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 <iostream>
#include <cmath>
using namespace std;
#define MAX 1000000

int main(){
	bool p[MAX+1];
	int n, ans;

	for(int i=0 ; i<=MAX ; i++){
		p[i] = (i<=1)? false : true ;
	}

	for(int i=2 ; i<=sqrt(MAX)+1 ; i++){
		if(p[i]){
			for(int j=i*2 ; j<=MAX ; j+=i){
				p[j] = false;
			}
		}
	}

	while( cin >> n ){		
		ans = 0;
		for(int i=0 ; i<=n ; i++){
			if(p[i]){
				ans++;
			}
		}
		cout << ans << endl;
	}
}

C++: #include<stdio.h>

int	main(){
	int	i,	k,	x,	cnt[1000001],	data[1000000];
	cnt[0]	=	0;
	data[0]	=	0;
	data[1]	=	0;
	for(i	=	2;	i	<=	1000000;	i++){
		data[i]	=	1;
	}
	for(i	=	2;	i	*	i	<=	1000000;	i++){
		if(data[i]){
			for(k	=	2	*	i;	k	<=	1000000;	k	+=	i){
				data[k]	=	0;
			}
		}
	}
	for(i	=	0;	i	<=	1000000;	i++){
		cnt[i	+	1]	=	cnt[i]	+	data[i];
	}
	while(scanf("%d",	&x)	!=	EOF){
		printf("%d\n",	cnt[x	+	1]);
	}
	return	0;
}

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

int judgeprime(long num){
  int primeflag=1,i;

  if(num%2==0){primeflag=0;}
  else for(i=3;i<sqrt(num);i+=2){

    if( num%i==0 ){
      primeflag=0;
      break;
    }

  }

  if(num==0||num==1||num==4){primeflag=0;}
  else if(num==2){primeflag=1;}

  return primeflag;

}

int main(void){
  long i,num;
  int much;

  while( scanf("%ld",&num) != -1 ){

    much=0;

    for(i=1;i<=num;i++){
      much+=judgeprime(i);
    }

    printf("%d\n",much);

  }

  return 0;  
}
04:57:02 ~/programming/AOJ 
$

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

int p_num[1000000] = {} ;
int d[1000000] = {} ;

void cal(){
	for( int i=2 ; i<1000000 ; i++ ){
		if( p_num[i] == 0 ){
			for( int j=2 ; j<1000000 ; j++ ){
				if( i*j > 1000000 ) break ;
				p_num[i*j] = 1 ;
			}
		}
	}
	
	for( int i=2 ; i<1000000 ; i++ ){
		if( p_num[i] == 0 ) d[i] = d[i-1] +1 ;
		else d[i] = d[i-1] ;
	}
}

int main(){
	
	cal() ;
	
	int n ;
	while(1){
		cin >> n ;
		if( cin.eof() ) break ;
		
		cout << d[n] << endl ;
	}
	
return 0 ;
}

C++: #include <iostream>
#include <vector>
#include <cmath>
using namespace std;
 
//素数かどうかが真のものの数（カウントアップ）を返す
int countup(vector<bool>::iterator begin, vector<bool>::iterator end) 
{
    int counter = 0;
    while (begin++ != end) {
        if (*begin) 
            counter++;
    }
    return counter;
}
int main()
{
    int n;
    
    while (cin >> n) {
        int                     max = sqrt(static_cast<double>(n));
        vector<bool>            is_prime(n, true);
        vector<bool>::iterator  begin, end;
 
        is_prime[0] = is_prime[1] = false;
        
        for (int i=2; i<=max; i++) {
            if (!is_prime[i]) {
                continue;
            }
            else { // iがもし素数だったら、(自分以外の)その倍数全部潰す
                for (int cntr = 2, j = 2*i; j<=n; j = i*(cntr++)) { 
                    is_prime[j] = false;
                }
            }
        }
        begin   = is_prime.begin();
        end     = is_prime.end();
        cout << countup(begin, end) << endl;
    }
}

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

int main(){
	const int max_num=1000000;
	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 <iostream>
#include <cmath>
using namespace std;
// have you mooed today?
bool isprime(int& num)
{
    static int c;
    static int max;
    max = sqrt(num) + 5;
    if(num == 2) return true;

    for(c=3; c < max; c+=2)
        if(num % c ==0) return false;
    return true;
}

main()
{
    int ans;
    int buff;
    while(cin >> buff)
    {
        ans  = 0;
        for(int d =2; d <= buff; d++)
        {
            if(isprime(d)) ans++;
        }
        cout << ans << endl;
    }
}

C++: #include <iostream>
#include <vector>
#define MAX 100000
using namespace std;

int main(){
  //bool num[MAX];
  pair<bool,int> num[MAX];
  int count=0;
  int input=0;
  cin >> input;
  for(int i=2; i<MAX; i++){
    num[i].first=true;
    num[i].second=i;
  }
  for(int i=2;i<input+1;i++){
    if(num[i].first){
      //cout << num[i].second <<",";
      count++;
      for(int j=i+i; j<MAX; j+=i){
        num[j].first=false;
      }
    }
  }
  cout << count << endl;
//cout << MAX <<"までの素数の個数:"<< count  << endl;
  return 0;
}

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

#define pb push_back

using namespace std;

const int MAX=100000;
int main(void)
{
	int x;
	bool p[MAX];
	int cnt;
	int i1,i2;

	fill(p,p+MAX,false);

	for(i1=2;i1<MAX;i1++){
		for(i2=2;i2*i2<=i1;i2++){
			if(i1%i2==0) break;
		}
		if(i2*i2>i1) p[i1]=true;
	}

	while(cin>>x){
		cnt=0;
		for(i1=0;i1<=x;i1++){
			if(p[i1]) cnt++;
		}
		cout<<cnt<<endl;
	}
	return 0;
}

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

int main(){
	int n;
	int a[10000];
	int b;
	int i, j, p;
	int c[30];
	p = 0;
	while (scanf("%d", &n) != EOF){
		b = (int)pow((double)n, 0.5);
		c[p] = 0;
		a[0] = -1;
		for (i = 2; i <= b; i++){
			for (j = 2; j <= n / i; j++){
				a[i*j - 1] = -1;
			}
		}
		for (i = 1; i <= n; i++){
			if (a[i - 1] != -1){ c[p]++; }
			a[i - 1] = 0;
		}
		p++;
	}
	for (i = 0; i < p; i++){
		printf("%d\n", c[i]);
	}
	return 0;
}

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

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	int n;
	while(cin >> n){
	
	int cnt = 0;
	for(int i = 2; i <= n; ++i){
		bool check = true;
		for(int j = 2; i >= j * j; ++j){
			if(i % j == 0){
				check = false;
				break;
			}
		}
		if(check)
			cnt++;
	}
	
	cout << cnt << endl;
	
	}
	
	return 0;
}

C++: #include<iostream>
#include<map>
#define M 999999
using namespace std;
int Prime[999999];
void sang()
{
for(long i=3;i<=M;i+=2) Prime[i]=1;
Prime[0]=Prime[1]=0;
Prime[2]=1;
for(long i=2;i<=M/i;i++)
if(Prime[i]==1)
{
for(long j=i*i;j<=M;j+=i) Prime[j]=0;
}
for(long long i=0;i<=M;i++){
			Prime[i]+=Prime[i-1];
	    }
}
int main()
{
	sang();
	long long n;
	while(1){
		cin>>n;
	    cout<<Prime[n]<<endl;
	}
    return 0;
}