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>
using namespace std;

int a[999999]={};

int main(){
	int n,ans=0;
	vector<int> v;
	for(int i = 2; i*i < 999999; i++){
		for(int j = 0; j < 999999; j+=i){
			if(i!=j) a[j]=1;
		}
	}
	while(cin >> n){
		v.push_back(n);
	}
	for(int i = 0; i < v.size(); i++){
		for(int j = 2; j <= v[i]; j++){
			if(a[j]==0) ans++;
		}
		cout << ans << endl;
		ans=0;
	}
}

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

int Prime_number(int n){
	int i = 2;
	int ans = 0;

	while(n >= 2){
		while(1){
			if(i == n){
				ans++;
				break;
			}else if(n % i == 0){
				break;
			}
				i++;
		}
		if(n % 2 == 0)
			n--;
		else
			n -= 2;
		i = 2;
	}

	return ans;
}

int main(){
	int n;

	while(cin >> n){
		if(n == 1){
			cout << "0" << endl;
		}else if(n == 2){
			cout << "1" << endl;
		}else{
		cout << Prime_number(n) << endl;
		}
	}

}

C++: #include <stdio.h>
#include <vector>
#include <algorithm>
using namespace std;

const int size=1000000;
vector<int> map(size,1);
void sieve(int p)
{
    for(int j=p*2;j<size;j+=p)
    {
        map[j]=0;
    }
}
int main(void)
{
    int n;
    scanf("%d",&n);
    map[0]=0;
    map[1]=0;
    sieve(2);
    sieve(3);
    for(int i=7;i<1000;i+=6)
    {
        sieve(i);
        sieve(i+4);
    }
    printf("%d\n",count(map.begin(),map.begin()+n+1,1));
    return 0;
}

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

int main()
{
	const int N = 1000000;
	static bool P[N];
	for ( int i=2; i<N; i++)
		P[i]=true;
	for(int i=2; i*i<N; i++ )
	if(P[i])
		for(int j=i+i;j<N;j+=i)
			P[j]=false;

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

}

C++: #include <stdio.h>

int main(void){
	bool a[1000000];
	int n = 0;
	int c = 0;
	while(scanf("%d", &n) != EOF){
		switch(n){
		case 1:
			c = 0;
			break;
		case 2:
			c = 1;
			break;
		default:
			c = 1;
			for(int i=3; i<=n; i+=2){
				a[i] = true;
			}
			for(int i=3; i<=n; i+=2){
				if(a[i] == false){ continue; }
				c++;
				for(int j=i; j<=n; j+=i){
					a[j] = false;
				}
			}
			break;
		}
		printf("%d\n", c);
	}
	return 0;
}

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

int main(){
  bool prime[1000000];
  prime[0]=false;prime[1]=false;
  for(int i=2;i<1000001;i++){
    prime[i]=true;
  }

  for(int i=2;i<=sqrt(999999);i++){
    if(prime[i]){
      for(int j=1;j*i<=999999;j++){
        prime[j*i]=false;
      }
    }
  }
  int n;
  while(cin>>n){
    int cnt=0;
    for(int i=2;i<=n;i++){
      if(prime[i])cnt++;
    }
    cout<<cnt<<endl;
  }
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <math.h>
#pragma warning(disable:4996)

int main()
{
	
	int num, cnt;
	unsigned int i, j, k, data_num;
	int data[30];
	unsigned int sq_num;
	unsigned int *prime;

	for (data_num = 0; data_num <30; data_num++)
	{
		data[data_num] = -1;
		scanf("%d", &data[data_num]);
		if (data[data_num] < 0)
			break;
	}

	for (k = 0; k < data_num; k++)
	{

		sq_num = (int)sqrt((double)data[k]);
		prime = (unsigned int*)malloc(sizeof(unsigned int) * data[k]);
		prime[0] = 0;

		for (i = 1; i < data[k]; i++)
		{
			prime[1] = 1;
		}

		for (i = 1; i < sq_num; i++)
		{

			if (prime[i] == i)
			{
				for (j = (i + 1); (i + 1)* j <= data[k]; j++)
				{
					prime[(i + 1) * j - 1] = 0;
				}
			}
		}

		cnt = 0; 
		for (i = 0; i < data[k]; i++)
		{
			if (prime[j] == 1)
			{
				cnt++;
			}
		}
		printf("%d\n", cnt);
		free(prime);
	}

	return 0;
}

C++: #include<iostream>

using namespace std;

int main(){
  int n,flag=1,sum = 0;

  while(cin >> n){

    for(int i=2; i <= n; i++){
      if(i != 2 && i%2 == 0) flag = 0;
      else if(i != 3 && i%3 == 0) flag = 0;
      else if(i != 5 && i%5 == 0) flag = 0;
      else{
	for(int j=3; j < i/2; j++){
	  if(i%j == 0 ){
	    flag = 0;
	    j = i/2;
	  }
	}
      }
      if(flag == 1) sum++;
      flag = 1;
    }
    cout << sum <<endl;
    sum = 0;
  }
}

C++: #include<stdio.h>
int num[1000001]={};
int main ()
{
int p,n,m,cou;m=0;cou=0;p=3;


for(int i=2;i<=10000001;i++)
	num[i]=1;
for(int i=2;i<=1001;i++){
for(int k=2;k*k<=i;k++){
if(i%k==0){num[i]=0;}}
}
while(scanf("%d",&n)!=EOF)
{
for(int i=2;i<=n;i++)
	cou+=num[i];

if(n==2)cou=1;
if(n==1)cou=0;
//else cou++;
printf("%d\n",cou);
cou=0;
}
return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main()
{
    int i,j,flag,n,c=0;
    scanf("%d",&n);
    for(i=2;i<=n;i++)
    {
        flag=0;
        for(j=2;j<=sqrt(i);j++)
        {
            if(i%j==0)
            {
                flag=1;
            }
        }
        if(flag==0)
        {
            c++;
        }
    }
    printf("%d",c);
    return 0;
}

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

typedef int TInt;
typedef std::vector<TInt> TList;
bool IsPrime(TInt Value, const TList& Prime)
{
    TInt Max = std::sqrt(Value);
    for(TList::const_iterator It = Prime.begin();
        It != Prime.end() && *It <= Max;
        ++It)
    {
        if(Value % (*It) == 0) return false;
    }
    return true;
}
TList GetPrimeList(TInt Max)
{
    TList Prime;
    Prime.reserve(Max);
    Prime.push_back(2);
    for(TInt i = 3; i <= Max; i += 2){
        if(IsPrime(i, Prime)){
            Prime.push_back(i);
        }
    }
    return Prime;
}
int main()
{
    TList List = GetPrimeList(999999);
    while(true){
        int n;
        std::cin >> n;
        if(std::cin.eof()) break;
        TList::iterator It = std::upper_bound(List.begin(), List.end(), n);
        std::cout << std::distance(List.begin(), It) << std::endl;
        
    }
    return 0;
}

C++: //*-*-coding:utf-8-*-*/
#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <sstream>
#include <iterator>
#include <cmath>
#include <stack>
#include <functional>
#include <iomanip>
using namespace std;

int main()
{
  int n;

  while(cin >> n, !cin.eof()) {
    int ans = 0;
    if(n >= 2) ans++;    
    for(int i = 3; i <= n; i+=2) {
      int flag = 0;
      for(int j = 3; j <= sqrt(i); j+=2) {
	if(i % j == 0) {
	  flag = 1;
	  break;
	}
      }
      if(flag == 0) ans++;
    }    
    cout << ans << endl;
  }
  
  return 0;
}