Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		a, _ := strconv.Atoi(sc.Text())
		fmt.Println(countPrime(a))
	}
}

func countPrime(a int) int {
	primes := make([]bool, a+1)
	n := 0
	for i := 2; i <= a; i++ {
		if primes[i] == false {
			n++
			for j := i * i; j <= a; j += i {
				primes[j] = true
			}
		}
	}
	return n
}

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

int main(){
  int a;
  int i, j;
  int past=1, a_sqrt;
  int count;
  bool chk[1000000];

  for(i=1; i<1000000; i++) chk[i]=true;
  while(cin>>a){
    count=0;
    if(past>=a){
      for(i=1; i<=a; i++){
        if(chk[i]) count++;
      }
      printf("%d\n", count);
    }
    else {
      a_sqrt=(int)sqrt((double)a);
      for(i=2; i<=a_sqrt; i++){
        for(j=past+1; j<=a; j++){
          if(j%i==0) chk[j]=false;
        }
      }
      past=a;
      for(i=1; i<=a; i++){
        if(chk[i]) count++;
      }
      printf("%d\n", count);
    }
  }

  return 0;
}

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

using namespace std;

int main()
{
	int n;
	int count;
	while (cin >> n, !cin.eof())
	{
		count = 0;
		vector<int> p(n + 1, 1);
		p[1] = 0;
		for (int i = 2; i * i <= n; i++)
		{
			if (p[i] == 0) continue;
			for (int j = 2; i * j <= n; j++)
			{
				p[i * j] = 0;
			}
		}
		for (int i = 1; i <= n; i++)
		{
			count += p[i];
		}
		cout << count << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<stdio.h>
using namespace std;
int phi[1000000];
bool vis[1000000];
int p[1000000];
int sieve(int n) {
		int i, j,cnt=0;
		phi[1] = 1;
		for (i = 2; i < n; ++i) {
			if (!vis[i]) {
				p[cnt++] = i;
				phi[i] = i - 1;
			}
			for (j = 0; j < cnt && i * p[j] < n; ++j) {
				vis[i * p[j]] = true;
				if (i % p[j]) phi[i * p[j]] = phi[i] * phi[p[j]];
				else {
					phi[i * p[j]] = phi[i] * p[j];
					break;
				}
			}
		}
		return cnt;
	}
	
	


int main()
{
	int n;
	while (scanf("%d",&n)) {
		printf("%d\n", sieve(n));
	}
			
	

    return 0;
}

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

#define MAX_NUM (1000000)

static int getPrimeNum(int maxVal);

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

static int getPrimeNum(int maxVal)
{
  static int searchList[MAX_NUM] = {0};
  static int primeList[MAX_NUM] = {0};
  
  memset(searchList, 0, sizeof(int) * MAX_NUM);
  memset(primeList, 0, sizeof(int) * MAX_NUM);
  
  double sqMaxVal = sqrt((double)maxVal) + 1;
  
  for (int i = 2; i <= sqMaxVal; i++) {
    if (searchList[i] == 0) {
      searchList[i] = 1;
      primeList[i] = 1;
      for (int j = i + 1; j <= maxVal; j++) {
        if (j % i == 0) {
          searchList[j] = 1;
        }
      }
    }
  }
  
  for (int i = 2; i <= maxVal; i++) {
    if (searchList[i] == 0) {
      primeList[i] = 1;
    }
  }
  
  int count = 0;
  for (int i = 2; i <= maxVal; i++) {
    if (primeList[i] == 1) {
      count++;
    }
  }
  
  return count;
}

C++: #include <iostream>
using namespace std;
#define N 1000000
int a[N];
int main(){
	int i,j;
	a[0]=0;a[1]=0;
	for(i=2;i<N;i++){
		a[i]=1;
	}
	for(i=2;i<N/2+1;i++){
		j=2;
		while(i*j<N){
			a[i*j]=0;
			j+=1;
		}
	}
	int n;
	while(cin>>n){
		j=0;
		for(i=2;i<=n;i++){
			j+=a[i];
		}
		cout<<j<<endl;
	}
}

C++: #include <stdio.h>
#include <math.h>
int sosuu(int x){
    int n,m,sosuu;
    sosuu=x-1;
    for(n=2;n<=x;n++){//nは素数を調べる。入力された数字まで１からスライドしていく。
        for(m=2;m<n;m++){//mはnが素数かどうか調べる。素数かどうかをnより小さい数字で数えていく。2からなのは2から素数だから。1はふくめない。
            if(n%m==0){//素数じゃなかったら、つまり割り切れてしまったら。
                sosuu--;
            }
            
            if(n%m==0) break;
        }
    }
    if(x==1||x==0) sosuu=0;
    return sosuu;
}

int main ( void ) {
    int s,result;
    while(scanf("%d",&s)!=EOF){
        result = sosuu(s);
        printf("%d\n",result);
    }
    return 0;
}

C++: #include<iostream>
#include<vector>
#define MAX 1000000
using namespace std;
int main(){
    vector<int> v(MAX);
    for(int i = 0;i<MAX;i++)
      v[i]=true;

      v[0]=false;
      v[1]=false;
    for(int i = 2;i<MAX;i++){
      if(v[i]==true){
        for(int j = i;j<MAX;j=j*j){
         v[j]=false;
       }
      }
    }
    for(int i = 2;i<MAX;i++)v[i]+=v[i-1];
    while(true){
      int a;
      cin >> a;
      if(cin.eof())return 0;
      cout << v[a]<<endl;
    }
}

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

using namespace std;

int A[1000];
int Pream(int n)
{
	int kusa = (int)sqrt((double)n);
	int mama = 0;

	for (int i = 0; i < n; i++)
		A[i] = 1;  // ??¨????????°???1????????????
	A[0] = 0;    // 1????´???°??§?????????

	for (int i = 1; i < kusa; i++) {
		if (A[i] == 1)   // prime[i]????´???°??????
		{
			for (int j = (i + 1); (i + 1) * j <= n; j++)
				A[(i + 1) * j - 1] = 0;  // ?´???°????????°????´???°??§?????????
		}
	}
	for (int i = 0; i < n; i++)
		if (A[i] == 1)  // 1????????£???????????????????´???°
			mama++;

	return mama;
}

int main()
{
	int n;
	cin >> n;
	while (n != -1)
	{
		cout << Pream(n) << endl;
		cin >> n;
	}
	return 0;
}

C++: #include <math.h>
#include <algorithm>
#include <string>
#include <iostream>
using namespace std;

#define MAX 1000000
int a[MAX];

int main()
{
	int n;
	a[0] = a[1] = 0;
	a[2] = 1;
	a[3] = 2;
	for (int i = 4; i < MAX; i++)
	{
		bool ok = true;
		for (int j = 2; j < sqrt(i) + 0.1; j++)
		{
			if (i % j == 0)
			{
				a[i] = a[i - 1];
				ok = false;
				break;
			}
		}
		if (ok)
			a[i] = a[i - 1] + 1;
	}

	while (cin >> n)
		cout << a[n] << endl;
	
	return 0;
}

C++: #include <iostream>
using namespace std;
const int maxn = 1000000;
int dp[maxn];
bool ar[maxn];
int main()
{
	for(int i = 2;i<=maxn;i++)
	{
		if(!ar[i])
		{
			for(int j = 2;j*i<maxn;j++)
				ar[i*j] = true;
		}
		dp[i] = dp[i-1] + !ar[i];
	}
	for(int n;cin >> n;)
		cout << dp[n] <<endl;
	return 0;
}

C++: #include <stdio.h>
int prime_finder(int i){
    int j;
    for(j=2;j<=i/2;j++){
        if(i%j==0){
            return 0;
        }
    }
    return 1;
}
int main(){
    int n,i,k,j;
    while (scanf ("%d",&n) != EOF && n<=999999 )
    {
        j=0;
        for(k=2;k<=n;k++){
            if(prime_finder(k)==1){
                j++;
            }
        }
        printf("%d\n",j);
    }
    return 0;
}

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

using namespace std;

int main()
{
	int n;

	while (cin >> n, n != EOF){
		int pn = 0;
		for (int i = 2; i <= n; i++){
			bool flag = true;
			for (int j = 2; j <= n; j++){
				if (i % j == 0 && i != j){
					flag = false;
					break;
				}
			}
			if (flag) pn++;
		}
		
		cout << pn << endl;
	}

	return 0;
}

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

int main() {
	int n, count;
	bool *data;

	while (cin >> n) {
		data = new bool[n+1];
		count = 0;
		data[0] = data[1] = false;
		for (int i = 2;i <= n;i++) {
			if (data[i]) {
				count++;
				for (int j = i + 1;j <= n;j += j) {
					data[j] = false;
				}
			}
		}
		cout << count << endl;
		delete[] data;
	}
	return 0;
}

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

int main()
{
	int n;
	int a[MAX];
	int counter;
	
	a[0] = 0;
	a[1] = 0;
	for (int i = 2; i < MAX; i++) {
		a[i] = 1;
	}
	
	int j = 2;
	while (j < MAX) {
		if (a[j] == 0) {
			j++;
		} else {
			for (int k = j *2; k < MAX; k += j)
				a[k] = 0;
			j++;
		}
	}
	
	while (cin >> n) {
		counter = 0;
		for (int i = 2; i <= n; i++)
			if (a[i])
				counter++;
		cout << counter << "\n";
	}
		
	return 0;
}