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 <cstdio>

int main(){
	int n;
	static bool c[1000000]={true};
	static int t[1000000];//t[0] = 0;
	for(int i = 2;i < 1000000;i++){
		if(!c[i]){
		for(int j = 2;i*j < 1000000;j++){
			c[i*j] = true;
		}
		}
		t[i]=t[i-1]+!c[i];
	}
	/*for(int i = 1;i < 1000000;i++){
		if(c[i])t[i]=t[i-1];
		else t[i] = t[i-1]+1;
	}*/
	for(int n;scanf("%d", &n);n++){
		printf("%d\n", t[n]);
	}
	
	while(scanf("%d", &n)){
		printf("%d\n", t[n+1]);
	}
	return 0;
}

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

int main(){
    int n;
    int result;
    int i, j;
    bool flug = false;

    while(cin >> n){
        for(i=2; i<n; i++){
            for(j=2; j<(int)sqrt(i)+1; j++){
                if(i % j == 0){
                    flug = true;
                    break;
                }
            }
            if(!flug) result++;
            flug = false;
        }

        cout << result << endl;
        result = 0;
    }

    return 0;
}

C++: #include <iostream>

using namespace std;
int TestCase;



int main()
{
   while(cin >> TestCase)
   {
       int PrimeNumbers = 0;
       if(TestCase >= 2)
       {
           PrimeNumbers++;
       }
       for(int Sample = 3; Sample <= TestCase; Sample+=2)
       {
           if(Sample > 10)
           {
                if(Sample%3 != 0 && Sample%5 != 0 && Sample%7 != 0)
               {
                   PrimeNumbers++;
               }
           }
           else
           {
               if(Sample != 9)
               {
                   PrimeNumbers++;
               }
           }

       }
       cout << PrimeNumbers << endl;
   }

}

C++: #include<stdio.h>
const int MAX_V = 100;
int prime[MAX_V+1];
int main() {
  int i, k;
  for(i = 2; i <= MAX_V; i++) {
    prime[i] = 1;
  }
  for(i = 2; i*i <= MAX_V; i++) {
    if(prime[i]) {
      for(k = 2 * i; k <= MAX_V; k += i) {
        prime[k] = 0;
      }
    }
  }

  int right;
  int sum[MAX_V+1] = {0};
  sum[1] = 0;
  for(i = 1; i < MAX_V+1; i++){
    if(prime[i] == 1){
      sum[i + 1] = sum[i] + 1;
    }else{
      sum[i + 1] = sum[i];
    }
  }
  while(scanf("%d", &right) != EOF){
    printf("%d\n", sum[right + 1]);
  }
  return 0;
}

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 ++;
  }
 
  return 0;
}

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

#define REP(i, a, b) for (int i = (int)(a); i < (int)(b); ++i)
#define rep(i, n) REP (i, 0, n)
#define mset(a, n) memset(a, n, sizeof(a))

using namespace std;

const int MAX_N = 100000000;
bool prime[MAX_N];
void eratos() {
	rep(i, MAX_N) prime[i] = true;
	prime[0] = false;
	prime[1] = false;
	REP(i, 2, sqrt(MAX_N)) {
		if (prime[i]) {
			for (int j = 0; i * (j + 2) < MAX_N; ++j) {
				prime[i * (j + 2)] = false;
			}
		}
	}
}

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

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

int main()
{
	const size_t MAX = 1000000;

	bitset<MAX> primebox;
	primebox.set();
	primebox.reset(0);
	primebox.reset(1);

	for(int i = 2; i < MAX; i++)
	{
		if(!primebox.test(i))
			continue;

		for(int j = i*2; j < MAX; j += i)
			primebox.reset(j);
	}
		cout << "ok" << endl;

	size_t n;
	while(cin >> n)
	{
		int found;
		if(n < 2)
			found = 0;
		else if(n == 2)
			found = 1;
		else
		{
			found = 1;
			for(size_t i = 3; i <= n; i += 2)
				if(primebox.test(i))
					found++;
		}

		cout << found << endl;
	}

	return 0;
}

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

using namespace std;

int prime(int a){
    int sqt = sqrt(a);
    int array[a+1];
    array[1] = 1;
    array[2] = 0;
    array[3] = 0;
    if(a >= 4){
        for(int i = 2; i <= sqt; i++){
            for(int j = 2; j*i <= a; j++){
                array[i*j] = 1;
            }
        }
    }
    int count = 0;
    for(int i = 1; i <= a; i++){
        if(array[i] == 0) count++;
    }
    return count;
}

int main(){
    int n;
    while(cin >> n){
        cout << prime(n) << endl;
    }
}

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

int main(){

	const int N_MAX=1000000;
	int i,j,n[32],ni,soi,max;
	int so[N_MAX],count[N_MAX];
	
	for(i=0;i<N_MAX;i++)
		so[i]=1;
	ni=0;
	max=0;
	while(cin>>n[ni])
		if(n[ni++]>max)
			max=n[ni-1];
	so[0]=so[1]=0;
	for(soi=0,i=2;i<=1000;i++){
		if(so[i]==0)continue;
		for(j=i*2;j<=max;j+=i)
			so[j]=0;
	}
	for(i=2;i<=max;i++){
		count[i] = count[i-1]+so[i];
	}
	for(i=0;i<ni;i++)
		cout << count[n[i]] << endl;

	return 0;
}

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

int main()
{
	int n, i, j, k=1, wareru=0,cnt;
	long int *sosu = (long int*)malloc(sizeof(long int) * 1000000);
	sosu[0] = 2;
	for (i = 3; i < 1000000; i += 2)
	{
		wareru = 0;
		for (j = 3; j <= sqrt(i*1.0); j += 2)
		{
			if (i%j == 0)
			{
				wareru = 1;
				break;
			}
		}
		if (wareru == 0)
		{
			sosu[k++] = i;
		}
	}

	while (scanf("%d", &n) != EOF)
	{
		if (n == 1)
		{
			printf("0\n");
			continue;
		}
		if (n == 2)
		{
			printf("1\n");
			continue;
		}
		for (i = 0; sosu[i] <= n; i++);
		printf("%d\n", i);
	}
}

C++: #include<iostream>
#include<iomanip>
#include<algorithm>
#include<cassert>
#include<cctype>
#include<cmath>
#include<cstdio>
#include<cstring>
#include<functional>
#include<limits>
#include<list>
#include<map>
#include<numeric>
#include<set>
#include<stack>
#include<string>
#include<sstream>
#include<queue>
#include<vector>
using namespace std;

#define MOD 1000000007
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL

#define MAX 999999

//?´???°??????
bool isPrime(int x) {
	if (x < 2)return 0;
	else if (x == 2)return 1;
	if (x % 2 == 0)return 0;
	for (int i = 3; i*i <= x; i += 2)
		if (x%i == 0)return 0;
	return 1;
}

//??¨????????????????????????
//n??\????????£??´??°????´???°???????????°???????????????is_prime[]???????´?
void eratos(int n, bool is_prime[]) {
	fill(is_prime, is_prime + n, true);
	is_prime[0] = is_prime[1] = false;
	for (int i = 2; i*i <= n; i++) {
		if (is_prime[i]) {
			int j = i + i;
			while (j <= n) {
				is_prime[j] = false;
				j = j + i;
			}
		}
	}
}

int main() {
	bool is_prime[MAX];
	eratos(MAX, is_prime);

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

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

using namespace std;

#define MAX_VALUE 1000000

int get_numprime( int num );

int main(void)
{
	int num;

	while(1)
	{
		cin >> num;
		if( cin.eof() ) break;
		int result = get_numprime( num );
		cout << result << endl;
	}

	return 0;
}

int get_numprime( int num )
{
	int numprime = 1; // 2はカウント済み

	bool erat[MAX_VALUE];

	for( int i = 0 ; i < MAX_VALUE ; i++ )
	{
		erat[MAX_VALUE] = true;
	}
	erat[0] = false; erat[1] = false;
	for( int i = 2 ; i <= num ; i++ )
	{
		if( erat[i] )
		{
			for( int j = i*2 ; j <= num ; j+=i )
			{
				erat[j] = false;
			}
		}
	}
	for( int i = 3 ; i <= num ; i+=2 )
	{
		if( erat[i] ) numprime++;
	}

	return numprime;
}