Go: package main

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

func main() {
    var limit = 1000000
    var root = 1000
    var isPrime [1000000]bool
    for i := 0; i < limit; i++ {
        isPrime[i] = true
    }
    isPrime[0] = false
    isPrime[1] = false
    for i := 2; i <= root; i++ {
        if isPrime[i] {
            for j := i * i;j < limit; j+=i {
                isPrime[j] = false
            }
        }
    }
    var primeCnt [1000000]int
    var cnt = 0
    for i := 0; i < limit; i++ {
        if isPrime[i] {
            cnt++
        }
        primeCnt[i] = cnt
    }

    var sc = bufio.NewScanner(os.Stdin)
    for sc.Scan() {
        n, _ := strconv.Atoi(sc.Text())
        fmt.Println(primeCnt[n])
    }
}

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

int main(){
	const int max_num=10000000;
	int prime[max_num];
	int prime2[max_num];
	int min_prime[168]={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 };
	int sosuu=0;
	int k=168;

	for(int i=0;i<168;i++){
		prime[i]=min_prime[i];
	}
	for(int i=1001;i<max_num;i+=2){
		sosuu=0;
		for(int j=0;j<168;j++){
			if(i%min_prime[j]==0){
				sosuu++;
			}
		}
		if(sosuu==0){
			prime[k]=i;
			k++;
		}
	}
	int l=0;
	for(int i=0;i<max_num;i++){
		if(i>=prime[l]){
			l++;
		}
		prime2[i]=l;
	}

	int n;

	while(cin>>n){
		cout<<prime2[n]<<endl;
	}

	return 0;
}

C++: #include	<stdio.h>

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

int main(int argc, char** argv){
	
	int search, counter;
	
	while((scanf("%d", &search)) != EOF){
		counter = 0;
		for(int i = 2; i <= search; i++){
			counter += isPrimeNumber(i) ? 1 : 0;		
		}
		printf("%d\n", counter);
	}
	
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <iomanip>
#include <string>
#include <set>
#include <functional>

static int const MAX(1000000);

class Mod{
	const int n;
	public:
		Mod(const int _n):n(_n){}
		bool operator()(const int _src){
			return ! (_src % n);
		}
};

const std::vector<int> Eratosthenes(std::set<int>& src){
	std::vector<int> result;
	int cnt(0);
	while(!src.empty()){
	  int n(*src.begin());
		result.push_back(n);
		Mod mod(n);
		//std::set<int>::iterator it= std::remove_if( src.begin(), src.end(), mod );
		// n>(MAX/2) ? std::remove( src.begin(), src.end(), n ) :
		for(int i(n);i<MAX;i+=n)  {
		std::set<int>::iterator it= src.find(i);
		if(it!=src.end())
		   src.erase(it );
		}    
		//src.erase(it,src.end());
	}
	return result;
}


int main(int,char**){
	std::set<int> vec;
	int max_2(MAX-MAX/2-MAX/3-MAX/5+MAX/6+MAX/10+MAX/15-MAX/30+3);
	//	vec.reserve(max_2);
	for(int i(3);i<=MAX;i+=2){
		if(i!=3 && i%3==0) continue;
		if(i!=5 && i%5==0) continue;
		vec.insert(i);
	}
	std::vector<int> primes;
	primes.reserve(10000);
	primes =Eratosthenes(vec);
	int n;
	while(std::cin >>n){
		std::vector<int>::const_iterator const it= std::upper_bound(primes.begin(),primes.end(),n);
		int off=it -primes.begin();
		if(n>=2){ ++off; }
		std::cout << off <<std::endl;
	}
	return 0;
}

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)){
        int count = 0;
        for(int i = 1; i <= n; i++){
            if(p[i])count++;
        }
        printf("%d\n", count);
    }
    return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <string.h>

int prime(int n) {
  int k;
  for(k = 2; k * k <= n; k++) {
    if(n % k == 0) return 0;
  }
  return 1;
}

int cntPrime(int n) {
  int cnt = 0, i, k;
  for(i = 2; i <= n; i++) {
    cnt += prime(i);
  }
  return cnt;
}

int main() {
  int a;
  int arg[1024] = { 0 };
  int cnt = 0;
  while(scanf("%d", &a) != EOF) {
    printf("%d\n", cntPrime(a));
  }
  int i;
  for(i = 0; i < cnt; i++) {
  }
}
~                                                                                                       
~                                                                                                       
~

C++: #include <stdio.h>
const int MAX_V = 999999;//999999?????§????´???°??¨?????????
int main(){
	int prime [MAX_V];//??¨?????????????????????????????????
	int sum[MAX_V+1];//?´????????????????
	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] == 1){
			for(k = 2*i; k <= MAX_V; k += i){
				prime[k] = 0;//?´???°????????°????´???°??§??????
			}
		}
	}
	for(i = 2; i < MAX_V; i++){//?´????????????????
		sum[i + 1] = sum[i] + prime[i];//
	}
	while(scanf("%d", &n) == EOF){
		printf("%d\n", sum[n + 1]);
	}
	return 0;
}

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

using namespace std;

int prime[100000];
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;
    cin>>n;
    cout<<sosuu(n)<<endl;
    return 0;
}

C++: #include <stdio.h>

int isprime(int n){
    for(int i = 2; i * i <= n; i++) {
        if(n % i == 0) return 0;  //素数でない
    }
    return 1; //素数である
}

int main() {
    int n;
    int ans[2] = {};
    while(scanf("%d", &n) != 0) {
        for(int i = n; i >= 5; i--) {
            if(isprime(i) == 1) {
                ans[0] = i;
                if(isprime(i - 2) == 1) {
                    ans[1] = i - 2;
                    break;
                }
            }
        }
        printf("%d %d", ans[0], ans[1]);
    }
}

C++: #include<stdio.h>

int main(void)
{
	int data,i,j,k=0,l=0,kai[100]={0},s[100000]={0};

	while(scanf("%d",&data) != EOF)
	{
		if(data<=1)
		   kai[k]=0;
		else
		{
			for(i=2;i<=data;i++)s[i]=i;

		    i=2;
			while(i*2<data)
			{
			  if(s[i] != 0)
			  {  
				j=2;
				while(j*i<=data)
				{
					s[j*i]=0;
					j+=1;
				}
			  }
			  i+=1;
			}

			
			for(i=2;i<=data;i++)
			{	
				if(s[i]!=0)
				{
					kai[k]+=1;
				}
			}
		}
		k+=1;
	}

  for(j=0;j<k;j++)
    printf("%d\n",kai[j]);
  return 0;
}

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

bool nonprime[1000000];

int main(){
	long i,j,n;
	for(i=2;i<=999999;i++){
		for(j=2;i*j<=999999;j++) nonprime[i*j]=1;
	}
	while(scanf("%ld",&n)!=EOF){
		j=0;//j?????????????????¨?????????????????????
		for(i=2;i<=n;i++) if(!nonprime[i]) j++;
		cout << j << endl;
	}
	return 0;
}