Go: package main

// https://ja.wikipedia.org/wiki/%E3%82%A8%E3%83%A9%E3%83%88%E3%82%B9%E3%83%86%E3%83%8D%E3%82%B9%E3%81%AE%E7%AF%A9

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() (int, error) {
	return strconv.Atoi(nextString())
}

func main() {
	scanner.Split(bufio.ScanWords)

	for {
		n, err := nextInt()
		if err != nil {
			break
		}
		maybePrime := make([]bool, n+1)
		for i := 0; i <= n; i++ {
			maybePrime[i] = true
		}

		maybePrime[0] = false
		maybePrime[1] = false
		for i := 2; i <= int(math.Sqrt(float64(n)))+1; i++ {
			j := i * 2
			for j <= n {
				maybePrime[j] = false
				j += i
			}
		}

		nPrimes := 0
		for _, p := range maybePrime {
			if p {
				nPrimes++
			}
		}
		// fmt.Println(maybePrime)
		fmt.Println(nPrimes)
	}
}

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

using namespace std;

typedef long long ll;
typedef pair<ll, ll> P;

#define MOD 1000000007
#define REP(i, N) for (int i = 0; i < N; ++i)
#define REP1(i, N) for (int i = 1; i <= N; ++i)
#define RREP(i, N) for (int i = N - 1; i >= 0; --i)
#define ALL(a) a.begin(), a.end()

int main() {
  bool is_prime[1000000];
  REP1(i, 999999) is_prime[i] = true;
  is_prime[0] = is_prime[1] = false;
  for (int i = 2; i * i <= 999999; ++i) {
    if (is_prime[i]) {
      for (int j = 2; i * j <= 999999; ++j) is_prime[i * j] = false;
    }
  }
  int sum[1000000];
  sum[0] = 0;
  REP1(i, 999999) sum[i] = sum[i - 1] + is_prime[i];
  int n;
  while (cin >> n) cout << sum[n] << endl;
  return 0;
}

C++: #include<iostream>

#define LIM 1000000
using namespace std;

int main(){
  bool num_map[LIM];
  int in_num,prime_count;
  for(int i=0;i<LIM;i++){
    num_map[i]=1;
  }
  
  for(int i=2;i<LIM;i++){
    if(num_map[i]==1)
      for(int j=i*2;j<LIM;j+=i){
	num_map[j]=0;
      }   
  }
  while(cin>>in_num){
    prime_count = 0;
    //    cin>>in_num;
    //cout<<in_num<<endl<<endl;
    for(int i=2;i<=in_num;i++){
      if(num_map[i]==1)
	prime_count++;
    }
    cout<<prime_count<<endl;  
  }
  
}

C++: #include<stdio.h>
int main(){
	long int a,b,n;
	long int p[80000];
	p[0]=2;
	n=1;
	for(a=3;a<1000000;a++){
		for(b=0;b<n;b++){
			if(a%p[b]==0){break;}
			if(a<p[b]*p[b]){
				p[n]=a;
				n++;
				break;
			}
		}
	}
	for(;;){
		if(scanf("%ld",&a)==EOF){break;}
		n=0;
		while(a>=p[n]){
			n++;
		}
		if(a<2){n=0;}
		printf("%ld\n",n);
	}
	return 0;
}

C++: #include<iostream>
#include<vector>
using namespace std;
int main(){
	int time;
	vector<int> v;
	v.push_back(2);
	while(cin>>time){
		int abcd=0;
		if(time>v[v.size()-1]){
			for(int abc=1+v[v.size()-1];abc<=time;abc++){
				for(int bcd=0;bcd<(int)v.size() ;bcd++){
					if(abc%v[bcd]==0){
						goto end;
					}
				}
				v.push_back(abc);
				end:;
			}
		}
		for(int abc=0;v[abc]<=time;abc++){
			abcd++;
		}
		cout<<abcd<<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
#include <numeric>
#define MAX 1000000
using namespace std;

bool isPrime(int x) {
  if(x == 2) {
	return true;
  }
  
  if(x < 2 || x % 2 == 0) {
	return false;
  }
  for(int i = 3; i <= sqrt(x); i += 2) {
	if(x % i == 0) {
	  return false;
	}
  }
  return true;
}

int main() {
  int n;
  bool a[MAX] = {};

  for(int i = 1; i < MAX; ++i) {
	if(isPrime(i)) {
	  a[i] = true;
	}
  }
  /*
  for(int i = 1; i < MAX; ++i) {
	if(isPrime(i)) {
	  cout << i << " ";
	}
  }
  */
  while(cin >> n) {
	cout << accumulate(&a[1], &a[n + 1], 0) << endl;
  }
  return 0;
}

/*-- primitive algorithm --*/
/*
bool isPrime(int n) {
  for(int i = 2; i < n; ++i) {
	if(n % i == 0) return false;
  }
  return true;
}
*/

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

bool a[1000001] ;
int main(){
    a[0] = 1 ;
    a[1] = 1 ;
    vector<int> v ;
    for(int i=2;i<=1000000;i++){
        if ( a[i] == 0 ){
            v.push_back(i);
            for(int j=i+i;j<=1000000;j+=i){
                a[j] = 1 ;
            }
        }
    }

    int x ;
    while(cin >> x){
        long long c = 0 ;
        for(int i=0;i<v.size();i++){
            if ( v[i] > x ) break;
            c++;
        }
        cout << c << endl;
    }



    return 0 ;
}

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

constexpr int LIM = 1000000;

static char isprime[LIM];
static int numofprime[LIM];

void setprime(int N) {
    fill(isprime, isprime + N + 1, 1);
    isprime[0] = isprime[1] = 0;
    for(int x = 2, lim = (int)sqrt(N); x <= lim; ++x) {
        if(isprime[x]) {
            for(int y = x * x; y <= N; y += x) {
                isprime[y] = 0;
            }
        }
    }
    numofprime[0] = 0;
    for(int x = 0; x < N; ++x) {
        numofprime[x + 1] = numofprime[x] + isprime[x + 1];
    }
}

int main() {
    setprime(999999);
    int n;
    while(scanf("%d", &n) != EOF) {
        printf("%d\n", numofprime[n]);
    }
    return 0;
}

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

using namespace std;

int prime[10000];
bool is_prime[1000000];

int sosuu(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=i*2; j<=n; j+=i) {
                is_prime[j]=false;
            }
        }
    }
    return p;
}

int main(void){
    int n;
    while(cin>>n)
    cout<<sosuu(n)<<endl;
    return 0;
}

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

int main()
{
	int n = 0, x = 0;
	int countdata[1000];
	while (cin >> n) {
		if (n / 1000000 > 0) {
			int count = 0;
			for (int a = 2; a <= n; a++) {
				if (a % 2 != 0 && a % 3 != 0 && a % 5 != 0 && a % 7 != 0) 
					count++;
				if (a == 2 || a == 3 || a == 5 || a == 7)
					count++;
			}
			countdata[x] = count;
			x++;
		}
	}
	for (int a = 0; a < x; a++) {
		cout << countdata[a] << endl;
	}
}

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

using namespace std;

bool isprime_ge3(int n) {
	if (!(n&1))
		return false;
	int lim = (int)sqrt((double)n);
	for (int i = 3; i <= lim; i+=2)
		if (!(n%i))
			return false;
	return true;
}

int main(void) {
	int n;
	while (cin >> n) {
		if (n < 2)
			cout << 0 << endl;
		else if (n == 2)
			cout << 1 << endl;
		else {
			int total = 1;
			for (int i = 3; i <= n; i++) {
				if (isprime_ge3(i))
					total++;
			}
			cout << total << endl;
		}
	}
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <math.h>
#define REP(i,n) for(int (i) = 0; i <= n; ++i)
#define N 1000000
using namespace std;

vector<int>prime;

void Eratosthenes(){
    bool f[N];
    for(int i = 0; i < N; i++){
        f[i] = true;
    }
    f[0]=f[1]=false;
    
    for(int i = 2; i < sqrt(N); i++){
        if(f[i]){
            for(int j = 0; i * (j + 2) < N; j++){
                f[i *(j + 2)] = false;
            }
        }
    }
    
    for(int i = 2; i < N; i++){
        if(f[i]){
            prime.push_back(i);
        }
    }
}

int main() {
    int n;
    Eratosthenes();
    while(cin>>n){
        int ret = 0;
        for (int j = 0; j < prime.size(); j++) {
            if (prime[j] > n)break;
            ret++;
        }
        cout<<ret<<endl;
    }
    return 0;
}