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<stdio.h>
using namespace std;
int prime[1000005];
bool is_prime[1000005];
int sieve(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 = 2 * i;j <= n;j += i)is_prime[j] = false;
		}
	}
	return p;
}


int main()
{
	int n;
	int f = 0;
	
	while (scanf("%d", &n)) {
		if (f == 29) break;
		printf("%d\n", sieve(n));
		f++;
		
		
	}
	
	


	return 0;
}

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

int pn[1000002];

int main(){
int n, Count;
cin.tie(0);
while(scanf("%d",&n)!=EOF){
Count=0;
memset(pn,0,n);
pn[0]=pn[1]=1;
for(int i=2;i<=n;i++) if(!pn[i]) for(int j=1;i*j<=n;j++) pn[i*j]=1;
Count=count(pn,pn+n,0);
cout<<Count<<endl;
}
return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <cstdlib>
#include <map>

using namespace std;

bool prime(int p){
	if ((p==0)||(p==1)){return false;}
	else if(p==2){return true;}
	else{
		for(int i=2; i*i<=p; i++){
			if(p%i == 0){return false;}
		}
	}
	return true;
}

int main()
{
	int n;
	map<int,bool> p;

	for(int i=0; i<1000000; i++){
		if(prime(i)){ p[i]=true;}
	}

	while(cin >> n){
		int count = 0;
		if(n>=2){count++;}
		for(int i=1; i<n+1; i=i+2){
			if(p[i]){count++;}
		}

		cout << count << endl;
	}
}

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

int main(void){
	long n;
	int a;
	while(scanf("%d", &n) != EOF){
		vector<bool>p;
		for(int i = 0; i <= n; i++) p.push_back(true);
		p[0] = false;
		p[1] = false;
		for(int i = 2; i < sqrt(n)+1; i++){
			if(p[i]){
				for(int j = 0; i*(j+2) <= n; j++){
					p[i*(j+2)] = false;
				}
			}
		}
		int count = 0;
		for(int i = 2; i <= n; i++){
			if(p[i]) count++;
		}
		cout << count << endl;
	}
	return 0;
}

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

bool is_prime(int x)
{
  for (int i = 2; i <= round(sqrt(x)) && i < x; i++)
    if (x % i == 0) return false;
  return true;
}

int main()
{
  for (int n; cin >> n; ) {
    int ans = 0;
    for (int i = 2; i <= n; i++)
      if (is_prime(i))
        ans++;
    cout << ans << endl;
  }

  return 0;
}

C++: #include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;
int sum[1000005];
bool shai[1000005];
void dabiao(){
	for(int i=2;i<=1000000;i++){
		sum[i]=sum[i-1];
		if(!shai[i]){
			for(int j=2*i;j<=1000000;j+=i)
				shai[j]=1;
			sum[i]++;
		}
	}
}
int main()
{
	dabiao();
	int n;
	while(scanf("%d",&n)==1){
		printf("%d\n",sum[n]);
	}
}

C++: #include <bits/stdc++.h>
using namespace std;

#define REP(i, s, n) for (int i = s; i < n; ++i)
#define rep(i, n) REP(i, 0, n)
#define SORT(c) sort((c).begin(), (c).end())
#define IINF INT_MAX
#define LLINF LLONG_MAX
#define DEBUG false


int prime[100000]={0};
int k=1;
void
 makeprime(int n)
{
    prime[0]=2;
    for (int i = 3; i < n; i++)
    {
        double temp = sqrt(i);
        for (int j = 0;; j++)
        {
            if (i % prime[j] == 0)
            {
                break; //this is no prime.
            }
            if (temp < prime[j])
            {
                prime[k++]=i;
                break;
            }
        }
    }
}

int main()
{
    makeprime(999999);
    int n;
    int ans=0,i;
    while(cin>>n){
        ans=0;
        i=0;
        while(true){
            if(prime[i]<=n){
                ans++;
            }
            if(n<prime[i]){
                break;
            }
            i++;
        }
        cout<<ans<<endl;
    }
    return 0;
}

C++: #include <cstdio>
bool d[1000000];
int c[999999];
int main() {
    int i,j,r=0,n;
	for(i=4;i<=999999;i+=2)d[i]=true;
    for(i=2;i*i<=999999;i++) {
        if(d[i])continue;
        for(j=i*2;j<=999999;j+=i)d[j]=true;
    }
	for(i=1;i<999999;i++) {
		c[i]=c[i-1];
		if(!d[i+1])c[i]++;
	}
    while(scanf("%d",&n)!=EOF) {
        printf("%d\n",c[n-1]);
    }
}

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

bool d[1000000];

int main()
{
  int n;
  int ans;
  while(cin >> n)
  {
    n++;
    for(int i = 0; i < 1000000; i++)
      d[i] = false;
    d[0] = d[1] = true;
    ans = 0;
    for(int i = 2; i < n; i++)
    {
      if(d[i])
        continue;
      for(int j = i + i; j < n; j+= i)
        d[j] = true;
    }
    for(int i = 0; i < n; i++)
    {
      //cout << d[i] << endl;
      ans += !d[i] ? 1 : 0;
    }
    cout << ans << endl;
  }
  return 0;
}

C++: #include<iostream>

typedef int moe;
typedef long lolita;
typedef char cuffs;
typedef float frill;
typedef double ruffle;
typedef void voile;
typedef unsigned tasogare;

#define katyusya const
#define apron volatile
#define pleats static
#define fancy virtual

using namespace std;

#define YUU 1000000

voile make_prime(moe n_prime[]){
  pleats moe is_prime[YUU];

  for( moe ai = 0; ai < YUU; ++ai )
    is_prime[ai] = 1;

  is_prime[0] = 0;
  is_prime[1] = 1;
  is_prime[2] = 1;

  for( moe ai = 2; ai < YUU; ++ai )
    if( is_prime[ ai ] )
      for( moe mai = 2; ai * mai < YUU; ++mai)
	is_prime[ ai * mai ] = 0;

  for( moe ai = 0; ai < YUU; ++ai )
    n_prime[ ai ] = 0;

  for( moe ai = 2; ai < YUU; ++ai ){
    n_prime[ ai ] = n_prime[ ai - 1 ];
    if( is_prime[ ai ] )
      n_prime[ ai ]++;
  }
}

moe main(voile){
  tasogare yuu;
  pleats moe n_prime[YUU];

  make_prime( n_prime );
  while(true){
    cin >> yuu;
    if( yuu == 0 || cin.eof() ) break;
    cout << n_prime[ yuu ] << endl;
  }

  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>

bool divisible( int u, int v )
{
    return (u % v) == 0 ? true : false;
}

int num_divisors( unsigned int start, unsigned int end )
{
    if ( start == end ) return 1;
    return divisible( end, start ) + num_divisors( start+1, end );
}

int no_of_divisor( unsigned int n )
{
    return num_divisors( 1, n );
}

bool is_prime( unsigned int n )
{
    return (no_of_divisor(n) == 2);
}

int count_prime( unsigned int n )
{
    int count = 1;
    for ( unsigned int i = 3; i <= n; i += 2 )
    {
        if ( is_prime(i) ) ++count;
    }
    return count;
}

int main()
{
    int n;
    while( std::cin >> n)
        std::cout << count_prime(n) << std::endl;
    return 0;
}