Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		input := nextInt()
		fmt.Println(IsPrime(input))
	}
}

func IsPrime(num int) int {
	eratos := make([]bool, num+1)
	c := 0
	for i := 2; i < len(eratos); i++ {
		if eratos[i] == false {
			c++
			eratosCalc(i, eratos)
		}
	}
	return c
}

func eratosCalc(primeNum int, eratos []bool) {
	for i := primeNum * primeNum; i < len(eratos); i += primeNum {
		eratos[i] = true
	}
}

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

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

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++)
	{
		prime[a] = 1;
	}

	for (int a = 3; a<1001; a++)
	{
		if (checked[a] == 0)
		{
			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;
					prime[c] = 0;
				}
			}
		}
	}

	for (int a = 3; a<100; a++)
	{
		primesum[a] = primesum[a - 1] + prime[a];
		cout << "prime[" << a << "] = " << prime[a] << endl;
		cout << "primesum[" << a << "] = " << primesum[a] << endl;
	}

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

	return 0;
}

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

using namespace std;

int sosuu(int n);

int main(){
    int n=0;
    int num=0;
    int a[30][2]={0};
    int max=0;
    while(scanf("%d",&n)!=EOF){
        if(max<n){
            max=n;
        }
        a[num][0]=n;
        num++;
    }
    
    int b[max];
    int c[max];
    int so=0;
    for(int i=0; i<=max; i++){
        b[i]=i;
    }
    b[1]=0;
    
    for(int i=2; i<sqrt(max); i++){
        if(b[i]!=0){
            for(int j=i+1; j<=max; j++){
                if(j%i==0){
                    b[j]=0;
                }
        }
        }
    }
    
    for(int i=0; i<=max; i++){
        if(b[i]!=0){
            c[so]=i;
            so++;
        }
    }
    
    
//    for(int j=0; j<num; j++){
//        cout<<a[j][0]<<"\n";
//    }
//    
//    for(int j=0; j<=so; j++){
//        cout<<c[j]<<"\n";
//    }
    
    
    
    
    for(int i=0; i<num; i++){
        for(int j=0; j<so; j++){
            if(c[j]<=a[i][0]){
                a[i][1]++;
            }else{
                j=so;
            }
        }
    }

    
    for(int j=0; j<num; j++){
        cout<<a[j][1]<<"\n";
    }
    
}

C++: #include <iostream>
#include <string>
#include <stack>
#include <math.h>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <vector>

using namespace std;






int main()
{
	int n[30];
	int count;
	bool prime[1000000];
	for (int i = 0; i < 30; i++) {
		cin >> n[i];
		count = 0;
		
		for (int i = 0; i <= n[i]; i++) {
			prime[i] = true;
		}
		prime[0] = prime[1] = false;
		for (int j = 2; j <= n[i]; j++) {
			if (prime[j]) {
				for (int k = 2 * j; k <= n[i]; k += j) {
					prime[k] = false;
				}
			}
		}

		for (int j = 1; j <= n[i]; j++) {
			if (prime[j]) count++;
		}
		cout << count << endl;


	}
	

}

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

int main(){
	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 n_prime[1000000];
	int cnt = 0; 
	for(int i = 0; i < 1000000; i++){
		if(is_prime[i]){
			cnt++;
		}
		n_prime[i] = cnt;
	}
	while(true){
		int n;
		cin >> n;
		if(cin.eof()){
			break;
		}
		cout << n_prime[n] << endl;
	}
	return 0;
}

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

using namespace std;

int main()
{
	int i,j;
	char array[1000010];
	
	for(i=0;i<1000001;i++)
		array[i]='1';
	
	array[0]=array[1]='0';
	
	for(i=0;i<1000001;i++)
	{
		if(array[i]=='1')
		{
			for(j=2;i*j<1000001;j++)
				array[i*j]='0';
		}
	}
	
	vector<int> ans;
	
	for(i=0;i<1000001;i++)
	{
		ans.push_back(0);
	}
	for(i=1;i<1000001;i++)
	{
		ans[i]+=ans[i-1];
		ans[i]+=(int)array[i]-48;
	}
	
	int inp;
	
	cin >> inp;
	cout << ans[inp] << endl;
}

C++: #include <iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
const int maxn=10000000+5;
int prime[maxn];
bool p[maxn];
int cnt=0;
void is_prime()
{
    memset(p,true,sizeof(p));
    p[0]=p[1]=false;
    for(int i=2;i<=maxn;i++)
        if(p[i]) {
            prime[cnt++]=i;
            for(int j=i*i;j<=maxn;j+=i)
                 p[j]=false;
        }
}
int main()
{
    is_prime();
    int n;
    while(scanf("%d",&n)==1)
    {
        int k=lower_bound(prime,prime+cnt,n)-prime;
        if(prime[k]==n) k++;
        printf("%d\n",k);
    }
    return 0;
}

C++: /*#include <cstdio>
#define MAX 1000

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;
}*/

#include <cstdio>

int main(){
	int n;
	static bool c[1000000]={true};
	static int t[1000000];t[0] = 0;
	for(int i = 2;i < 1000000;i++){
		for(int j = 2;i*j < 1000000;j++){
			c[i*j-1] = true;
		}
	}
	for(int i = 1;i < 1000000;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;
 
bool IsPrime(int n){
    for(int i=2;i*i<=n;i++){
        if(n%i == 0){return false;}
    }
    return true;
}
 
int main(void) {
	int n;
 while(cin >> n){	
    int count = 1;
    
    for(int i=3;i<=n;i=i+2){
        if(IsPrime(i)) count++;
    }
    cout << count << endl;
    count = 0;
}

    return 0;
}

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

int count_prime( unsigned int const& n )
{
    if ( n <= 0 || 1000000 <= n ) return -1;
    std::vector<bool> memo_prime( n+1, true );
    for ( unsigned int i = 2; i < n; ++i )
    {
        for ( unsigned int j = 2; i*j <= n; ++j )
        {
            memo_prime.at(i*j) = false;
        }
    }
    memo_prime.at(0) = memo_prime.at(1) = false;
    memo_prime.at(2) = true;
    return std::count( memo_prime.begin(), memo_prime.end(), true );
}

int main()
{
    int n;
    while( std::cin >> n)
        std::cout << count_prime(n) << std::endl;
    return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<cmath>
using namespace std;
#define N 100000

int main() {
	int p[N] = {}, i, j,n,cnt;
	for (i = 2; i <= sqrt(N); i++) if (!p[i]) for (j = i*i; j < N; j += i)p[j] = 1;
	while (cin >> n) {
		cnt = 0;
		for (i = 2; i <= n; i++) if (!p[i]) cnt++;
		cout << cnt << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <vector>
//#include <math.h>
//#include <string>
#include <cmath>
#include <iomanip>

int main();
int count_prime_number(int n);

int main(){
	int input;
	std::vector<int> answer;
	
	
	//for (int i = 0; i < 3; i++){
		//std::cin >> input;
	while (std::cin >> input){
		answer.push_back(count_prime_number(input));
	}

	for (int i = 0; i < answer.size(); i++){
		std::cout << answer[i] << std::endl;
	}

	//while (1);
	return 0;
}

/**
* nまでの素数の個数を出力する
* args:数値n
* return:素数の個数
* 参考:http://tsg.ne.jp/TT/tsg/c/mansaku/P01.html
*/
int count_prime_number(int n){
	int rtn = 0;

	//2だけ個別チェック
	if (n >= 2){
		rtn++;
	}

	for (int i = 3; i <= n; i += 2)
	{
		bool is_prime_num = true;
		for (int j = 3; j <= sqrt(i); j += 2)		//ルートi以上のiの約数はルートi以下のiの約数と対になっているので範囲はルートiまででいい
		{
			if (i % j == 0)
			{
				is_prime_num = false;
				break;
			}
		}

		if (is_prime_num){
			rtn++;

			//デバッグ
			//std::cout << i << std::endl;
		}
	}

	return rtn;
}