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++: /// http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=0009
/// http://www.tsg.ne.jp/TT/tsg/c/mansaku/P01.html
#include<iostream>
#include<cmath>
using namespace std;

int main(){
    int n;
    while(~scanf("%d", &n)){
        int cnt = 0;
        for(int i = 3; i<=n; i+=2){
            int k= 0;
            for(int j=3; j<sqrt(i); j+=2){
                if(i%j==0){
                    k=1;
                    break;
                }
            }
            if(k==0) cnt++;
        }
        printf("%d\n", cnt);

    }

    return 0;
}

C++: #include <cstdio>

const int NUM = 1000000;
bool prime[NUM];
int primeNum[NUM];

int main(){

  prime[0] = false;prime[1] = false;
  for(int i=2;i<NUM;i++)prime[i] = true;
	for(int i=2;i*i<NUM;){
		for(int k=2;i*k<NUM;k++)
			prime[i*k]=false;
		do{i++;}while(!prime[i]);
	}
	
	for(int i=0,c=0;i<NUM;i++){
		prime[i]&&c++;
		primeNum[i] = c;
	}
	for(int n;scanf("%d",&n)+1;printf("%d\n",primeNum[n]));
}

C++: #include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <string>
#include <cctype>
#include <stack>
#include <queue>
#include <vector>
#include <cstdlib>

using namespace std;

int main(void){
	
	int i, j, n, count = 0;
	bool arr[99999] = {false};

	while (cin >> n) {

		for (i = 2; i <= n; i++) {
			arr[i] = true;
		}

		for (i = 2; i < sqrt(n); i++) {
			if(arr[i])
				for (j = 0; i*(j + 2) <= n; j++) {
					arr[i*(j + 2)] = false;
				}
		}

		for (i = 2; i <= n; i++) {
			if (arr[i] == true)
				count++;
		}

		cout << count << endl;
		count = 0;

	}

	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long LL;
#define REP(i,N) for(int i = 0; i < (int)N; i++)

int main(){
	int max = 1000100;
	bool prime[1000100];
	int dp[1000100];
	int ret=0;
	memset(prime,true,sizeof(prime));
	memset(dp,0,sizeof(dp));
	int hako[30];
	memset(hako,0,sizeof(hako));
	prime[0] = prime[1] = false;
	REP(i,30) cin >> hako[i];
	for(int i=2;i<sqrt(max);i++)for(int j=i*2;j<max;j=j+i){
		if(prime[i])prime[j] = false;
	}
	for(int i=1; i<max ;i++){
		dp[i]=dp[i-1];
		if(prime[i])dp[i]++;
	}
	REP(i,30)	cout << dp[hako[i]] << endl;
	return 0;
}

C++: #include <stdio.h>

int main() {
    int i, k, v,scan,an;
    int MAX_V = 1000000;
    int count[MAX_V];
    int sum[MAX_V];
    int prime[MAX_V];
    for(i = 2; i <= MAX_V; i++) {
        prime[i] = 1;
    }
    for(i = 2; i*i <= MAX_V; i++) {
        if(prime[i]) {
            for(k = 2 * i; k <= MAX_V; k += i) {
                prime[k] = 0;   
            } 
        }
    }
    count[0]=0;
    for(i=1;i<=MAX_V;i++){
        count[i]= count[i-1]+prime[i];
    }
    while(scanf("%d",&scan)!= EOF){
        an=count[scan]-count[0];
        printf("%d\n",an);
    }
    return 0; 
}

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

using namespace std;

bool isprime(int n)
{
	for (int i = 2; i <= sqrt(n); i++)
	{
		if (n % i == 0) return false;
	}

	return true;
}

int main()
{
	static int N[31];
	static int C[31];

	for (int i = 0; i < 3; i++)
	{
		for (int j = 2; j <= N[i]; j++)
		{
			if (isprime(j) == true)
			{
				C[i] += 1;
			}
		}
	}

	for (int i = 0; i < 31; i++)
	{
		cout << C[i] << endl;
	}

	return 0;
}

C++: #include<iostream>
using namespace std;
#define rep(i,a) for(int i = 0 ; i < a ; i ++)
#define loop(i,a,b) for(int i = a ; i < b ; i ++)
const int M = (1e6);
bool p[M];
short cnt[M];
void isPrime(){
  rep(i,M)p[i] = true;
  p[0] = p[1] = false;
  cnt[0] = cnt[1] = 0;
  loop(i,2,M){
    cnt[i] = cnt[i-1] + p[i];
    if(p[i])
      for(int j = i + i ; j < M ; j += i)
	p[j] = false;
  }
}
int main(void){
  isPrime();
  int n;
  while(cin>>n)cout<<cnt[n]<<endl;
}

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

int main()
{
	int n, i, i2;
	int prime[10000];
	for(int i=2; i<= 10000; i++){
		prime[i]=1;
		for(i2 = 2; i2<=sqrt(i); i2++)
		{
			if(i % i2 == 0)
			{
				prime[i] = 0;
				break;
			}
		}
	}
	while(1)
	{
		scanf ("%d",&n);
		if(n==0)
		{
			return 0;
		}
		for(i=n; 1; i--)
		{
			if(prime[i] && prime[i-2])
			{
				printf("%d %d\n",i-2 ,i);
				break;
			}
		}
	}
	return 0;
}

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

using namespace std;

bool a[1000000];

int main()
{
    memset(a, true, sizeof(a));

    for (int i = 2; i < 1000000; i += 1 + (i & 1)) {
        if (!a[i]) continue;
        for (int j = i*2; j < 1000000; j += i) a[j] = false;
    }

    int n;
    while (cin >> n) {
        int cnt = 0;
        for (int i = 2; i <= n; ++i) cnt += a[i];
        cout << cnt << endl;
    }
}

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

//2,3,5????????°????¨?????????????????????????????¨??????????????????¨??????


int main(void){

	int n,count_num,count_pnum;

	while (cin >> n){
		count_num = 0;
		for (int i = 2; i < n+1; i++){
		
			//2,3,5?????¨????????????
			if (i == 2 || i == 3 || i == 5){ count_num++; }

			if (i % 2 != 0 && i % 3 != 0 && i % 5 != 0){
			//2,3,5??§?????????????????¨???????´???°??§??????????????§?¨?????????????
				count_pnum = 0;
				for (int j = 7; j < i; j++){
					if (i%j != 0)count_pnum++;
				}if (count_pnum == i - 7)count_num++;


			}
		}cout << count_num << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <list>
#include <algorithm>

using namespace std;

int main() {
  int n;
  
  while(!cin.eof()) {
    cin >> n;
    list<int> search_list(n);
    list<int> prime_number;
    while(n!=1) {
      search_list.push_front(n);
      n--;
    }
    while(!search_list.empty()) {
      int m = search_list.front();
      prime_number.push_back(m);
      search_list.pop_front();
      list<int>::iterator it = search_list.begin();
      while(it!=search_list.end()) {
	if((*it % prime_number.front()) == 0){
	  it = search_list.erase(it);
	  continue;
	}
	it++;
      }				   
    }
    cout << prime_number.size()-1 << endl;
  }
}

C++: #include<iostream>
using namespace std;
int main(int argc,char* argv[]){
unsigned x[30];
int count=0;
for(int i=0;i<argc;I++){
cin>>x[i];
}
for(int i=0;i<argc(i++){
for(int j=1;j<x[i];j++){
if(x[i]%j!=0)
count++;
}
cout<<count<<endl;
count=0;
}
return 0;
}

C++: #include<iostream>

using namespace std;

int main(){
        int n;
        int d; //素数を入れるための添字
        bool count; //割りきれるかどうかの判定
        int number[400000]; //素数を入れておく
        number[0] = 2;
        
        
        while(cin >>n != 0){
                d = 0;
                for(int i = 3; i <= n; i++){
                        count = true;
                        for(int j = 0; j <= d; j++){
                                if(i % number[j] == 0){
                                        count = false;
                                }
                        }
                        if(count == true){
                                d++;
                                number[d] = i;
                        }
                }
        cout << d+1 << endl;
        }
}

C++: #include<stdio.h>
int main(){
	int a,b,c,n;
	c=0;
	scanf("%d",&n);
	for(a=1;a<=n;a++){
		for(b=2;b<=n;b++){
			if(a%b==0&&a!=b){
				break;
			}else if(a%b==0&&a==b){
				c++;
			}else{
			}
		}
	}
	printf("%d\n",c);
	return 0;
}