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 <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<stdio.h>
const int MAX_V=1000000;
int prime[MAX_V+1];
int sum[MAX_V+2];
int main(){
  int i,k,n;
  for(i=2;i<=MAX_V;i++){
    prime[i]=1;
  }
  for(i=2;i*i<=MAX_V;i++){
    if(prime[i]){
      for(k=2*i;k<=MAX_V;k+=i){
        prime[k]=0;
      }
    }
  }
  for(i=0;i<MAX_V+1;i++){
    sum[i+1]+=sum[i]+prime[i];
  }
  while(scanf("%d",&n)!=EOF){
    printf("%d\n",sum[n+1]);
  }
  return 0;
}

C++: #include <bits/stdc++.h>
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define RFOR(i,a,b) for(int i=(b) - 1;i>=(a);i--)
#define REP(i,n) for(int i=0;i<(n);i++)
#define RREP(i,n) for(int i=n-1;i>=0;i--)

#define PB push_back
#define INF INT_MAX/3
#define ALL(a) (a).begin(),(a).end()
#define CLR(a) memset(a,0,sizeof(a))

typedef long long int ll;

using namespace std;

int main(){
    vector<int> primes;
    primes.PB(2);
    FOR(i,3,1000000){
	bool flg = true;
	for(int j=0;primes[j]*primes[j]<=i;j++){
	    if(i % primes[j] == 0 ) {
	    flg = false;
	    break;
	    }
	}
	if(flg) primes.PB(i);
	i++;
    }

    int n;
    while(true){
	cin >> n;
	if(cin.eof())break;
	int res = upper_bound(ALL(primes),n) - primes.begin();
	cout << res << endl;
    }

    return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <algorithm>
#include <functional>

using namespace std;

#define MAXN 1000016

int isprime[MAXN];

void sieve(int n)
{
	int i, j, num, p = 0;
	fill(isprime, isprime + MAXN + 1, 1);
	for (i = 2, num = 1; i <= n; i++)
	{
		if (isprime[i])
		{
			isprime[i] = num++;
			for (j = 2 * i; j <= n; j += i)
				isprime[j] = 0;
		}
	}
}

int main()
{
	int n;
	sieve(MAXN);
	while (scanf("%d", &n) != EOF)
	{
		if (n == 1)
		{
			printf("0\n");
			continue;
		}
		while (!isprime[n])
		{
			n--;
		}
		printf("%d\n", isprime[n]);
	}
	return 0;
}

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

long length;
long prime[1000000];
bool isPrime(long n)
{
    bool isprimep = true;
    for(int i=0; i < length-1; i++)
        {
            if (!(n % prime[i]))
                {
                    isprimep = false;
                    break;
                }
        }
    return isprimep;
}

int main(void)
{
    long num;

    while(cin >> num)
    {
        length = 0;
        for (int n = 2; n <= num; n++)
        {
            if (isPrime(n))
            {
                prime[length] = n;
                length++;
            }
        }
        cout << length << endl;
    }
    return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<set>
#include<map>
#include<vector>
#include<queue>
#include<cstring>
#include<cmath>
#include<cstdio>
#include<cstdlib>
using namespace std;
#define REP(i,a,n) for(int i = a ; i < n ; i++)
#define rep(i,n) REP(i,0,n)

typedef long long ll;

const int N = 1000000;  //MAXPRIME
const int M = 1000; //SQRT(N)
const int K = 600000;   //NUMBER OF PRIMES, CHOOSE 9/8 * N / LOG(N)

vector<int> iterativeSieve(){
  static int p[K],table[M];
  REP(i,2,M) p[i] = i;
  for(int i=2;i*i<M;i++)
    if(p[i])
      for(int j=i*i;j<M;j+=i)
        p[j] = 0;
  p[0] = p[1] = 0;

  int num = remove(p,p+M,0) - p;
  for(int m=M;m<N;m+=M){
    for(int x=m;x<m+M;x++)
      table[x-m] = x;
    for(int i=0,j;p[i]*p[i]<m+M;i++){
      if(p[i] >= m) j = p[i] * p[i];
      else if(m % p[i] == 0) j = m;
      else j = m - (m % p[i]) + p[i];
      for(;j<m+M;j+=p[i]) table[j-m] = 0;
    }
    num = remove_copy(table,table+M,p+num,0) - p;
  }
  return vector<int>(p,p+num);
}

int main(){
  vector<int> p = iterativeSieve();
  int n;
  while(cin>>n){
    cout<<upper_bound(p.begin(),p.end(),n) - p.begin()<<endl;
  }
}

C++: #pragma  warning (disable:4996)
#include<iostream>
#include<cstdio>
#include<queue>
#include<cstring>
#include<string>
#include<cmath>
#include<algorithm>
#define MAX 1000000
using namespace std;
int amount[MAX];
int vis[MAX];
int prime[MAX];
int main(){
	//memset(vis, 0, sizeof(vis));
	memset(prime, 1, sizeof(prime));
	prime[0] = prime[1] = 0;
	for (int i = 2; i*i<= MAX; i++){
		if (prime[i]){
			
			for (int j = i *i; j <= MAX; j += i){
				//vis[j] = 1;
				prime[j] = 0;
			}
		}
	}
	
	int n, m;
	while (cin >> n>>m){
		if (n==m) cout <<0 << endl;
		else {

		}
	}
	return 0;
}

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

template <class T> int streamvar(std::istream& in, T& var)
{
    if (!in) {
        cerr << "invalid istream" << endl;
        return 1;
    }
    in >> var;
    if (in.fail()) {
        cerr << "no var" << endl;
        return 1;
    }
    return 0;
}
template <class T> int streamvec(std::istream& in, std::vector<T> &v)
{
    v.clear();
    string s;

    if (!in) {
        cerr << "invalid istream" << endl;
        return 1;
    }
    getline(in, s);
    if (in.fail()) {
        cerr << "no line" << endl;
        return 1;
    }
    istringstream is(s);
    while (1) {
        T tmp;
        is >> tmp;
        if (is.fail())
            break;
        v.push_back(tmp);
    }
    return 0;
}

#define NUM 1000000
int main(void)
{
    bool prime[NUM];
    int array[NUM] = {};
    for (int i = 0; i < NUM; i++) 
        prime[i] = 1;
    prime[0] = prime[1] = 0;

    for (int i = 0; i < NUM; i++) {
        if (prime[i]) {
            for (int j = i*2; j < NUM; j+=i) {
                prime[j] = 0;
            }
        }
    }
    array[0] = 0;
    for (int i = 1; i < NUM; i++) 
        array[i] = array[i-1] + prime[i];

    int n;
    while (cin >> n) {
        cout << array[n] << endl;
    }
    return 0;
}

C++: #include <iostream>
#include <cmath>
#include <algorithm>
#include <map>
#include <string>
#include <vector>
#define FOR(i, a, b)  for(int i=a; i<b; ++i)
#define rFOR(i, a, b) for(int i=a-1; i>=b; --i)
#define REP(i, n) for(int i=0; i<n; ++i)
#define rREP(i, n) for(int i=n-1; i>=0; --i)
#define all(a)  a.begin(), a.end()
using namespace std;

int main(){
  int n;
  while(cin >> n){
    int ans=0;
    vector<int> vec;
    vec.push_back(2);
    for(int i = 3; i<=n; i+=2){
      bool isPrimeN = true;
      REP(j, vec.size()){
        if(i%vec[j]==0){
           false;
        }
      }
      for(int j=3; j<=floor(sqrt(i)); j++){
        if(i%j==0){
          isPrimeN = false;
          break;
        }
      }
      if(isPrimeN){
        vec.push_back(i);
      }
    }
    cout << vec.size() << endl;;
  }
  return 0;
}

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

int main()
{
   int n,count=0,m=0,i,j;
   
    while(scanf("%d",&n)!=EOF){
        for(i=2;i<=n;i++){
            for(j=1;j<i;j++){
                if(i%j==0)count++;
            }
            if(count==1)m++;
            count=0;
        }
        printf("%d\n",m);
        m=0;
    }

                return 0;
}