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>
using namespace std;

int main()
{
	int input;
	int i, j, k, cnt;
	bool fg;
	for(i = 0; i < 30; i++)
	{
		cin >> input;
		cnt = 0;

		for(k = 2; k <= input; k++)
		{
			fg = false;
			for(j = 2; j <= sqrt(k); j++)
			{
				if(k % j == 0)
				{
					fg = true;
					break;
				}
			}
			if(!fg) cnt++;
		}
		cout << cnt << endl;
	}
}

C++: #include <cstdio>
#include <cmath>
#define MAX_N 1000000

int main(){
    bool p[MAX_N+1];    // p[NUM] : NUM is Prime?
    int n;

    for(int i = 0; i <= MAX_N; i++) p[i] = true;

    for(int i = 1; i <= MAX_N; i++){
        if(i == 1) p[i] = false;
        else{
            if(p[i]){
                for(int j = 2; i * j <= MAX_N; j++){
                   p[i*j] = false;
                }
            }
        }
    }
 
    // for(int i = 0; i <= MAX_N; i++) if(p[i]) printf("%d\n",i);
    
    while(scanf("%d", &n) == 1){
        int count = 0;
        for(int i = 1; i <= n; i++){
            if(p[i])count++;
        }
        printf("%d\n", count);
    }
    return 0;
}

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

bool is_prime(int n){
for(int i=2;i*i<=n;i++){
if(n % i == 0) return false;
}
return n!=1;
}

int main(){
int n;

while(~scanf("%d",&n)){
int* table =  new int[1000000];
fill((int*)table,(int*)table+sizeof(table),1);
table[0]=0;

for(int i=1;i<=n;i++){
if(!is_prime(i)) {
for(int j=i;j<=n;j*=i){
table[j] = 0;
}
}
}
printf("%d\n",count(table,table+1000000,1));
}
}

C++: #include <iostream>

#define N 1000000

using namespace std;

int main() {
	bool prime[N];
	prime[0] = prime[1] = false;

	//initialize
	for(int i=2; i<N; i++) prime[i] = true;
	//Sieve of Eratosthenes
	for(int i=2; i<=N/2; i++){
		if(prime[i]){
			for(int j=2; i*j<=N; j++)
			{
				prime[i*j] = false;
			}
		}
	}

	int countPrime[N];
	int cnt = 0;
	for(int i=0; i<N; i++){
		if(prime[i]){
			cnt++;
		}
		countPrime[i] = cnt;
	}

	int input;
	while(true) {
		cin>>input;
		if(cin.eof()){break;}
		cout<<countPrime[input]<<endl;
	}
	return 0;
}

C++: #include <iostream>

using namespace std;

int main()
{
	unsigned int  n;
	cin >> n;
	unsigned int num, count = 0;
	for(int i = 0; i < n; i++)
	{
		cin >> num;
		for(int j = 2; j <=num; j++)
		{
			if(num == j)
                        {
                                count++;
                                break;
                        }
			if(num % j == 0)
			{
				break;
			}
		}
	}
	cout << count << endl;
	return 0;
}

C++: #include<stdio.h>

int	main(){
	int	i,	k,	x,	cnt,	data[1000000];
	for(i	=	2;	i	<=	1000000;	i++){
		data[i]	=	1;
	}
	for(i	=	2;	i	*	i	<=	1000000;	i++){
		if(data[i]){
			for(k	=	2	*	i;	k	<=	1000000;	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 <stdio.h>
#include <vector>
#include <algorithm>
using namespace std;

const int size=1000000;
vector<int> map(size,1);
void sieve(int p)
{
    for(int j=p*2;j<size;j+=p)
    {
        map[j]=0;
    }
}
int main(void)
{
    int n;
    scanf("%d",&n);
    map[0]=0;
    map[1]=0;
    sieve(2);
    for(int i=7;i<1000;i+=6)
    {
        sieve(i);
        sieve(i+4);
    }
    printf("%d\n",count(map.begin(),map.begin()+n+1,1));
    return 0;
}

C++: #include <cstdio>
#define max_n 999999
int prime[max_n];
bool is_prime[max_n+1];
int main(){
    for(int i=0; i<max_n; i++){
        is_prime[i]=true;
    }
    int p = 0;
    is_prime[1]=false;
    is_prime[0]=false;
    for(int i=2;i<max_n;i++){
        if(is_prime[i]){
            prime[p++] = i;
            for(int j=2*i;j<max_n;j+=i)is_prime[j]=false;
        }
    }
    int n;
    while(scanf("%d",&n)!=EOF){
        
        
        int ans = 0;
        for(int i =0; i<max_n; i++){
            if(prime[i]>n){
                ans = i;
                break;
            }
        }
        printf("%d\n",ans);
    }
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

#define REP(i, s, n) for (int i = s; i < n; ++i)
#define rep(i, n) REP(i, 0, n)
#define SORT(c) sort((c).begin(), (c).end())
#define IINF INT_MAX
#define LLINF LLONG_MAX
#define DEBUG false

int main()
{
    vector<int> prime;
    prime.push_back(2);
    for (long long int i = 3; i < 999999; i++)
    {
        double temp = sqrt(i);
        for (int j = 0;; j++)
        {
            if (temp < prime[j])
            {
                prime.push_back(i);
                break;
            }
            if (i % prime[j] == 0)
            {
                break; //this is not prime
            }
        }
    }
    int n;
    int i;
    while (cin >> n)
    {
        for (i = 0; i<prime.size()&&prime[i] <= n; i++);
        cout << i << endl;
    }
    return 0;
}

C++: #include<cstdio>
#include<cstring>
int main(){
 int n,cnt=0;
 scanf("%d",&n);
 int a[n+1];
 memset(a,-1,sizeof(int)*(n+1));
 for(int i=2;i<=n;i++){
 if(a[i]==-1){ cnt++;
 for(int j=i;j<=n;j=j+i) a[j]=0;}
}
printf("%d",cnt);
return 0;
}

C++: #include<stdio.h>
int flg[1000000];
int main(void)
{
    int n,i,j,x;
	
    while(scanf("%d",&n)!=EOF){
		for(i=2;i<=n;i++)	{
			flg[i]=1;
		}
		for(i=2;i*i<=n;i++){
			if(flg[i]==1){
				for(j=2;j*i<=n;j++){
					flg[i*j]=0;
				}
			}
		}
		x=0;
		for(i=2;i<=n;i++)	{
			if(flg[i]==1){
				x++;
			}
		}
		printf("%d\n",x);
	}
	return 0;
}

C++: #include<iostream>
#include<cmath>
using namespace std;
#define MAX 100

bool prime[MAX]={};

void run(){
  for(int i=2;i<MAX;i++){
    prime[i]=true;
  } 

  for(int i=2;i<MAX;i++){
    if(!prime[i])continue;
    for(int j=2;j*i<MAX;j++){//i*1 is prime number
      prime[i*j]=false;
    }
  }
  return;
}

int main(){

  run();
  
  int n;
  while(cin>>n){
    int sum=0;
    for(int i=2; i<=n; i++){
      if(prime[i])sum++;
    }
    cout<<sum<<endl;
  }

}