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

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

using namespace std;

int main() {
    int MAX = 1000000;
    int lim = sqrt(MAX);
    vector<int> is_prime(MAX, 1);
    is_prime[0] = is_prime[1] = 0;
    for(int i = 2; i < MAX; ++i) {
        if(is_prime[i])
            if(i < lim) 
                for(int j = i*i; j < MAX; j += i) is_prime[j] = 0;
    }

    vector<int> primes;
    for(int i = 0; i < MAX; ++i)
        if(is_prime[i]) primes.push_back(i);

    int n;
    while(cin >> n) 
        cout << upper_bound(primes.begin(), primes.end(), n)-primes.begin() << endl;
    return 0;
}

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

const int MX = 1000000;
bool prime[MX];
int cnt[MX];

int main() {
    for (int i=0; i<MX; ++i) prime[i] = true;
    prime[0] = prime[1] = false;
    for (int i=2; i<MX; ++i) {
        if ( ! prime[i]) continue;
        for (int j=i; (long long)i*j<MX; ++j) {
            prime[i*j] = false;
        }
    }
    cnt[0] = 0;
    for (int i=1; i<MX; ++i) {
        cnt[i] = cnt[i-1] + prime[i];
    }
    int N;
    while (cin >> N) {
        cout << cnt[N] << endl;
    }
    return 0;
}

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

using namespace std;

int main() {
  int n;
  
  while(1) {
    cin >> n;
    if(cin.eof()) {
      break;
    }
    int m = (int)sqrt(n);
    
    vector<bool> h(n+1, true);
    h[0] = false;
    h[1] = false;
    
    for(int i = 2; i <= m; i++) {
      if(h.at(i)) {
	for(int s = 2; s*i < h.size(); s++) {
	  if(h.at(s*i))
	    h[s*i] = false;
	}
      }
    }
    
    int ans=0;
    for(auto f : h) {
      if(f)
	ans++;
    }
    cout << ans << endl;
  }
  
  return 0;
}

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

using namespace std;

int solist[999999];

int main(int argc, char* argv[])
{
	int num;
	int count;
	int i, j;
	int val;

	count = 8;
	solist[0] = 2;
	solist[1] = 3;
	solist[2] = 5;
	solist[3] = 7;
	solist[4] = 11;
	solist[5] = 13;
	solist[6] = 17;
	solist[7] = 19;

	for (num = 3; num < 1000000; num += 2) {
		for (i = 0; i < count; i++) {
			if (num % solist[i] == 0) break;
		}
		if (i == count) solist[count++] = num;
	}

	while (cin >> val) {
		for (i = 0; i < count; i++) {
			if (solist[i] > val) {
				cout << i + 1 << endl;
				break;
			}
		}
		if (i == count) cout << count << endl;
	}
	
	return 0;
}

C++: #include <iostream>
#include <cstdio>
using namespace std;
const int MAX_N = 1000000; // いくつまで調べるか
bool prime[MAX_N + 1]; // 添字が素数ならtrue、それ以外はfalse
void sieve()
{
  prime[0] = prime[1] = false; // 例外処理
  // 配列prime の初期化
  for(int i = 2; i <= MAX_N; ++i)
    prime[i] = true;
  // 素数でないものを調べる
  for(int i = 2; i <= MAX_N; ++i)
    if(prime[i])
      for(int j = i * 2; j <= MAX_N; j += i)
	prime[j] = false;
	}

int is_prime(int n)
{
  return prime[n];
  }

/*bool is_prime(int n)
{
  if(n == 1)return false;
  for(int i = 2; i < n; ++i)
    if(n % i == 0)return false;
  return true;
  }
*/
int main()
{
  sieve();
  int a;
  while(cin >> a){
    //cin >> a;
    //if(a==EOF)return 0;
    int count=0;
    //int tmp;
    for(int i=1;i<=a;i++){
      if(is_prime(i))count++;
    }
    printf("%d\n",count);
  }
}

C++: #include<iostream>
using namespace std;
#define L long long int
int main(){
int n,ans,i,j;
while(cin>>n){
	ans=0;
	for(i=2;i<=n;i++){//search for 2 to n
		for(j=2;j<=i;j++)if(i%j==0)break;
		if(j==i)ans++;
	}
	cout<<ans<<endl;
}
return 0;
}

C++: #include <iostream>
#include <cmath>
#include <algorithm>
#include <map>
#include <string>
#include <vector>
#define FOR(i, a, b)  for(int i=a; i<b; ++i)
#define rFOR(i, a, b) for(int i=a-1; i>=b; --i)
#define REP(i, n) for(int i=0; i<n; ++i)
#define rREP(i, n) for(int i=n-1; i>=0; --i)
#define all(a)  a.begin(), a.end()
using namespace std;

int main(){
  int n;
  while(cin >> n){
    int ans=0;
    vector<int> vec;
    vec.push_back(2);
    for(int i = 3; i<=n; i+=2){
      bool isPrimeN = true;
      for(int j=3; j<=floor(sqrt(i)); j++){
        if(i%j==0){
          isPrimeN = false;
          break;
        }
      }
      if(isPrimeN){
        vec.push_back(i);
      }
    }
    cout << vec.size() << endl;;
  }
  return 0;
}

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




int main(){


	int Nmax = 1000000;
	bool isp[Nmax]={};
	int count[Nmax]={};

	isp[2] = true;
	for(int i=3;i<Nmax;i+=2){
		isp[i] = true;
	}

	for(int i=3;i<Nmax;i+=2){
		if(isp[i]){
			int j = i*2;
			while(j<Nmax){
				isp[j] = false;
				j += i;
			}
		}
	}

	for(int i=1;i<Nmax;i++){
		count[i] = count[i-1] + isp[i];
	}

	int n;
	while(cin >> n){
		cout << count[n] << endl;
	}

}

C++: #include<stdio.h>

int main(void){

	int sosuu[1000000],kazu,n,i,j;

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

		kazu=n-1;

		for(i=2;i<=n;i++){
			sosuu[i]=1;
		}
		sosuu[0]=0;sosuu[1]=0;

		for(i=2;i<=n;i++){
			if(sosuu[i]==1){
				for(j=i*2;j<=n;j+=i){
					if(sosuu[j]==1){
						kazu--;
						sosuu[j]=0;
					}
				}
			}
		}

		printf("%d\n",kazu);
		
	}

	return 0;
}

C++: #include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
vector<int> prime;
void init(){
	for(int i=2;i<999999;i++){
		bool isprime=true;
		for(vector<int>::iterator p(prime.begin());p!=prime.end();p++){
			if(i%*p==0){
				isprime=false;
				break;
			}
		}
		if(isprime){
			prime.push_back(i);
		}
	}
}
int main(){
	int n;
	init();
	while(cin>>n){
		cout<<lower_bound(prime.begin(),prime.end(),n)-prime.begin()+1<<endl;
	}
	return 0;
}

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

#define N 1000000L
bool num[N];

void furui() {
	long int i;
	int divide;
	int rt = 1000;
	for (i = 0L; i < N; i++) {
		num[i] = true;
	}
	num[0] = false;
	num[1] = false;
	for (divide = 2; divide <= rt; divide++) {
		if (num[divide] == false) continue;
		for (i = (long int)divide; i < N; i += divide) {
			num[i] = false;
		}
		num[divide] = true;
	}
}

int main(void) {
	int c, i, j;
	long int k;
	char buf[7];
	long int n;
	long int count;

	furui();

	for (j = 0; j < 30; j++) {
		for (i = 0; ; i++) {
			c = getchar();
			if (c == EOF || c == '\n') {
				buf[i] = '\0';
				break;
			}
			buf[i] = c;
		}
		n = atol(buf);
		count = 0L;
		for (k = 1L; k <= n; k++) {
			if (num[k] == true) count++;
		}
		std::cout << count << std::endl;
		if (c == EOF) break;
	}
	return 0;
}

C++: #include<stdio.h>
int main(void)
{
	int a,x,i;
	while((scanf("%d",&a))!=EOF){
		x=0;
		for(i=0;i<a;i++){
			if((a-i)==1){
			}
			else if((a-i)%2!=0&&(a-i)%3!=0&&(a-i)%5!=0&&(a-i)%7!=0){
				x++;
			}
			else if((a-i)==2||(a-i)==3||(a-i)==5||(a-i)==7){
				x++;
			}
		}
		printf("%d\n",x);
	}
	return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<cstring>
int cnt,n,prime[1000000];
using namespace std;

void getprime()
{
	/*int tt=0,flag;
	prime[tt++]=2;
	for(int i=3;i<=1000000;i++) 
	{
		flag=1;
		for(int j=0;prime[j]*prime[j]<=i;j++)
		{
			if(i%prime[j]==0)
			{	
			    flag=0;
		     	break;
			}
		}
		if(flag==1) prime[tt++]=i;
	}*/
	memset(prime,0,sizeof(prime));  
    for(int i = 2; i<=1000000; i++)  
    {  
        if (!prime[i]) prime[++prime[0]] = i;  
        for(int j = 1;j<=prime[0] &&prime[j] < 1000000/i;j++ )  
        {  
            prime[prime[j]*i] = 1;  
            if(i%prime[j]==0) break;  
        }  
    } 
}

int main()
{	getprime();
	while(scanf("%d",&n))
	{
       for(int i=1;;i++){
       	if(prime[i]>n){
       		printf("%d\n",i-1);
       		break; 
		   }
	   }
    }
}