????°?????? p ??¨?????????
p ??\?????? p ????????°?????¨????¶????????????????
3???2??????????????°?????????????????????p ??? ???n ????¶??????????????????????????????????????????£?????????????´???°???
*/
// ??¨????????????????????????
int SieveOfEratosthenes(int num) {
	
	if (num < 2) {
		return 0;
	}
	// num???????´???°?????°
	int count = 0;
	bool fl[1000001];
	memset(fl, true, sizeof(fl));
	for (int i = 2; i <= sqrt(num); i++) {
		if (fl[i]) {
			for (int j = 2; j*i <= num; j++) {
				fl[i*j] = false;
			}
		}
	}
	for (int i = 2; i <= num; i++) {
		if (fl[i]) {
			count++;
		}
	}
	return count;
}

int main() {

	int num = 0;
	while (cin >> num) {
		//clock_t start = clock();    // ??????????????????
		cout << SieveOfEratosthenes(num) << endl;
		//clock_t end = clock();     // ????????????
		//std::cout << "duration = " << (double)(end - start) / CLOCKS_PER_SEC << "sec.\n";
	}
	return 0;
}

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

using namespace std;

#define N 100000

int main(){

    bool primes[N];
    int  n; 

    fill(primes, primes + N, true);
    primes[0] = false;
    primes[1] = false;
    
    for(int i = 2; i < N; ++i){
        if(primes[i]){
            for(int j = i + i; j < N; j += i)primes[j] = false;
        }
    }

    while(cin >> n){
        int  count = 0;
        for(int i = 2; i <= n; ++i){
            if(primes[i])count++;
        }
        cout << count << endl;
    }
    
	return 0;
}

C++: #include<iostream>
using namespace std;
int main() {
	int n,ans;
	while (cin >> n) {
		ans = 0;
		for (int i = 2; i <= n; i++) {
			if (i != 2) {
				if (i % 2 == 0) {
					goto a;
				}
			}
			else {
				ans = ans + 1;
				goto a;
			}
			for (int z = 2; z <= i/2+1; z++) {
				if(i!=z){
					if (i%z == 0) {
						goto a;

					}
				}
			}
			ans = ans + 1;
		a:;
		}
		cout << ans << endl;
	}
}

C++: #include <cstdio>
#define MAX 1000000

int main(){
	int n;
	bool c[MAX]={true};
	int t[MAX];t[0] = 0;
	for(int i = 2;i < MAX;i++){
		for(int j = i;i*j < MAX;j++){
			c[i*j-1] = true;
		}
	}
	for(int i = 1;i < MAX;i++){
		if(c[i])t[i]=t[i-1];
		else t[i] = t[i-1]+1;
	}
	while(scanf("%d", &n)){
		printf("%d\n", t[n-1]);
	}
	return 0;
}

C++: #include <iostream>

using namespace std;

int main()
{
    int n;
    long cnt;
    bool flag;

    while (cin >> n) {
        if (n < 2) {
            cout << "0\n";
            continue;
        }
        else if (n == 2) {
            cout << "1\n";
            continue;
        }
        else if (n == 3) {
            cout << "2\n";
            continue;
        }
        
        cnt = 4;
        for (int i = 5; i <= n; i += 2) {
            flag = false;
            for (int j = 3; j < n; j += 2) {
                if (!(i % j)) {
                    flag = true;
                    break;
                }
            }
            if (!flag)
                cnt++;
        }
        cout << cnt << endl;
    }
    
    return 0;
}

C++: #define _CRT_SECURE_NO_WARNINGS
#include "bits/stdc++.h"
using namespace std;

int const N = 1000010;
int ans[N];

bool isprime(int x){
    for (int i = 2; i*i <= x; i++){
        if (x%i == 0) return false;
    }
    return true;
}

int main(int argc, char ** argv) {
    ans[0] = ans[1] = 0;
    for (int i = 2; i < N; ++i) {
        if (isprime(i)) ans[i] = ans[i - 1] + 1;
        else ans[i] = ans[i - 1];
    }
    int n;
    while (~scanf("%d\n", &n)){
        printf("%d\n",ans[n]);
    }
}

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

int isPrime(int a){
	int i;     
	int isTrue;     
	for(i = 3, isTrue = 1; i < a; i+=2){ 
		if( !(a%i) ){ isTrue = 0; break; } 
	}     
	return a==2||isTrue&&a%2; 
}   

int main(){
	int n,ans=0;
	while(cin >> n){
		for(int i = 2; i <= n; i++){
			if(isPrime(i))ans++;
		}
		cout << ans << endl;
		ans=0;
	}
}

C++: import math

def check_prime(goal):
    true_or_false=False
    for i in range(2,int(math.sqrt(goal))+1):
        if goal%i==0:
             true_or_false=True
    if true_or_false==False:
        return True
    else:
        return False
anss=[]
while True:
    try:
        num=int(input())
        num_of_prime=0
        for i in range(2,num+1):
           if check_prime(i):
               num_of_prime+=1
        anss.append(num_of_prime)
    except:
        break

for ans in anss:
    print(ans)