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 <stdio.h>

int main(void){
int n;
while(scanf("%d",&n)!=EOF){
int count=0;
for(int m=2;m<=n;m++){
bool check=true;
for(int i=2;i<m;i++){
if(i!=m && m%i==0){
check=false;
}
}
if(check==true){
count++;
}
}
printf("%d",count);
}
return 0;
}

C++: #include <iostream>

int main() {
	int input;
	while (std::cin >> input) {

	int count = 0; // input??\???????????¶??°????????¨???????´???°???????¨?
	for (; input > 1; input--) {// input???1???????????????????????????????????¨???????????????
		/*
		 * 5
		 * 4
		 * 3
		 * 2
		 */
		bool fl = true;
		for (int i = 2; i < input; i++) // 1 < i < input ????????¶??????????????°????????§?????????
			/*
			 * 5 -> 2, 3, 4
			 * 4 -> 2, 3
			 * 3 -> 2
			 * 2 ->
			 */
			 // ?????????????????°????????¨??????????????????????????????
			if (!(input % i))
				fl = false;

		if (fl)
			count++;
	}

			std::cout << count << std::endl;
	}
}

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<iostream>
#include<stdio.h>
using namespace std;

bool p[1000010];
int input;

void erato(){
	fill(p, p+1000010, true);
	p[0] = p[1] = false;

	for(int i=0; i * i < 1000010; i++){
		if(!p[i]) continue;

		for(int j = i + i; j < 1000010; j += i){
			p[j] = false;
		}
	}
}

int main(){
	erato();
	int input;

	while(scanf("%d", &input) != EOF){
		int ans = 0;
		for(int i=0; i <= input; i++){
			if(p[i]) ans++;
		}
		cout << ans << endl;
	}
}

C++: //http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=0009

#include <algorithm>
#include <iostream>
#include <vector>
#include <cmath>
using namespace std;
vector<int> prime;

void Prime() {
	prime.emplace_back(2);
	for (int i = 3; i <= 999999; i += 2) {
		bool f = false;
		for (int j = 3; j <= sqrt(i); j += 2) {
			if (i%j == 0) {
				f = true;
				break;
			}
		}
		if (!f) prime.emplace_back(i);
	}
}

int main() {
	cin.tie(0); ios::sync_with_stdio(false);
	Prime();
	int n;
	while (cin >> n) {
		cout << upper_bound(prime.begin(), prime.end(), n) - prime.begin() << endl;
	}
}

C++: #include<iostream>
#include<vector>
using namespace std;
int main(){
    int n,k,sum,check;
    int t=1;
    vector<int> prime;
    prime.push_back(0);
    prime.push_back(0);
    for(int i=2;i<1000000;i++){
        prime.push_back(1);
    }
    while(t<1000000){
        for(int j=t+1;j<=1000000;j++){
            if(prime[j]==1){
                t=j;
                for(k=2*j;k<=1000000;k+=j){
                    prime[k]=0;
                }
                break;
            }
            else if(j==1000000){
                t=1000000;
            }
        }
    }
    while(cin>>n){
        sum=0;
        for(check=1;check<=n;check++){
            if(prime[check]==1){
                sum+=1;
            }
        }
        cout << sum << endl;
    }
    return 0;
}

C++: #include<iostream>

using namespace std;

int main(void){
	
	long int n,f,k,i;
	int count;
	bool chk;
	while(cin>>n){
		count=0;
		for(i=2;i<=n;i++){
		
			chk=0;
			f=i/2;
			k=2;
		
			while(f>=k){
				if((i%k)==0){
					chk=true;
					break;
				}
				k++;
			}
		
			if(chk==false){
				count++;
			}
		}
		cout<<count<<endl;
	}
}

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

int main()
{
	int n[30];
	int i=0;
	while(cin >> n[i]) i++;
		
	for (int k=0;k<i;k++){
		int s=1;
	
		for (int j=3;j<=n[k];j++){
				//judgement of prime number
				for(int m=2;m<j;m++){
					if(j%m==0){break;}
					if(m==j-1){s=s+1;}
				}
		}
	cout << s <<endl;
	}
	
    return 0;
}

C++: // BCC
#include <cstdio>
#include <algorithm>
#include <iostream>
#include <utility>
#include <vector>
#include <queue>

// GCC
// #include <bits/stdc++.h>

using namespace std;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rrep(i, n) for (int i = (int)(n) - 1; i >= 0; i--)
#define each(i, c) for (__typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define chmin(a, b) a = min(a, b)
#define chmax(a, b) a = max(a, b)
#define pb push_back
#define mp make_pair

typedef long long ll;

const int INF = 1 << 28;
const ll INFLL = 1ll << 56;


/**
 * ??¨????????????????????????
 * ?´???°??¨??¨?´???°?????????????????????
 */

const int SIEVE_SIZE = 2000000;

bool is_prime[SIEVE_SIZE];
vector<int> primes;

void eratosthenes()
{
	primes.clear();
	for (int i = 2; i < SIEVE_SIZE; i++) is_prime[i] = true;
	
	for (int i = 2; i * i < SIEVE_SIZE; i++){
		if (!is_prime[i]) continue;
		primes.pb(i);
		for (int j = i * i; j < SIEVE_SIZE; j += i){
			is_prime[j] = false;
		}
	}
}

int sum[1000000];

int main()
{
	eratosthenes();
	each(p, primes) sum[*p]++;
	rep(i, 999999) sum[i + 1] += sum[i];
	
	int n;
	while (~scanf("%d", &n)){
		printf("%d\n", sum[n]);
	}
}

C++: #include<stdio.h>
#define N 1000000

int main() {
	int ipt;
	int root;
	int cnt = 0;
	int A[N];
	for (int i = 1; i <= N; i++) {
		A[i] = i;
	}
	for (int pos = 2; pos*pos <= N; pos++) {
		root = A[pos];
		if (!root)continue;
		for (int k = 2; k*root <= N; k++) {
			A[k*root] = 0;
		}
	}

	while(scanf("%d",&ipt)==1) {
		for (int i = 2; i <= ipt; i++) {
			if (A[i] != 0)cnt++;
		}
		printf("%d\n", cnt);
		cnt = 0;
	}

	return 0;
}

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

using namespace std;

const int kSieveSize = 1000001;
bool sieve[kSieveSize] = {true, true};

int main()
{
  vector<int> primes;
  primes.reserve(kSieveSize);
  for (int i = 2; i * i < kSieveSize; ++i){
    if (sieve[i]) continue;
    primes.push_back(i);
    for (int j = i * i; j < kSieveSize; j += i){
      sieve[j] = true;
    }
  }
  for (int n; ~scanf("%d", &n); ){
    printf("%d\n", upper_bound(primes.begin(), primes.end(), n) - primes.begin());
  }

  return (0);
}