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 <algorithm>
#include <array>
#include <cmath>
#include <iostream>
#include <set>
#include <vector>

std::vector<std::size_t> prime_table = {2};

void make_prime_table()
{
    std::set<std::size_t> undefined_table;
    for(std::size_t i = 3; i <= 999999; ++i)
    {
        undefined_table.insert(i);
    }

    const std::size_t sn = 1000;
    for(std::size_t p = 0; !undefined_table.empty() && p < prime_table.size(); ++p)
    {
        for(auto u = undefined_table.begin(); u != undefined_table.end();)
        {
            if(0 == *u % prime_table[p])
            {
                u = undefined_table.erase(u);
            }
            else
            {
                ++u;
            }
        }

        if(!undefined_table.empty())
        {            
            prime_table.push_back(*undefined_table.begin());
            undefined_table.erase(undefined_table.begin());
            if(sn <= prime_table.back())
            {
                break;
            }
        }
    }
    prime_table.insert(prime_table.end(), undefined_table.begin(), undefined_table.end());
}

std::array<std::size_t, 1000000> prime_acc = {{0, 0}};

void make_prime_acc()
{
    std::size_t acc_found = 1;
    for(auto p = prime_table.begin(); p != prime_table.end(); ++p)
    {
        std::fill_n(prime_acc.begin() + acc_found + 1, *p - acc_found - 1, prime_acc[acc_found]);
        acc_found = *p;
        prime_acc[acc_found] = prime_acc[acc_found - 1] + 1;
    }
    std::fill(prime_acc.begin() + acc_found + 1, prime_acc.end(), prime_acc[acc_found]);
}

int main()
{
    make_prime_table();
    make_prime_acc();

    std::size_t n;

    while(std::cin >> n, !std::cin.eof())
    {
        std::cout << prime_acc[n] << std::endl;
    }
}

C++: #include <iostream>
using namespace std;
#define x 1000000
int main(){
	bool q[x];
	for(int i=2;i<x;i++) q[i] = true;
	for(int i=2;i<x;i++) if(q[i]) for(int j=2;j<=x/i;j++) q[i*j] = false;
	int n;
	while(cin >> n){
		int a = 0;
		for(int i=2;i<x;i++) if(q[i]) a++;
		cout << a << endl;
	}
}

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

int main(){
	int  n,i,j,sqrtn,sum;
	while(scanf("%d",&n)!=EOF){
		sum=(n-1)/2;
		for(i=3;i<=n;i+=2){
			sqrtn=(int)sqrt(i);
			for(j=1;j*2+1<sqrtn;j++){
				if(i%(j*2+1)==0){
					sum--;
					break;
				}
			}
		}
		printf("%d\n",sum);
		
	}
	return 0;
}

C++: #include<stdio.h>
#define size 1000000
int a[size],c[size];
void seive()   {    int i,j;
    //for(i=0;i<size;i++)    a[i]=1;    a[0]=a[1]=0;
    for(i=4;i<size;i+=2)     a[i]=1;
    for(i=3;i*i<size;i+=2)   {
        if(!a[i])   {
            for(j=i*i;j<size;j+=2*i)
                a[j]=1;  }   }    }

int main()
{
    seive();
    int i,n=0,x;
    for(i=2;i<size;i++)
    {
        if(!a[i])
        {
            n++;
        }
        c[i]=n;
    }
    while(scanf("%d",&x)==1)
    {
        printf("%d\n",c[x]);
    }
    return 0;
}

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

int main(){
	int NUM;
	while(std::cin >> NUM){
		std::list<int> numList,primeList;
		for(int i=2;i<=NUM;++i){
			numList.push_back(i);
		}
	
		for(int i=numList.front();i<std::sqrt(NUM);i=numList.front()){
			primeList.push_back(i);
			numList.erase(std::remove_if(numList.begin(),numList.end(),[&](int num){return num%i==0;}),numList.end());
		}
		int cnt=numList.size()+primeList.size();	
		std::printf("%d\n",cnt);

		}
	return 0;
}

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

int main() {
  const int N = 1000000;
  vector<bool> prime(N, true);
  prime[0] = prime[1] = false;
  for (int i = 2; i*i < N; ++i) {
    if (!prime[i]) { continue; }
    for (int j = i*i; j < N; j += i) {
      prime[j] = false;
    }
  }
  vector<int> ans(N);
  for (int i = 1; i < N; ++i) {
    ans[i] = ans[i - 1] + prime[i];
  }
  int n;
  while (cin >> n) {
    cout << ans[n] << endl;
  }
}

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

typedef long long LL;
typedef long double LD;
typedef pair<int,int> P;
typedef pair<int,P> P1;

#define fr first
#define sc second
#define mp make_pair
#define pb push_back
#define rep(i,x) for(int i=0;i<x;i++)
#define rep1(i,x) for(int i=1;i<=x;i++)
#define rrep(i,x) for(int i=x-1;i>=0;i--)
#define rrep1(i,x) for(int i=x;i>0;i--)
#define sor(v) sort(v.begin(),v.end())
#define rev(s) reverse(s.begin(),s.end())
#define lb(vec,a) lower_bound(vec.begin(),vec.end(),a)
#define ub(vec,a) upper_bound(vec.begin(),vec.end(),a)
#define uniq(vec) vec.erase(unique(vec.begin(),vec.end()),vec.end())
#define min_3(a,b,c) min(a,min(b,c))
#define max_3(a,b,c) max(a,max(b,c))
#define mp1(a,b,c) P1(a,P(b,c))
#define pque(a) priority_queue<a>
#define rpque(a) priority_queue<a,vector<a>,greater<a>>

const int INF=1000000000;
const int dre_4[4][2]={{1,0},{0,1},{-1,0},{0,-1}};
const int dre_8[8][2]={{1,0},{1,1},{0,1},{-1,1},{-1,0},{-1,-1},{0,-1},{1,-1}};
const int kaijou[10]={1,1,2,6,24,120,720,5040,40320,362880};

int main(){
	static bool prim[1000010]; rep(i,1000010)prim[i]=true;
	static int ans[1000010];
	ans[1]=0;
	
	int i=2;
	for(;i*i<=1000000;i++){
		ans[i]=ans[i-1];
		if(prim[i]){
			ans[i]++;
			for(int j=i*i;j<=1000000;j+=i){
				prim[j]=false;
			}
		}
	}
	for(;i<=1000000;i++){
		ans[i]=ans[i-1];
		if(prim[i])ans[i]++;
	}
	
	int n;
	while(~scanf("%d",&n))printf("%d\n",ans[n]);
}

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

const int MAX_N = 1000000;

vector<int> init(void){
    vector<int> prime;
    bool is_prime[MAX_N];
    memset(is_prime, true, MAX_N);
    
    is_prime[0] = is_prime[1] = false;
    for(int i=2; i<MAX_N; i++){
        if(is_prime[i]){
            prime.push_back(i);
            for(int j=2*i; j<MAX_N; j+=i) is_prime[j] = false;
        }
    }
    return prime;
}

int main(int argc, const char * argv[])
{
    int n;
    vector<int> prime = init();
    while(cin>>n){
        cout << upper_bound(prime.begin(), prime.end(), n)-prime.begin() << endl;
    }
    return 0;
}

C++: #include <iostream>

using namespace std;

typedef long long ll;

int main()
{
    int prime_num[1000001];
    memset(prime_num, 0, 1000001);
    prime_num[2] = 1;
    int ans = 1;
    for(int i = 3; i <= 1000000; i++) {
        if(i % 2 == 0) { 
            prime_num[i] = prime_num[i-1];
            continue;
        }
        bool is_prime = true;
        for(int j = 3; j*j <= i; j++) {
            if(i % j == 0) {
                is_prime = false;
                break;
            }
        }
        if(is_prime) {
            prime_num[i] = prime_num[i-1] + 1;
        } else {
            prime_num[i] = prime_num[i-1];
        }
    }
    int n;
    while(cin >> n) {
        cout << prime_num[n] << endl;
    }
    return 0;
}

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

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