Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		input := nextInt()
		fmt.Println(IsPrime(input))
	}
}

func IsPrime(num int) int {
	eratos := make([]bool, num+1)
	c := 0
	for i := 2; i < len(eratos); i++ {
		if eratos[i] == false {
			c++
			eratosCalc(i, eratos)
		}
	}
	return c
}

func eratosCalc(primeNum int, eratos []bool) {
	for i := primeNum * primeNum; i < len(eratos); i += primeNum {
		eratos[i] = true
	}
}

func next() string {
	return sc.Text()
}

func nextInt() int {
	a, _ := strconv.Atoi(next())
	return a
}

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

using namespace std;
int main() {
  long n;
  vector<long> primes;
  primes.push_back(2);
  while (cin >> n) {
    if (n == 1) {
      cout << 0 << endl;
      return 0;
    }
    for (int i = 3; i <= n; i += 2) {
      bool prime = true;
      for (unsigned int j = 0; j < primes.size() && primes[j] * primes[j] <= n; j++)
        if (i % primes[j] == 0) {
          prime = false;
          break;
        }
      if(prime && find(primes.begin(), primes.end(), i) == primes.end()) {
        primes.push_back(i);
      }
    }
    int x = 0;
    for (long& e: primes)
      if (e <= n) x++;
    cout << x << endl;
  }
}

C++: #include <cstdio>
#define MAX 100000

int main(){
	int n;
	bool c[MAX]={true};
	int t[MAX];t[0] = 0;
	for(int i = 2;i < MAX;i++){
		for(int j = i;i*j < MAX;j++){
			c[i*j-1] = true;
		}
	}
	for(int i = 1;i < MAX;i++){
		if(c[i])t[i]=t[i-1];
		else t[i] = t[i-1]+1;
	}
	while(scanf("%d", &n)){
		printf("%d\n", t[n-1]);
	}
	return 0;
}

C++: 
/* vo00_09.cpp */

#include <iostream>
using namespace std;

const int N = 1000000;
int prime[N];

void init();

int main()
{
	init();

	int n;
	while (cin >> n) {
		
		cout << prime[n] << '\n';

	}

	return 0;
}

void init()
{
	for (int i = 0; i < N; ++i) { prime[i] = 1; }

	prime[0] = prime[1] = 0;

	for (int i = 2; i < N; ++i) {
		if (prime[i])
			for (int j = i * 2; j < N; j += i) { prime[j] = 0; }
	}

	int count = 0;
	for (int i = 0; i < N; ++i) { if (prime[i]) { count++; } prime[i] = count; }
}

C++: #include "bits/stdc++.h"
#include<unordered_map>
#include<unordered_set>
#pragma warning(disable:4996)
using namespace std;
using ld = long double;
const ld eps = 1e-9;

vector<int>primes;
void hurui(const int amax) {
	static bool flag = false;
	if (flag)return;
	vector<int>sos;
	sos = vector<int>(amax + 1, true);
	sos[0] = false; sos[1] = false;
	for (int i = 2; i <= amax; ++i) {
		if (sos[i]) {
			for (int j = 2 * i; j <= amax; j += i)sos[j] = false;
		}
	}
	for (int i = 0; i <= amax; ++i) {
		if (sos[i]) {
			primes.push_back(i);
		}
	}
	flag = true;
}

int main() {
	hurui(1e6 + 5);
	int num;
	while (cin >> num) {
		auto it = upper_bound(primes.begin(), primes.end(),num);
		cout << it - primes.begin() << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <cmath>
#include <algorithm>
#include <string>
#include <stdio.h>
using namespace std;
int isprime(int x){
    if(x==2) return true;
    if(x<2||x%2==0) return false;
    else{
        int i=3;
        while(i<=sqrt(x)){
            if(x%i==0) return false;
            i = i+2;
        }
        return true;
    }
}
int main() {
    int n;
    while(cin >> n){
        int ans=0;
    for (int j=n; j>1; j--) {
        if(isprime(j)!=0) ans++;
    }
    cout << ans << endl;
    }
}

C++: #include <bits/stdc++.h>

using namespace std;

#define MAX 1000000

int main()
{
    vector<int> p;
    bool prime[MAX];
    fill(prime, prime + MAX, 1);
    prime[0] = prime[1] = 0;
    for (int i = 2; i < MAX; i++) {
        if (prime[i]) {
            p.push_back(i);
            for (int j = 2*i; j < MAX; j += i) {
                prime[j] = 0;
            }
        }
    }

    int N;
    while (cin >> N) {
        cout << upper_bound(p.begin(), p.end(), N) - p.begin() << endl;
    }
    return 0;
}

C++: #include <stdio.h>
const int MAX_V = 100000;
int prime[MAX_V+1],sum[MAX_V+2]={0};
int main(){
  int n;
  for(int i = 2;i <= MAX_V;i++){
    prime[i] = 1;
  }
  for(int i = 2;i * i <= MAX_V;i++){
    if(prime[i]){
      for(int k = 2* i;k <= MAX_V;k += i)
      prime[k]=0;
    }
  }
  for(int i = 2;i < MAX_V;i++){
    sum[i] +=sum[i-1]+prime[i];
  }
  while(1){
    scanf("%d",&n);
    printf("%d\n",sum[n]);
  }
  return 0;
}

C++: #include <iostream>
#include <vector>
using namespace std;
#define MAX 1000000
int main( void )
{
	vector <bool> prime( MAX, false );
	prime[2] = true;
	for ( int i = 3; i <= MAX; i++ )
	{
		prime[i] = true;
	}

	for ( int i = 3; i <= MAX; i += 2 )
	{
		if ( prime[i] )
		{
			for ( int j = i + i; j <= MAX; j += i )
			{
				prime[j] = false;
			}
		}
	}

	while ( !cin.eof() )
	{
		int n;
		cin >> n;
		if ( cin.eof() )
			break;

		int cnt = 1;
		for ( int i = 3; i <= n; i += 2 )
		{
			if ( prime[i] )
			{
				cnt++;
			}
		}

		cout << cnt << endl;

	}

	return 0;
}

C++: #include<iostream>
using namespace std;
int d[1000000]={},p[1000000]={};
void calc(){
	for(int i=2;i<1000000;i++){
		if(d[i]==0){
			for(int j=2;i*j<1000000;j++){
					d[i*j]=1;
			}
		}
	}
	for(int i=2;i<1000000;i++){
		if(d[i]==0)
		p[i]=p[i-1]+1;
		else
		p[i]=p[i-1];
	}
}
int main(){
	calc();
	int n;
	while(cin>>n,n){
		cout<<p[n]<<endl;;
	}
	return 0;
}

C++: /*
 * main.cpp
 *
 *  Created on: 2011/04/08
 *      Author: satoki
 *      5:09 ~ 6:03
 *      Wrong Answer : 2
 *      nÈºÌfð¢ÅÀ
 *      Runtime Error : 4
 *      PÂ½ßÉfðZbg
 */
#include <iostream>
#define MAX_iP 1000004 // 664,580 ÔÚÌf = 10,000,019
#define MAX_nP 78499 // ÔºÌÓ¡àßÄ (KvÈfÌ + 1) ðmÛ
using namespace std;

int iN;

// fe[u
bool bPrime[MAX_iP];
int iPrime[MAX_nP];
int nPrime;

void setPrime() {
  // fe[uðì¬
  int i, j;

  nPrime = 0;
  fill(bPrime, bPrime + MAX_iP, true);
  bPrime[0] = bPrime[1] = false;

  for (i = 2; i < MAX_iP; i++) {
    if (bPrime[i]) {
      iPrime[nPrime++] = i;
      for (j = 2; i * j < MAX_iP; j++) {
        bPrime[i * j] = false;
      }
      //if (nPrime == MAX_nP) break;
    }
  }
  //printf("nPrime = %d i = %d\n", nPrime, i);
}

void init() {
  setPrime();
}

bool input() {
  if (scanf("%d", &iN) == EOF) return false;
  return true;
}

int manage() {
  int i;
  for (i = 0; iPrime[i] <= iN; i++);
  return i;
}

int main() {
  init();
  while (1) {
    if (!input()) break;
    printf("%d\n", manage());
  }
  return 0;
}

C++: #include <stdio.h>
int main(void){
	int k,j,a;
	//flag 0=素数 1=そすうじゃない
	int flag[1000000]={1,1};//0,1は除外する
	for(k=2;k<1000;k++){
		if(flag[k]==0){
			for(j=k*k;j<1000000;j+=k)
				flag[j]=1;
		}
	}
	while(~scanf("%d",&a)){
		k=0;
		for(j=0;j<a;j++) if(flag[j]==0) k++;
		printf("%d\n",k);
	}
}

C++: #include<stdio.h>

int pr[50847534];
int x = 0;

int nextpr(int p){
  int nxp = p+2;
  int i;
  int ok;
  x++;
  if(p==2)return 3;
  while(1){
      /*printf("p=%d->",nxp);*/
    for(i=0;pr[i]*pr[i]<=nxp;i++){
      ok = 1;
      if(!(nxp%pr[i])){
        ok = 0;
        break;
      }
    }
    if(ok){
        /*printf("O\n");*/
      return nxp;
    }else{
        /*printf("X\n");*/
      nxp+=2;
    }
  }
}

int main(){
    int n;
    int i;
    pr[0] = 2;
    while(scanf("%d",&n)!=EOF){
        printf("n=%d\n",n);
        i=0;
        if(n-1){
            while(pr[i]<=n){
                printf("prime:%d(%d)\n",pr[i],i+1);
                i++;
                if(i>=x){
                    pr[i] = nextpr(pr[i-1]);
                }
            }
        }
        printf("%d\n",i);
    }
}

C++: #include <iostream>

using namespace std;

bool is_prime(int x)
{
	if (x == 2) return true;
	if (x % 2 == 0) return false;
	for (int i = 3; i*i <= x; i += 2)
		if (x % i == 0)
			return false;

	return true;
}

int main()
{ 
	int n;

	while (cin >> n)
	{
		int cnt = 0;

		if (n > 1) cnt++;
		for (int i = 3; i <= n; i += 2)
			cnt += is_prime(i);

		cout << cnt << endl;
	}

	return 0;
}