Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	var primeCount int
	for sc.Scan() {
		input := nextInt()
		for i := 0; i <= input; i++ {
			if IsPrime(i) {
				primeCount++
			}
		}
		fmt.Printf("%d\n", primeCount)
		primeCount = 0
	}
}

func IsPrime(num int) bool {
	switch {
	case num == 2 || num == 5:
		return true
	case num < 2 || num%2 == 0 || num%5 == 0:
		return false
	}
	sqrt := math.Sqrt(float64(num))
	for i := 3; i <= int(sqrt); i += 2 {
		if num%i == 0 {
			return false
		}
	}
	return true
}

func next() string {
	return sc.Text()
}

func nextInt() int {
	a, _ := strconv.Atoi(next())
	return a
}

C++: #include <iostream>
#include <vector>
using namespace std;
vector <int> wow(100001);

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(){

	for(int i = 0; i < 100001; ++i){
	if(prime(i)){
					wow[i] = 1;
				}else{
					wow[i] = 2;
				}
	}


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

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

int main()
{
	int n, i, k, j, h;
	bool flag = false;

	for(i=0; i<30; i++)
	{
		cin >> n;
		h = 0;
		for(k = 2; k <= n; k++)
		{
			flag = false;
			for(j = 2; j <= sqrt(k); j++)
			{
				if(k % j == 0)
				{
					flag = true;
					break;
				}
			}
			if(flag == false)
				h++;
		}
		cout << h << endl;
	}
}

C++: #include<cstdio>
#include<algorithm>
using namespace std;
int prime[333333],cur;
bool isprime[1000000];
int main()
{
	for(int i=2;i<=1000000;i++)
		if(!isprime[i])
		{
			prime[cur++]=i;
			for(long long j=1LL*i*i;j<1000000LL;j+=1LL*i)
				isprime[j]=1;
		}
	int n;
	while(~scanf("%d",&n))
		printf("%d\n",lower_bound(prime,prime+cur,n)-prime+(!isprime[n]));
	return 0;
}

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

int main(){
    long maxy = 200000;
    int prime[maxy];
    //ツエツδ可トツスツテツネツスツづ個づ督づゥツつ「
    
    for(int i = 0; i < maxy; i++){
        prime[i] = 1;
    }
    prime[0] = prime[1] = 0;
    for(int i = 2; i <= maxy; i++){
        if(prime[i] == 1){
            for(int j=i*2; j <= maxy; j+=i){
                prime[j] = 0;
            }        
        }
    }
    int n;
    while(cin >> n){
        int ans = 0;
        for(int i=0; (i <= maxy && i <= n); i++){
            if(prime[i] == 1){
                ans++;
            }
        }
        cout << ans << endl;
    }
    return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
const int INF = 1000000000;
#define REP(i,s,n) for(int i=(int)(s);i<(int)(n);i++)
#define rep(i,n) REP(i, 0, n)
int prime[1000000];
bool is_prime[1000001];
int main(){
    int n;
    while(cin >> n){
        int p = 0;
        rep(i, n + 1) is_prime[i] = true;
        is_prime[0] = is_prime[1] = false;
        REP(i, 2, n + 1){
            if(is_prime[i]){
                prime[p++] = i;
                for(int j = i * 2; j <= n; j += i)
                  is_prime[j] = false;
            }
        }
        cout << p <<endl;
    }
    return 0;
}

C++: #include<stdio.h>
int isprime(int n){
  for(int i=2;i*i<=n;i++){
    if(n%i==0)return 0;
  }
  return 1;
}
int main(){
  int k,n,p,q;
  while(1){
    scanf("%d",&n);
    if(n==0)break;
    for(k=2;k<=n-2;k++){
      if(isprime(k)&&isprime(k+2)){
        p=k;
        q=k+2;
      }
    }
    printf("%d %d\n",p,q);
  }
  return 0;
}

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

int main() {
	int a;
	while (cin >> a) {
		int sosuu = 0;
		if (a > 1)sosuu++;
		if (a > 2)sosuu++;
		for (int b = 5; b <= a; b+=2) {
			for (int c = 3;; c+=2) {
				if (b%c==0)break;
				if ((c + 1)*(c + 1) > b) { sosuu++;   break; }
			}
		}
		cout << sosuu << endl;
	}
}

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

int main(){
	bool is_prime[1000000];
	
	is_prime[0]=is_prime[1]=false;
	for(int i=2;i<1000000;i++){
		is_prime[i]=true;
	}
	
	for(int i=2;i*i<1000000;i++){
		if(is_prime[i]){
			for(int j=i*i;j<1000000;j+=i){
				is_prime[j]=false;
				}
			}
		}
		
		int n_prime[1000000];
		
		int cnt=0;
		for(int i=0;i<1000000;i++){
			if(is_prime[i]){
				cnt++;
			}
			n_prime[i]=cnt;
		}
		
		while(true){
			int n;
			cin>>n;
			if(cin.eof())
				break;
				
			cout<<n_prime[n]<<endl;
		}
		
		return 0;
}

C++: 
#include <iostream>
#include <cmath>
#include <vector>
#include <string>
#include <map>
#include <algorithm>
#include <tuple>
#include <set>

using namespace std;
using ll = long long;
using ull = unsigned long long;
#define out(S) cout<<(S)<<endl;
#define REP(i,b) for(size_t i=0;i<(b);i++)
#define rREP(i,b) for(int i=(b)-1;i>=0;i--)
#define FOR(i,a,b) for(size_t i=(a);i<(b);i++)
#define rFOR(i,a,b) for(int i=(b)-1;i>=0;i--)
#define Foreach(item,collection) for(auto item:collection)
#define mod(i) ((i) % (ll)(1e9 + 7))
ll modpow(ll i, ll j) { ll tmp = 1; while (j) { if (j % 2)tmp = mod(tmp*i); i = mod(i*i); j /= 2; }return tmp; }
#define divmod(a,b) (mod(a * modpow((ll)b,(ll)(1e9 + 5))))
#define Yes out("Yes")
#define No out("No")
#define NO out("NO")
#define YES out("YES")
#define INF 1e10;
#define ShowAll(collection) for(auto i:collection){out(i);}
#define IfOut(condition,text) if((condition)){out(text);return 0;}
#define IfeOut(condition,itext,etext) if(condition){out(itext);return 0;}else{out(etext);return 0;}
#define Select(collection,condition,result) Foreach(i,collection){result+=condition;}
#define GetAuto(_N,vect) {cin>>N;vect.resize(N);REP(i,_N){cin>>vect[i];}}
void removeAt(string& s, int index) { s.erase(index, 1); }
template<typename T> void removeAt(vector<T>& v, int index) { v.erase(v.begin() + index); }
ll manhattanDistance(ll x1, ll y1, ll x2, ll y2) { return (abs(x2 - x1) + abs(y2 - y1)); }
#define beginend(vec) (vec).begin(),(vec).end()


int main() {
	ll n;
	vector<ll> ans;
	while (cin>>n)
	{
		int prime;
		double sqrtmax = sqrt(n);
		vector<ll> primeVec, searchVec;
		FOR(i, 2, n+1) searchVec.push_back(i);
		do
		{
			prime = searchVec[0];
			primeVec.push_back(prime);
			auto itr = remove_if(beginend(searchVec), [=](ll x)->bool {return x%prime == 0; });
			searchVec.erase(itr, searchVec.end());
		} while (prime<sqrtmax);
		ans.push_back(primeVec.size() + searchVec.size());
	}
	Foreach(i, ans) out(i);
	return 0;
}

C++: #include<stdio.h>
#include<math.h>
int gurobal[999999]={0};
int main(void)
{
	int n=0;
	int i=0,j=0;
	int heihoukon=0;
	int count=0;
	while(scanf("%d",&n)!=EOF)
	{
		count=0;
		gurobal[1]=0;
		for (i=2;i<=n;i++)
		{
			gurobal[i]=1;
		}
		(int)heihoukon=sqrt((double)n);
		for (i=2;i<=heihoukon;i++)
		{
			if (gurobal[i]!=0)
			{
				for (j=i*2;j<=n;j+=i)
				{
					gurobal[j]=0;
				}
			}
		}
		for (i=2;i<=n;i++)
		{
			if (gurobal[i]!=0)
			{
				count++;
			}
		}
		printf("%d\n",count);
	}

	return 0;
}