Go: package main

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

func main() {
	scanner := bufio.NewScanner(os.Stdin)

	for scanner.Scan() {
		num, _ := strconv.Atoi(scanner.Text())
		flag := false
		count := 0

		for i := 3; i <= num; i += 2 { //1を除く入力値以下の数字
			for j := 3; j < i; j += 2 {
				if i%j == 0 {
					flag = true
					//fmt.Println(i, "/", j)
					break
				}
			}
			if !flag {
				count++
			} else {
				flag = false
			}
		}
		fmt.Println(count + 1)
	}
}

C++: import sys


def prime(x):
    for i in p_list:
        if x % i == 0:
            return 0
    return 1


if __name__ == "__main__":
    a = 10000
    i = 2
    p_list = []
    while i < a:
        if prime(i):
            p_list.append(i)
        i += 1
    print(p_list)

    for i in sys.stdin:
        n = int(i)
        while 1:
            if n not in p_list:
                n -= 1
            else:
                print(p_list.index(n) + 1)

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

using namespace std;
static const int MAXV = 1000000;

int sosuu_list[MAXV];

void make_sosuu_list() {
  for(int i=0;i<MAXV;i++) sosuu_list[i] = 1;
  sosuu_list[0] = sosuu_list[1] = 0;
  for(int i=2;i<MAXV;i++) {
    if(sosuu_list[i]) {
      for(int j=i+i;j<MAXV;j+=i) sosuu_list[j] = 0;
    }
  }
  for(int i=1;i<MAXV;i++)sosuu_list[i]+=sosuu_list[i-1];
}

int main() {
  int n;
  make_sosuu_list();
  while(!feof(stdin)) {
    scanf("%d", &n);
    if(feof(stdin)) break;
    printf("%d\n", sosuu_list[n-1]);
  }
}

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

using namespace std;

bool p[1000000];

void primes() {
    for (int i = 0; i < 1000000; i++) {
        p[i] = true;
    }

    p[0] = p[1] = false;

    for (int i = 2; i < sqrt(999999); i++) {
        if (p[i]) {
            for (int j = i * 2; j < 1000000; j += i) {
                p[j] = false;
            }
        }
    }
}

int main() {
    primes();

    int n;
    while (cin >> n) {
        int ans = 0;
        for (int i = 2; i <= n; i++) {
            if (p[i]) {
                ans += 1;
            }
        }

        cout << ans << endl;
    }

    return 0;
}

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

bool isPrime(int x) {
    if (x==2) {
        return true;
    }
    if (x<2 || 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;
    int cnt;
    while (scanf("%d",&n)!=EOF) {
        cnt=0;
        for (int i=0; i<=n; i++) {
            if (isPrime(i)) {
                cnt++;
            }
        }
        printf("%d\n",cnt);
    }
    return 0;
}

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

const int MAX_N=1000000;
int prime[MAX_N+1];
bool isPrime[MAX_N+1];

int main(int N){
	int count=0;
	while(cin>>N){
		for(int i=0;i<=N;i++)
		isPrime[i]=true;
	isPrime[0]=isPrime[1]=false;
	for(int i=2;i<=N;i++){
		if(isPrime[i]){
			prime[count++]=i;
			for(int j=2*i;j<=N;j+=i)
				isPrime[j]=false;
		}
	}
	return count;
	cout<<count<<endl;
}
	
	}

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

int main()
{
 bool prime[1000000];
 for(int i=0; i<1000000; i++) prime[i] = true;

 prime[0] = prime[1] = false;
 for(int i=0; i<1000000; i++)
  if(prime[i]){
   for(int j=i; j<1000000; j += i)
    prime[j] = false;
  }

 int n;
 while(cin >> n){
  int c = 0;
  for(int i=0; i<=n; i++)
   if(prime[i]) c++;
  
  cout << c << '\n';
 }

 return 0;
}

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

using namespace std;
const int max_d = 30;

int sieve(int n) {
    int res = 0;
    bool prime[n + 1];
    for(int i = 0; i <= n; i++) {
        prime[i] = true;
    }
    for(int i = 2; i <= n ; i++) {
        if(prime[i]) {
            res++;
            for(int j = 2 * i; j <= n; j += i) {
                prime[j] = false;
            }
        }
    }
    return res;
}

int main() {
    int n;
    int dc = 0;
    while(cin >> n) {
        dc++;
        if(dc > max_d) break;
        cout << sieve(n) << endl;
    }
    return 0;
}

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

using namespace std;

#define MAX_N 10000000

int sosuu[MAX_N+1];

int main() {
	for (int i = 0; i < MAX_N; ++i)
		sosuu[i] = 1;

	sosuu[0] = sosuu[1] = 0;

	for (int i = 2; i < MAX_N; ++i)
		if (sosuu[i])
			for (int j = 2*i; j < MAX_N; j += i)
				sosuu[j] = 0;

	for (int i = 0; i < MAX_N; ++i)
		sosuu[i+1] += sosuu[i];

	int n;
	while ( scanf("%d", &n) != EOF )
		cout << sosuu[n] << endl;
}

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

int main(){
	int i=0,k=2,source[30];
	while(cin >> source[i]) i++;
	int m = max(source[0],source[i]);
	bool hurui[1000000];
	memset(hurui+2,1,sizeof(hurui));
	for (int j=2;j<=m;j++,k=2)
	{
		if (!hurui[j]) continue;
		while(j*k < m)hurui[j*k]=false,k++;
	}
	for (int j=0;j<i;j++){
		int count=0;
		for (k=2;k<=source[j];k++) count += !!hurui[k];
		cout << count << endl;
	}
}

C++: #include <iostream>
#include <math.h>
#define MAX 1000
using namespace std;

int main(){
	int n;
	int prime[MAX],i,j;
	int count;
	bool flag;

	while((cin >> n)!= NULL){
		count = 0;
		for(i = 3; i <= n; i+=2){
			flag = true;
			for(j = 0; j < count; j++){
				if(i % prime[j] == 0){
					flag = false;
					break;
				}
			}
			if(flag == true){
				prime[count++] = i;
			}
		}
		if(n > 1)	count++; //2
		cout << count << endl;
		//delete[] prime;
	}
	return 0;
}

C++: #include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <limits>
#include <map>
#include <string>
using namespace std;

bool is_prime(int n){
for(int i=2;i*i<=n;i++){
if(n % i == 0) return false;
}
return n!=1;
}

int main(){
int n;


while(~scanf("%d",&n)){
int* table =  new int[n+1];
fill((int*)table,(int*)table+sizeof(table),1);
table[0]=0;
table[1]=0;

for(int i=2;i*i<=n;i++){
if(!is_prime(i)) {
for(int j=i;j<=n;j*=i){
table[j] = 0;
}
}
}
printf("%d\n",count(table,table+1000000,1));
}
}

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

using namespace std;

int N,ans=1;

int IsPrime(int p) {
	double n = sqrt(p);
	for (int i = 3; i <= n; i++) {
		if (p%i == 0)return 0;
	}
	return 1;
}

int main() {
	while (cin >> N) {
		//double n = sqrt(N);
		for (int i = 3; i <= N; i = i + 2) {
			ans += IsPrime(i);
		}
		cout << ans << endl;
		ans = 1;
	}
	return 0;


}

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

map<int, bool> memo_of_prime;

bool check_prime_number (int n) {
    if ( n <= 1 ) return false;
    if (memo_of_prime.find(n) != memo_of_prime.end()) {
        return memo_of_prime[n];
    }

    for (int i = 2; i <= sqrt(n); ++i) {
        if (n % i == 0) return memo_of_prime[n] = false;
    }

    return memo_of_prime[n] = true;
}

int count_prime_number (int n) {

    int count = 0;
    if (n == 1) return 0;
    for (int i = 2; i <= n; ++i) {
        if (check_prime_number(i)) {
            count++;
        }
    }

    return count;
}

int main() {
    int n;
    while (cin >> n) {
        cout << count_prime_number(n);
    }
    return 0;
}

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

int main(){
	int x=100000;
	bool isPrime[x];
	isPrime[0]=isPrime[1]=false;
	for(int i=2; i<x; i++)
		isPrime[i]=true;
	
	for(int i=2; i*i<x; i++){
		if(isPrime[i]){
			for(int j=i*i; j<x;j+=i)
				isPrime[j]=false;
		}
	}

	int nPrime[x];
	int cnt=0;

	for(int i=0; i<x; i++){
		if(isPrime[i])
			cnt++;

		nPrime[i]=cnt;
	}

	while(true){
		int n;
		cin >> n;

		if(cin.eof())
			break;

		cout << nPrime[n] << endl;
	}
	
	return 0;
}