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 <math.h>

int prime[1000000];


bool isPrime(int n) {
  int x = (int)sqrt(n);
  for (int i = 3; i <= x; i += 2) {
    if (n % i == 0) {
      return false;
    }
  }
  return true;
}


void get_prime(int max_num) {
  int primecount = 1;
  prime[2] = primecount;
  for (int i = 3; i <= max_num; i += 2) {
    if (isPrime(i))
      prime[i] = ++primecount;
    else
      prime[i] = primecount;
  }
}


int main () {
  int ns[31];
  int i = 0, max_num;
  while (std::cin >> ns[i]) {
    max_num = std::max(max_num, ns[i]);
    if (ns[i] % 2 == 0)
      ns[i] --;
    i ++;
  }
  get_prime(max_num);
  return 0;
}

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

int main()
{
  bool is_prime[1000000];fill(is_prime,is_prime+1000000,true);
  int count_prime[1000000];fill(count_prime,count_prime+1000000,0);
  is_prime[0]=false,is_prime[1]=false;
  for(int i=2;i<1000000;i++)
  {
    count_prime[i]=count_prime[i-1]+is_prime[i];
    if(!is_prime[i])continue;
    for(int j=i*2;j<1000000;j+=i)is_prime[j]=false;
  }
  int n;
  while(cin>>n)printf("%d\n",count_prime[n]);
}

C++: #include <iostream>

using namespace std;

int main(){
	int n, x;
	while(cin >> n){
		int x = 0;
		int sum;
	
		for(int i = 1;i <= n;i++){
			sum = 0;
			for(int j = 1; j <= i; j++){
				if( i % j == 0) sum += j;{
					if(sum == i + 1) x++;
				}
			}
		}
		
		cout << x << endl;
	}
	
	return 0;
}

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

#define MAX 1000000

int main(){
	bool p[MAX+1];
	int num,answer;

	while( scanf("%d" , &num) != EOF ){	
		p[0] = false;
		p[1] = false;
		for(int i=2 ; i<=MAX ; i++){
			p[i] = true;
		}
		answer = 0;

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

		for(int i=0; i<=num; i++){
			if(p[i]){
				answer++;
			}
		}
		printf("%d\n", answer);
	}
}

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

#include <math.h>

using namespace std;

static inline bool judge_prime(const int num) {
	if (num < 2) {
		return false;
	}
	else if (num == 2) {
		return true;
	}
	else if (num % 2 == 0) {
		return false;
	}
	else if (num == 3) {
		return true;
	}

	if (num % 3 == 0) {
		return false;
	}

	if(num % 6 != 1 && num % 6 != 5) {
		return false;
	}

	const double sqrtnum = sqrt(num);
	for(int i = 5; i <= sqrtnum; i += 6) {
		if(num % i == 0) {
			return false;
		}
		if (num % (i + 2) == 0) {
			return false;
		}
	}
	return true;
}

static int count_prime(const int input) {
	int counter_prime = 0;

	for (int i = input; 1 < i; i --) {
		if (judge_prime(i)) {
			counter_prime ++;
		}
	}
	return counter_prime;
}

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

C++: #include<iostream>
using namespace std;
int main(){
	int n, cnt;	
	while(cin >> n){
		bool prime[n];
		prime[n] = {false};
		prime[1] = true;
		
		// ??¨??????????????????????????????
		for(int j = 2; j*j <= n; j++){
			if(prime[j] == false){
				for(int k = j; k*j <= n; k++){
					prime[j*k] = true;
				}
			}
		}
		cnt = 0;
		for(int i = 1; i <= n; i++){
			if(prime[i] == false) cnt++;
		}
		
		cout << cnt << endl;
	}
}

C++: #include<iostream>
#define N 999999
#define FOR(i,a,b) for( int i = ( a ); i < ( b ); i++ )
#define WHILE(n) while( std::cin >> n )
int p[(N+1)/2];int m[N+1];int mk();int main(){int n;p[0]=2;m[1]=0;m[2]=1;mk();WHILE(n){cout<<m[n]<<i'\n';}return 0;}int mk(){int i,j,e=1;REP(i,3,N_MAX){for(j=0;p[j]*p[j]<=i;j++){if(i%p[j]==0){goto tag;}}p[e]=i;e++;tag:m[i]=e;}}

C++: #include<cstdio>
int sosu[1000000]={};
bool f[1000000]={false};
int main(){
  sosu[2] = 1;
  for(int i=3;i<=999999;i+=2){
    if(!f[i]){
      f[i] = true;
      if(i >= 1000) continue;
      for(int j=i*i;j<=999999;j+=i) f[j] = true;
      sosu[i] = 1;
    }
    sosu[i] += sosu[i-1];
    sosu[i+1] += sosu[i];
  }
  int n;
  while(scanf("%d",&n)!=EOF) printf("%d\n",sosu[n]);
  return(0);
}

C++: #include <stdio.h>
const int MAX_V = 1000000;
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(scanf("%d",&n)!=EOF){
    printf("%d\n",sum[n]);
  }
  return 0;
}

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

bool isPrime(unsigned int n);

int main()
{
    int n;
    while(cin>>n){
        int count=0;
        for(int i=0; i<=n; i++){
            if(isPrime(i)) count++;
        }
        cout << count << endl;
    }

    return 0;
}

bool isPrime(unsigned int n)
{
    if( n==0 || n==1 ) return false;
    if( n==2 ) return true;
    if(!(n%2)) return false;
    
    int rn = static_cast<int>(sqrt(n));
    
    bool flag = true;
    for(int i=3; i<=rn; i+=2){
        if(!(n%i)){
            flag = false;
            break;
                }
    }
    return flag;
}

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

using namespace std;
bool IsPrime[1000000];
vector<int> Prime;

void calc()
{
	for(int i = 0; i < 1000000; ++i)
	{
		IsPrime[i] = true;
	}
	for(int i = 2; i < 1000000; ++i)
	{
		if(IsPrime[i])
		{
			Prime.push_back(i);
			for(int j = 2 * i; j < 1000000; j += i)
			{
				IsPrime[j] = false;
			}
		}
	}
}

void solve()
{
	calc();
	int n;
	cin >> n;
	int count = 0;
	int a = Prime.size() - 1;
	while(Prime[count] <= n)
	{
		if(count >= a)
		{
			break;
		}
		count++;
	}
	cout << count << endl;
}

int main()
{
	solve();
	return(0);
}

C++: #include <bits/stdc++.h>
using  namespace std;
#define ll    long long int
#define pai     pair<int,int>
#define vi      vector<int>
#define vd      vector<double>
#define mpi     map<int,int>
#define mps     map<string,int>
#define sti     stack<int>
#define stc     stack<char>
#define sei     set<int>
#define ses     set<string>
#define multii  multiset<int>
#define MAX      10000010
bool prime[MAX];
void seive(){
memset(prime,true,sizeof(prime));
prime[0]=prime[1]=false;
for(int p=2;p*p<=MAX;p++){
       if(prime[p]){
    for(int i=p*p;i<=MAX;i+=p){
       prime[i]=false;
    }
}

}
}
int main(){
seive();
ll n;
int c=0;
while(cin>>n){
 for(int i=2;i<=n;i++){
   if(prime[i]){
    c++;
   }
 }
 cout<<c<<endl;
 c=0;
}

}

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

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

int main(){
	int n;
	
	while(cin >> n){
		int a=0;
		for(int i = 1; i <= n; ++i){
			if(prime(i))++a;
	}
		cout << a << '\n';
	}
}