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 || num == 5:
		return true
	case num < 2 || num%2 == 0 || num%5 == 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<stdio.h>
#include<iostream>
#include<vector>
#include<string.h>
using namespace std;
const int N=1000000;
vector<int> prime;
bool arr[N];
void init(){
memset(arr,true,sizeof(arr));
arr[0]=arr[1]=false;
for(int i=2;i<N;++i){
    if(arr[i])
        prime.push_back(i);
    for(int j=0;j<prime.size()&&i*prime[j]<N;j++)
    {
        arr[i*prime[j]]=false;
        if(i%prime[j]==0)
            break;
    }
}
}
int main(){
int n;
init();
while(scanf("%d",&n)!=EOF){
    int j=0;
    for(;prime[j]<=n;j++);
    printf("%d\n",j);
}
}

C++: #include <iostream>
#define MAX_N 1000000
using namespace std;

int prime[MAX_N];
bool is_prime[MAX_N+1];

int sieve(int n) {
  int p = 0;
  for(int i=0;i<=n;i++) {
    is_prime[i] = true;
  }
  is_prime[0] = is_prime[1] = false;
  for(int i=2;i <= n;i++) {
    if(is_prime[i]) {
      prime[p++] = i;
      for(int j=2 * i;j<= n;j += i) is_prime[j] = false;
    }
  }
  return p;
}

int main() {
  int n;
  while(cin >> n) {
  cout << sieve(n) << endl;
  }
}

C++: #include<stdio.h>
#define MAX 1000010

int main(){
	bool prime[MAX];
	prime[0]=prime[1]=0;
	for(int i=0;i<MAX;i++){
		prime[i]=1;
	}
	prime[0]=prime[1]=0;
	for(int i=2;i<MAX;i++){
		if(prime[i]==1){
			for(int j=2;i*j<MAX;j++){
				prime[i*j]=0;
			}
		}
	}
	int prime_under_num[MAX];
	prime_under_num[0]=0;
	for(int i=1;i<MAX;i++){
		prime_under_num[i]=prime_under_num[i-1]+prime[i];
	}
	
	int n;
	while((scanf("%d",&n))!=EOF){
		printf("%d\n",prime_under_num[n]);
	}
	
	
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <cstdio>
#include <string>
#define rep(x,to) for(x=0;x<to;x++)
#define rep2(x,from,to) for(x=from;x<to;x++)
#define num  1000000

using namespace std;




int main(void){

int innum,i;

char Find[num]; // 探索リスト
	for(int i = 2; i <= num; i++){
		Find[i] = '0';
		if(i%2==0) Find[i] = '1';
	}
	for(int i = 2; i*i < num; i++){
    	if(Find[i] > '0') continue; // すでに除いた数は飛ばす
    	for(int j = i * 2; j< num; j += i) {
        	if(Find[j]=='0') Find[j] = '1';
    	}
	}
	Find[2]='0';
	while(cin >> innum){
		int cnt=0;
		rep(i,innum+1) if(Find[i]=='0') { 
		//	cout << i << " ";
			cnt++;
		}

		cout << cnt <<endl;
	}
	return 0;
}

C++: #include<bits//stdc++.h>
using namespace std;
int main(){
	int a[1000000];
	a[0]=2;
	int z=1;
	for(int i=2;i<1000000;){
		for(int j=0;j<z;j++){
			if(i%a[j]==0)goto stop;
		}
		a[z]=i;
		z++;
		stop:;
		if(i==2) i++;
		else i+=2;
	}
	int b;
	while(cin>>b){
		int c=0;
		for(int i=0;i<z;i++){
			if(a[i]>b)break;
			else c++;
		}
		cout<<c<<endl;
	}
}

C++: #include<iostream>
#include<algorithm>
#include<vector>
#include<cmath>
#include<cstdio>
#include<string>
#include<cmath>
typedef long long int ll;


using namespace std;
bool priN[1000000]={};
bool pri[1000000]={};

bool isPrime(int n){
	if(priN[n]==true)return pri[n];
	for(int i=3;i<=sqrt((double)n);i++){
		if(n%i==0){
			priN[n]=true;
			pri[n]=false;
			return false;
		}
	}
	priN[n]=true;
	pri[n]=true;
	return true;
}

int main(){
	while(1){
		int n;
		if( !(cin>>n) )break;
		ll sum=1;
		if n<2 sum--;
		for(int i=3;i<=n;i+=2){
			if(isPrime(i))sum++;
		}
		cout<<sum<<endl;
	}
	return 0;
}

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

using namespace std;

bool isprime(int N)
{
	for (int I = 2; I <= sqrt(N); I++)
	{
		if (N % I == 0) return false;
	}

	return true;
}

int main()
{
	int c = 0;

	int n;

	if (cin >> n)
	{
		for (int i = 2; i <= n; i++)
		{
			if (isprime(i) == true)
			{
				c += 1;
			}
		}
	}

	cout << c << endl;

	return 0;
}

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

#define Max 1000000

int isPrime[Max];

int main(){
    int num;
    int i,j;

    //配列の初期化
    isPrime[0] = isPrime[1] = 0;
    for(i = 2; i < Max; ++i){
        isPrime[i] = 1;
    }

    for(i = 2; i < Max; i++){
        if(isPrime[i] == 1){
            if(i % 2 == 0){
                isPrime[i] = 0;
            }else if(i % 3 == 0){
                isPrime[i] = 0;
            }else if(i % 5 == 0){
                isPrime[i] = 0;
            }else if(i % 7 == 0){
                isPrime[i] = 0;
            }
        }
    }

    isPrime[2] = 1;isPrime[3] = 1;isPrime[5] = 1;isPrime[7] = 1;

    while(cin >> num){
        int count = 0;
        for(i = 2; i <= num; ++i){
            if(isPrime[i] == 1){
                count++;
            }
        }
        cout << count << endl;
    }
}

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

int main(){
bool unti[1000000];
fill(unti,unti+1000000,0);
for(int n = 2;n < 1000000;n++)
{
if(unti[n] == 0)
{
for(int a = 2;(a * n) < 1000000;a++)
{
	unti[a * n]++;
}
}
}
int b;
while(cin >> b )
{
int cnt = (count(unti,unti+b+1,0) - 2);
cout << cnt ;
	}
return 0;
}

C++: #include<cstdio>
#include<iostream>
using namespace std;
int main(){ 
    int n;
    int prime[1000000];
    while(cin>>n){
    for(int i=0;i<1000000;i++)prime[i]=0;
    for(int i=2;i<=n;i++){ 
        for(int j=2;i*j<=n;j++){ 
            prime[i*j]=1;
        }
    }
    int ans=0;
    for(int i=2;i<=n;i++){
        if(prime[i]==0)ans++;
    }
    printf("%d\n",ans);
    }
}

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

#define pb push_back

using namespace std;

const int MAX=100000;
int main(void)
{
	int x;
	bool p[MAX];
	int cnt;
	int i1,i2;

	fill(p,p+MAX,false);

	for(i1=2;i1<MAX;i1++){
		for(i2=2;i2*i2<=i1;i2++){
			if(i1%i2==0) break;
		}
		if(i2*i2>i1) p[i1]=true;
	}

	while(1){
		cin>>x;
		if(cin.eof()) break;

		cnt=0;
		for(i1=0;i1<=x;i1++){
			if(p[i1]) cnt++;
		}
		cout<<cnt<<endl;
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
int main(void)
{
    int n;
    while (cin >> n)
    {
        int c = 0;
        for (int i = 2; i <= n; i++)
        {
            int t = 1;
            for (int j = 2; j < i; j++)
            {
                if (i % j == 0)
                {
                    t++;
                }
            }
            if (t == 1)
            {
                c++;
            }
        }
        cout << c << endl;
    }
}

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

int main() {
    long in, out;
    int p, count;
    
    do{
         cin >> in;
         out = 0; 
         
         for(int i = 1; i <= in; i++)
         {
                 count = 0;
                 for(p = 1; p <= i; p++)
                 {
                       if(i % p == 0){
                            count++;
                            }
                 }
                 
                 if(count == 2){
                          out++;
                 }
         }
         
         cout << out << endl;
         
    }while(in);  
    

    return 0;
}

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

using namespace std;

#define MAX_N 99999
#define MAX_D 30

bool prime_check(int n, int k = 2)
{
	if (n < k * k) { return true; }

	if (n % k == 0) { return false; }

	return prime_check(n, k + 1);
}

int main()
{
	int n;

	bool isprime[MAX_N + 1];

	for (int i = 2; i < MAX_N + 1; i++)
	{
		isprime[i] = prime_check(i);
	}

	while (cin >> n)
	{
		cout << count(isprime + 2, isprime + n + 1, true) << endl;
	}

	return 0;
}