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 <iostream>
const int MAX_V = 1000000; // 100????????§?´???°??¨?????????
int prime[MAX_V+1]; // 1????´???°???0????´???°??§?????? 
int pcount[MAX_V+1];
int main() {
    int i, k,a,sum=0;
    for(i = 2; i <= MAX_V; i++) {
        prime[i] = 1; // 2??\????´???°??¨??????
    } 
    for(i = 2; i*i <= MAX_V; i++) {
        if(prime[i]) {
            for(k = 2 * i; k <= MAX_V; k += i) {
                prime[k] = 0; // ?´???°2?????\????????°?´???°??§?????? 
            }
            
        } 
        
    }
    pcount[0]=0;
    pcount[1]=0;
    for(i=2;i<MAX_V;i++){
        if(prime[i]){
            sum++;
        }
        pcount[i]=sum;
    }
    for(i=0;i<30;i++){
        scanf("%d",&a);
        printf("%d\n",pcount[a]);
    } 

    return 0; 
    
}

C++: #include <cctype>
#include <cmath>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#define REP(i, n) for(LL i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(LL i = m;i < n;i++)
#define FORR(i, m, n) for(int i = m;i >= n;i--)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define LL long long
#define pb(a) push_back(a)
#define INF 999999999
using namespace std;
typedef pair<int, int> P;
typedef pair<LL, LL> LP;
typedef pair<int, P> PP;
typedef pair<LL, LP> LPP;

int dy[]={0, 0, 1, -1, 0};
int dx[]={1, -1, 0, 0, 0};

#define MAX_NUM 999999
vector<LL> vecsosuu;

void makesosuu(void){
	FOR(i, 2 ,MAX_NUM){
		bool flag = true;
		if(i == 2){
			vecsosuu.pb(i);
		}else if(i % 2 != 0){
			int j = 3;
			while(j <= sqrt(i)){
				if(i % j == 0){
					flag = false;
				}
				j += 2;
			}
			if(flag){
				vecsosuu.pb(i);
			}
		}
	}
}

LL sosuu(LL n){
	LL i = 0;
	while(n >= vecsosuu[i]) ++i;

	return i;
}

int main(){
	LL n;
	makesosuu();
	while(cin >> n){
		cout << sosuu(n) << endl;	
	}
}

C++: #include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
const int MAXN = 1000000+100;
const int maxn=90000+5;
int prime[maxn];
bool p[MAXN];
int cnt;
void getPrime()
{
    memset(p,true,sizeof(p));
    p[0]=p[1]=false;
    for(int i=2;i<=MAXN;i++)
       if(p[i]) {
       	prime[cnt++]=i;
       	for(int j=i*i;j<=MAXN;j+=i)
       	    p[j]=false;
	   }
}
int main()
{
	cnt=0;
	getPrime();
	/*int k=lower_bound(prime,prime+cnt,999999)-prime;
	for(int i=k;i>=0;i--)
	   if(prime[i]<999999) {
	   	printf("%d\n",i+1);
	   	break;
	   }*/
	int n;
	while(~scanf("%d",&n)){
		int k=lower_bound(prime,prime+cnt,n)-prime+1;
		printf("%d\n",k);
	}
	return 0;
}

C++: #include<iostream>
using namespace std;
bool p[1000000]={true,true};
int main(){
  for(int i=2; i<1000; i++)
    if(!p[i])
      for(int j=i*i; j<1000000; j+=i)
	p[j] = true;
  int n;
  while(cin >>n){
    int solve = 0;
    for(int i=0; i<=n; i++)
      if(!p[i])
	solve++;
    cout <<solve<<endl;
  }
  return 0;
}

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

int main(){

int i,j,n,count;

while(scanf("%d",&n) != EOF){

count=2;

if(n<5){
	count=n-1;
}else{
	for(i=5;i<n+1;i+=2){
		for(j=2;j<(int)sqrt(i)+1;j++)

			if(i%j==0){
				break;
			}else if(j==(int)sqrt(i))
				count+=1;
	}
}

printf("%d\n",count);

}
return 0;		
}

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

using namespace std;

const int MAX_N = 1000000;
bool prime[MAX_N];

void eratos() {
    for (int i = 0; i < MAX_N; ++i) prime[i] = true;
	prime[0] = false;
	prime[1] = false;
	for (int i = 2; i < sqrt(MAX_N); ++i) {
		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<cstdio>
#include<iostream>
#include<cstring>
using namespace std;
int n,m;
int prime(int n)
{
    for(int i=2;i*i<=n;i++)
        if(n%i==0) return 0;
    return 1;
}
int main()
{
   while(cin>>n)
   {
       int s=0;
       for(int i=2;i<=n;i++)
        if(prime(i)) s++;
       cout<<s<<endl;
   }
    return 0;
}

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

typedef long long ll;
 
ll mod_pow(ll x, ll n, ll mod){
  ll res = 1;
  while(n > 0){
    if(n & 1) res = res * x % mod;
    x = x*x % mod;
    n >>= 1;
  }
  return res;
}

ll mod_mul(ll x, ll n, ll mod){
  x %= mod;
  n %= mod;
  return x*n % mod;
}

bool millrr_rabin(ll n){
  if(n == 2) return true;
  if((n & 1) == 0 || n <= 1) return false;
  int a[] = {2, 3, 5, 7, 11, 13, 17, -1}, r;
  ll s = 0, d, x;
  d = n - 1;
  while((d & 1) == 0){
    s++;
    d = d >> 1;
  }
  
  for(int i = 0 ; a[i] != -1 && a[i] < n ; i++){
    if((x = mod_pow(a[i], d, n)) != 1){
      for(r = 0 ; r < s ; r++){
	if(x == n-1) break;
	x = mod_mul(x, x, n);
      }
      if(r == s) return false;
    }
  }
  return true;
}

int main(){
  
  int n;
  while(cin >> n){
    int ans = 0;
    for(int i = 2 ; i <= n ; i++){
      if(millrr_rabin(i)) ans++;
    }
    cout << ans << endl;
  }
  
  
  return 0;
}

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

using namespace std;

#define FOR(i,k,n) for(int i = (k); i < (n); i++)
#define REP(i,n) FOR(i,0,n)
#define INF 114514810
#define ELEM(array) (sizeof (array)/sizeof *(array))
#define MAX_N 100
#define SAFE_FREE(ptr) if( ptr != NULL ){ free(ptr); ptr = NULL; }
typedef unsigned int UINT;
#define ll long long
typedef pair<int, int> P;

bool prime[100005];
void solve()
{
	memset(prime, true, sizeof(prime));
	prime[0] = prime[1] = false;
	FOR(i, 2, (int)sqrt(100005))
	{
		if (prime[i] == true)
		{
			FOR(j, 2, (int)sqrt(100005)){
				if (prime[i*j] == true)prime[i*j] = false;
			}
		}
	}
}

int main()
{
	int n,ans=0;
	cin >> n;
	solve();
	REP(i, n+1){
		if (prime[i] == true){ 
			ans++; 
		} 
	}
	cout << ans << endl;
	return 0;
}

C++: #include <iostream>
using namespace std;
int main(){
  int NUM = 1000000;
  bool prime[NUM];
  prime[0] = false;prime[1] = false;
  for(int i=2;i<NUM;i++)prime[i] = true;
  for(int i=2;i*i<NUM;i++)
    for(int k=2;i*k<NUM;k++)
      prime[i*k]=false;
  int n,c=0,i=0;
  for(cin>>n;i<=n;i++)if(prime[i])c++;
  cout<<c<<endl;
  return 0; 
}

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

int main(){

  int n;

  while(cin>>n){

	int cnt=0;
	
	if(n <= 1){
	  cout<<0<<endl;
	  continue;
	}

	else
	  cnt++;
	
	for(int i=3;i<=n;i+=2){
	  int f = true;
	  for(int j=3;j<=sqrt(i);j+=2){
		if(i%j==0){
		  f = false;
		  break;
		}
	  }
	  if(f == true)
		cnt++;
	}

	cout<<cnt<<endl;
	
  }

  return 0;
  
}

C++: #include<iostream>

using namespace std;

int n;
int prime[1000000];
int checked[1000000];
int primesum[1000000];
int result[30];
int datasets;

int main()
{
	prime[0] = 0;
	prime[1] = 0;
	prime[2] = 1;
	primesum[0] = 0;
	primesum[1] = 0;
	primesum[2] = 1;

	for(int a = 3;a<1000000;a++)
	{
		prime[a] = 1;
	}
	
	for(int a = 3;a<1000;a++)
	{
		if(checked[a] == 0)
		{
			checked[a] = 1;
			for(int b = 2;b<a;b++)
			{
				if(a%b==0)
				{
					prime[a] = 0;
					break;
				}
			}
			if(prime[a] == 1)
			{
				for(int c = 2*a ;c<1000000;c+=a)
				{
					checked[c] = 1;
					prime[c] = 0;
				}
			}
		}
	}

	for(int a = 3;a<100;a++)
	{
		primesum[a] = primesum[a-1]+=prime[a];
	}
	
	while(cin >> n)
	{
		cout << primesum[n-1] << endl;
	}

	return 0;
}