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 <cstdio>
#include <utility>
#include <queue>
#include <vector>
#include <iostream>
#include <algorithm>
#include <math.h>
#include <map>
#include <string.h>

using namespace std;

int prime[999999];
bool is_prime[1000000];

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<iostream>
#include<cmath>
#define MAX 1000000
using namespace std;
int main(){
	bool num[MAX] = {false,false};
	for(int i=2;i<MAX;i++){
		num[i] = true;
	}
	for(int i=2; i<sqrt(MAX); i++){
		if(num[i]){
			for(int l=i*2;l<MAX;l+=i){
				num[l] = false;
			}
		}
	}
	int n;
	while(cin >>n){
		int ans = 0;
		for(int i=0;i<=n;i++){
			if(num[i]){ans++;}
		}
		cout <<ans<<endl;
	}
	return 0;
}

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

int main() {
	// your code goes here
	int nu[30];
	int num;
	int count = 0;
	int max = 29;
	for(int x=0;x<30;x++){
		nu [x] = -1;
		cin >> nu[x];
		if (nu[x] == -1) {
			max = x;
			break;
		}
	}
	for (int y=0;y<max;y++) {
		num = nu[y];
		count = 0;
		
		for (int i = 1;i<=num;i++) {
		if(i%2==0) {
			//cout <<"2: "<<i<<endl;
			count ++;
			continue;
		}
		for (int j=3;j<(int)sqrt(i)+1;j=j+2) {
			if (i%j==0) {
				count ++;
				//cout <<"i: "<<i<<" j: "<<j<<endl;
				break;
			}
		}
	}
	cout << (num-count) <<endl;
	//cout << endl;
	}
	return 0;
}

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

int main(){
	int n, temp, counter;
	temp = 0;
	counter= 0;
	while(cin >> n){
		vector<int> num(n);
		num[0], num[1] = 1;
		for(int i = 0; i < n; i++){
			if(num[i] == 0){
				for(int j = 0; temp < n; j++){
					temp = i * j;
					num[temp] = 1;
					counter++;
				}
			}
		}
		cout << counter << endl;
	}
	return(0);
}

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

int main()
{
  int n;
  while(cin >> n){
    double nd=(double)n;
    int root=(int)sqrt(nd);
    int count=0;
    for(int i=2;i<=n;i++){
      int j;
      for(j=2;j<=root;j++){
        if(i%j==0) break;
      }
      if(j==root) count++;
    }
    cout << count << endl;
  }
  
  return 0;
}

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

using namespace std;

int A[1000000];
int Eratosthenes(int n)
{
	int mama = 0;
	for (int i = 0; i < n; i++)
	{
		A[i] = 1;
	}
	for (int i = 2; i < sqrt(n); i++)
	{
		if (A[i])
		{
			for (int j = 0; i*(j + 2) < n; j++)
			{
				A[i*(j + 2)] = 0;
			}
		}
	}

	for (int i = 2; i < n; i++)
	{
		if (A[i])
		{
			mama++;
		}
	}
	return mama;
}

int main()
{
	int n;
	cin >> n;
	cout << Eratosthenes(n) << endl;
	return 0;
}

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

int main(void) {
    int N=999999;
    int p[N] , s[N];
    p[0]=0;
    for (int i=1; i<N; i++){
        p[i] = 1;
        s[i] = 0;
    }
    for (int i=1; i<N; i++){
        cout << i;
        if (p[i]==1){
            int n=i+1 , m=2*n;
            while (m<=N){
                p[m-1] = 0;
                m += n;
            }
        }
    }
    
    for (int i=1; i<N; i++){
        s[i] = s[i-1]+p[i];
    }
    int n;
    while (1){
        cin >> n;
        cout << s[n-1] << endl;
    }
    return 0;
}

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

using namespace std;
using ull = unsigned long long;
using ll = long long;
using lli = long long int;
using ld = long double;
using pa = pair<int,int>;
using ppa = pair<pa,int>;
#define SORT(v, n) sort(v, v+n);
#define ALL(v) v.begin(),v.end()
#define VSORT(v) sort(ALL(v))
#define GRESORT(v) sort(ALL(v),greater<ll>())
#define REVERSE(v) reverse(ALL(v))
#define overlap(v) v.erase(unique(ALL(v)),v.end())
#define debug(x) cout << #x << ": " << x << endl
#define FOR(i,a,b) for(int i = (a); i < (b); i++)
#define rep(i,n) FOR(i,0,n)
#define RFOR(i,a,b) for(int i = (b-1); i >= a; i--)
#define rrep(i,n) RFOR(i,0,n)
#define INF INT_MAX
#define SUP INT_MIN
#define mins(a,b) a = min(a,b)
#define maxs(a,b) a = max(a,b)

//4??????
//int dy[]={0, 1, 0, -1};
//int dx[]={1, 0, -1, 0};

//8??????
//int dy[]={0,0,1,-1,1,1,-1,-1};
//int dx[]={1,-1,0,0,1,-1,1,-1};

const ll MOD = 1000000007;

ll pow_mod(ll a,ll b,ll mod){
    ll tmp = 1;
    a %= mod;
    if(b > 0){
        tmp = pow_mod(a, b/2, mod);
        if(b % 2) tmp = (((tmp*tmp)%mod)*a)%mod;
        else tmp = (tmp*tmp)%mod;
    }
    return tmp;
}

int mm = 999999+1;
vector<bool> d(mm,true);

void hoge(){ FOR(i,2,sqrt(mm)) for(int j = 2; i*j <= mm; j++) d[i*j] = false; }

void solve() {
    vector<int> dp(mm,INF);
    int val;
    int m = 2;
    dp[m] = 1;
    d[0] = d[1] = dp[0] = dp[1] = 0;
    hoge();
    while(cin >> val){
        if(dp[val] == INF){
            FOR(i,m+1,val+1){
                dp[i] = dp[i-1] + (d[i] ? 1 : 0);
            }
            m = val;
        }
        cout << dp[val] << endl;
    }
}

int main(){
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);

    solve();

    return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <iostream>
#include <math.h>

using namespace std; 

int list[1000000]={2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73,
79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163,
 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251,
 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349,
 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443,
 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541, 547, 557,
 563, 569, 571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643, 647,
 653, 659, 661, 673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751, 757,
 761, 769, 773, 787, 797, 809, 811, 821, 823, 827, 829, 839, 853, 857, 859, 863,
 877, 881, 883, 887, 907, 911, 919, 929, 937, 941, 947, 953, 967, 971, 977, 983,
 991, 997, 1009, 1013, 1019, 1021, 1031, 1033, 1039, 1049, 1051, 1061, 1063, 1069};

long int a,b,loop,hasil,input,mid;
long int lo;
long int hi;

 bool Prima(unsigned long n) 
{
    for (unsigned short i = 0; list[i]*list[i] <= n; i++) 
	{
        if (n % list[i] == 0) 
		{
            return false;
        }
    }
 
    return true;
}

int main()
{
	lo=1087;
	a=180;
	while(lo<=1000000)
	{
		if(Prima(lo))
		{
			list[a]=lo;
			a++;
		}
		lo+=2;
	}
	while(scanf("%ld",&input)!= EOF)
	{
		
	lo=0;
	hi=78498;
	/*while(lo<=hi)
        {
            mid=(lo+hi)/2;
            if (list[mid]<input)
            {
                lo=mid+1; 
            } 
            else if(list[mid]>input)
            {
                hi=mid-1; 
            }
            else 
            {
                break; 
            } 
        }*/
	while(input>=list[lo])
	{
		lo++;
	}
	printf("%d\n",lo);
	}
	return 0; 
}

C++: #include<stdio.h>



int main(){
	int n,i,j;
	bool c[100000];
	int data[100000];
	int count;
	int premaxin=1;


	for(i=0;i<100000;i++){
		c[i]=1;
		data[i]=0;
	}
	c[0]=c[1]=0;

	while((scanf("%d",&n)!=EOF)&&(n!=0)){

		if(premaxin<n){
			for(i=0;i<=n;i++){
				if(c[i]==1){
					if(premaxin>i){
						j=premaxin;
					}else{
						j=i+1;
					}
					for(;j<=n;j++){
						if(j%i==0){
							c[j]=0;
						}
					}
				}
			}

			count=data[premaxin];
			for(i=premaxin+1;i<=n;i++){

				if(c[i]==1){
					count++;
				}
				data[i]=count;
			}
		}

		printf("%d",data[n]);
	}
	return 0;
}