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<stdio.h>

int	main(){
	int	i,	k,	x,	cnt,	data[10000];
	for(i	=	2;	i	<=	10000;	i++){
		data[i]	=	1;
	}
	for(i	=	2;	i	*	i	<=	10000;	i++){
		if(data[i]){
			for(k	=	2	*	i;	k	<=	10000;	k	+=	i){
				data[k]	=	0;
			}
		}
	}
	while(1){
		cnt	=	0;
		scanf("%d",	&x);
		if(x	==	0)break;
		for(i	=	2;	i	<=	x;	i++){
			if(data[x]	==	1)cnt++;
		}
		printf("%d\n",	cnt);
	}
	return	0;
}

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

bool nonprime[1000000];

int main(){
	long i,j,n;
	for(i=2;i<=999999;i++){
		for(j=2;i*j<=999999;j++) nonprime[i*j]=1;
	}
	while(1){
		cin >> n;
		j=0;//j?????????????????¨?????????????????????
		for(i=2;i<=n;i++) if(!nonprime[i]) j++;
		cout << j << endl;
	}
	return 0;
}

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

int main(){
	int n,k,p[100000],s,i,j;
	k=1;
	p[0]=2;
	for(i=3;i<999999;i+=2){
		s=0;
		for(j=0;j<k;j++){
			if((i%p[j])==0){
				s++;
			}
		}
		if(s==0){
			p[k]=i;
			k++;
		}
	}
	
	while(cin >> n){
		for(i=0;n>=p[i];i++){
		}
		cout << i <<"\n";
	}
return 0;
}

C++: 
//  test.cpp

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <string>
#include <algorithm>

using namespace std;


int main(){
    long j,s[1000000],d=1,i,n,k;
    
    for(i=2;i<1000000;i++){
        if(s[i]==0){
            
            for(j=2;j*j<i;j+=2){
                if(j%i==0)
                    break;
                if(j==2)
                    j=1;
            }
            
            if(j*j>=i){
                d++;
                for(k=2;k*i<1000000;k++){
                    s[k*i]=-1;
                }
            }
        }
        s[i] = d;
    }
    
    while(EOF != scanf("%ld",&n)){
        printf("%ld\n",s[n]-1);
    }
    
    
    return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <set>
#include <map>
#include <cmath>
//ranker
using namespace std;

#define REPS(i, a, n) for (int i = (a); i < (n); ++i)
#define REP(i, n) REPS(i, 0, n)
#define RREP(i, n) REPS(i, 1, n + 1)
#define DEPS(i, a, n) for (int i = (a); i >= n; --i)
#define DEP(i, n) DEPS(i, n, 0)

using ll = long long;
using ull = unsigned long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using pii = pair<int, int>;
using pis = pair<int, string>;
using psi = pair<string, int>;
using D = double;


const int Max=10000000;
bool prime[Max];
int main(void){
    int in;
    for(int i=0;i<Max;i++)prime[i]=true;
    prime[1]=false;
    for(int i=2;i<Max;i++){
        if(prime[i]){
            for(int s=i*2;s<=Max;s+=i)prime[s]=false;
        }
    }
    while(cin>>in){
        int cnt=0;
        for(int i=1;i<=in;i++){
            if(prime[i])cnt++;
        }
        cout << cnt << "\n";
    }
    return 0;
}

C++: #include <iostream>
using namespace std;
int main() {
	bool isprime[1000000];
	for (int i = 0; i < 1000000; i++)
		isprime[i] = 1;
	for (int i = 2; i < 1000; i++) {
		if (!isprime[i]) continue;
		for (int j = i * i; j < 1000000; j += i)
			isprime[j] = 0;
	}
	int n;
	while (cin >> n) {
		int ans = 0;
		for (int i = 2; i <= n; i++)
			if (isprime[i])
				ans++;
		cout << ans << endl;
	}
}

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

using namespace std;

int main()
{
	vector<long long int>D(1000001 , 1);
	D[0] = 0;
	D[1] = 0;
	for(long long int i = 2; i <= sqrtl(1000001); i++)
	{
		if(D[i])
		{
			long long int j = 2;
			while(i*j < 1000001)
			{
				D[i*j] = 0;
				j++;
			}
		}
		D[i] += D[i - 1];
	}
	long long int n;
	while(cin >> n)cout << D[n] << endl;
	return 0;
}

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

int arr[1000000];

int main()
{
	int x, cnt, j, i = 3;
	arr[2] = 1;
	for (; i < 1000000; ++i) {
		cnt = 1;
		for (j = 2; j <= sqrt(i); ++j) {
			if (i%j == 0) {
				arr[i] = arr[i - 1];
				--cnt;
				break;
			}
		}
		arr[i] = arr[i - 1] + cnt;
	}
	while (cin >> x) {
		cout << arr[x] << endl;
	}
	return 0;
}

C++: #include<stdio.h>
int a[1000001];

int Func(int n) {
	if (n == 0)return 0;
	else return a[n] + Func(n - 1);
}

int main() {
	a[0] = 0;
	a[1] = 0;
	a[2] = 1;
	for (int i = 3; i <= 1000000; i++) {
		if (i % 2 == 0)a[i] = 0;
		else {
			int h = 1;
			for (int j = 3; j*j <= i && h == 1; j += 2) {
				if (i%j == 0)h = 0;
			}
			a[i] = h;
		}
	}
	int n;
	while (~scanf("%d", &n)) {
		printf("%d\n", Func(n));
	}
}

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

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

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

int N;
int array[MAX];
int count = 0;
int main(){
	cin >> N;
	
	for(int i=0; i < N; i++){
		array[i]=1;
	}
	
	for(int i=2; i < sqrt(N); i++){
		for(int j=1; i*j < N; j++){
			array[i*j] = 0; 
		}
	}
	
	for(int i=0; i < N; i++){
		if(array[i] == 1){
			count++;
		}
	}
	cout << count << endl;
	
	return 0;
}

C++: #include<iostream>

using namespace std;

int n;
int prime[1000000];
int checked[1000000];
int primesum[1000000];
int result[30];
int datasets;

int main()
{
	prime[0] = 0;
	prime[1] = 0;
	prime[2] = 1;
	primesum[0] = 0;
	primesum[1] = 0;
	primesum[2] = 1;
	
	for(int a = 3;a<1000000;a++)
	{
		if(checked[a] == 0)
		{
			prime[a] = 1;
			checked[a] = 1;
			for(int b = 2;b<a;b++)
			{
				if(a%b==0)
				{
					prime[a] = 0;
					break;
				}
			}
			if(prime[a] == 1)
			{
				for(int c = 2*a ;c<1000000;c+=a)
				{
					checked[c] = 1;
				}
			}
		}
		primesum[a] = primesum[a-1]+prime[a];
	}
	
	while(cin >> n)
	{
		cout << primesum[n] << endl;
	}

	return 0;
}

C++: #include<iostream>

using namespace std;

int main(){
	int n;
	int p[1000000] = { 1 };
	int ans[1000000] = { 0 };
	p[0] = 0;
	p[1] = 0;

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

	for (int i = 0; i < 1000000; i++){
		ans[i] += p[i];
	}

	while (cin >> n)
		cout << ans[n] << endl;

}