Go: package main
import (
	"fmt"
	"os"
	"strconv"
	"bufio"
	//"math"
)

var sc = bufio.NewScanner(os.Stdin)

func main(){
	for sc.Scan(){
		n := nextInt()
		fmt.Println(calc(n))
	}
}

func calc(n int)int{
	// エラトステネスで数え上げる
	// 最初から999999まで用意しておいて入力のたびO(1)で出すのとどっちが速いのかはわからん
	eratos := make([]bool, n+1)
	c := 0
	for i:=2; i<len(eratos); i++{
		if eratos[i] == false{
			c++
			for j:=i*i; j<len(eratos); j+=i{
				eratos[j]=true
			}
		}
	}
	return c
}

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

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

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[1000000];
   int count=0;
   for(int i=0;i<1000000;i++){
      if(is_prime[i])
         count++;
      n[i]=count;
   }
   while(true){
      int x;
      cin >> x;
      if(cin.eof())
         break;
      cout << n[x] << endl;
   }
   
   return 0;
}

C++: #include <string>
#include <vector>
#include <sstream>
#include <iostream>
#include <algorithm>
#include <map>
#include <list>
#include <set>
#include <numeric>
#include <queue>
#include <stack>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <cstring>
#include <climits>
#include <cfloat>
using namespace std;
typedef long long LL;
typedef pair<int,int> P;
const int up=1000*1000;
int n[up];
bool comp[up];
int main() {
	int r=0;
	for(LL i=2;i<up;i++){
		if(comp[i]==false){
			r++;
			for(LL j=i*i;j<up;j+=i){
				comp[j]=true;
			}
		}
		n[i]=r;
	}
	int k;
	while(cin >> k)
	{
		cout << n[k] << endl;
	}
}

C++: #include <stdio.h>

int IsPrime(int n);

int main(void){
    int n, cnt;
    int list[100000];

    cnt=0;
    for (long i=2; i<1000000; i++){
        if (i%10==0) list[i/10] = cnt;
        cnt += IsPrime(i);
    }
	printf("input : ");
    while (scanf("%d", &n) && n!=EOF){
        cnt = list[n/10];
        for (long i=1; i<n%10; i++){
            cnt += IsPrime(i);
        }
        printf("%d\n", cnt);
    }
    return 0;
}

inline int IsPrime(int n){
    int i;

    for (i=2; i*i<=n; i++){
        if (n%i==0) return 0;
    }
    return 1;
}

C++: #include<iostream>

using namespace std;

int main(){
    static bool not_prim[1000000];
    static int num_prim[1000000];
    for(int i=2;i<1000000;i++)
    {
        if(!not_prim[i])
        {
            for(int j=2;j*i<1000000;j++)
            {
                not_prim[i*j]=true;
            }
        }
        num_prim[i]=num_prim[i-1]+!not_prim[i];
    }
    int n;
    while(cin >> n){
    cout<<num_prim[n]<<endl;
    }
    return 0;
}

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

int main(void){
	int n;
	while(scanf("%d",&n)!=EOF){
		int count=0;
		std::vector<int> num;
		int max=2;
		for(int m=2;m<=n;m++){
			bool check=true;
			for(unsigned int i=0;i<count;i++){
				if(m!=num[i] && m%num[i]==0){
					check=false;
					if(max<m){
						max=m;
					}
					break;
				}
			}
			if(check==true){
				if(count>0){
					if(max>num[count-1]){
						for(int i=max+1;i<m;i++){
							if(i!=m && m%i==0){
								check=false;
								if(max<m){
									max=m;
								}
								break;
							}
						}
					}
					else{
						for(int i=num[count-1]+1;i<m;i++){
							if(i!=m && m%i==0){
								check=false;
								if(max<m){
									max=m;
								}
								break;
							}
						}
					}
				}
				else{
					for(int i=2;i<m;i++){
						if(i!=m && m%i==0){
							check=false;
							if(max<m){
								max=m;
							}
							break;
						}
					}
				}
			}
			if(check==true){
				num.push_back(m);
				count++;
			}
		}
		printf("%d\n",count);
	}
	return 0;
}

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

using namespace std;

int main() {

	int n;
	int *k;
	
	while (cin >> n) {
		int counter = 0;
		k = (int *)malloc(sizeof(int) * n);
		if (k == NULL) {
			exit(0);
		}
		k[0] = 1;
		for (int i = 1; i < n; i++) {
			k[i] = 0;
		}

		for (int i = 1; i < n; i++) {
			if (k[i] == 0) {
				for (int j = 1; j < i; j++) {
					if (k[j] == 0) {
						if ((i+1) % (j+1) == 0) {
							k[i] = 1;
							break;
						}
					}
				}
				if (k[i] == 0) {
					for (int t = 2; t < n/(i+1); t++) {
						k[(i+1)*t-1] = 1;
					}
				}
			}
		}

		for (int i = 0; i < n; i++) {
			if (k[i] == 0) {
				counter++;
			}
		}

		cout << counter << endl;

		free(k);
	}

	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
int n;
const int MAX=1e6+5;
int pri[MAX];
bool py[MAX];
int counts[MAX];
void isprime()
{
	memset(py,true,sizeof(py));
	memset(counts,0,sizeof(counts));
	int c=0;
	pri[0]=pri[1]=false;
	for(int i=2;i<=MAX;i++)
	{
		if(py[i])
		{
			counts[i]=++c;
			for(int j=2*i;j<=MAX;j+=i)
			py[j]=false;
		}
		else counts[i]=c;
	}
}
int main()
{
	isprime();
	while(cin>>n) cout<<counts[n]<<endl;
	return 0;
}

C++: #include <algorithm>
#include <bitset>
#include <cassert>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <unordered_set>
#include <unordered_map>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)
#define FOR(i,b,e) for (int i=(int)(b); i<(int)(e); i++)
#define ALL(x) (x).begin(), (x).end()

bool pr[1000000];

template <size_t N>
void psieve(bool (&p)[N]) {
  for (int i = 0; i < N; i++)
    p[i] = true;
  p[0] = p[1] = false;
  for (int i = 2; i * i < N; i++) {
    if (!p[i])
      continue;
    for (int j = i * i; j < N; j += i)
      p[j] = false;
  }
}

int solve(int n) {
  int r = 0;
  REP (i, n+1) r += pr[i];
  return r;
}

int main() {
  ios::sync_with_stdio(false);
  psieve(pr);
  int n;
  while (cin >> n) {
    cout << solve(n) << endl;
  }
  return 0;
}

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

using namespace std;

int x;
const int MAX_N = 1000000;
int prime[MAX_N];
bool is_prime[MAX_N + 1];

void sieve(int n){
	int p = 0;
	for (int i = 0; i <= n; i++) is_prime[i] = true;
	is_prime[0] = is_prime[1] = false;
	for (int i = 2; i <= n; i++){
		if (is_prime[i]){
			prime[p++] = i;
			for (int j = 2 * i; j <= n; j += i) is_prime[j] = false;
		}
	}
	cout << p << endl;
}

int main()
{
	while (scanf("%d",&x)!= EOF){
		sieve(x);
	}
	
	return 0;
}

C++: /* All men have limits. They learn what they are and learn
   not to exceed them, I ignore mine. -Batman           */
#include <bits/stdc++.h>
//#include <iostream>
//#include <Windows.h>
using namespace std;
bool prime(int n){
	if(n==0||n==1)return 0;
	else if(n==2)return 1;
	else
	for(int i=2;i<=sqrt(n);i++){
		if(n%i==0)return 0;
	}
	return 1;
	
}

int main(){int n;
while(cin>>n){int ans(0);
for(int i=1;i<=n;i++){
	if(prime(i))ans++;
}
	cout<<ans<<endl;

}

return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<cstdio>
using namespace std;
 
int main(){
int unti[1000001] = {0};
fill(unti,unti + 1000001,0);
for(int n = 2;n < 1000001;n++)
{
if(unti[n] == 0)
{
for(int a = 2;(a * n) < 1000001;a++)
{
    if((a * n  ) < 1000001 ) unti[a * n]++;
}
}
}
int b;
while(cin >> b )
{
int cnt = (count(unti,unti+b+1,0) - 2);
cout << cnt ;
    }
 
 
}

C++: #include <stdio.h>
  
int main(){
   long int e[30];
  
    int k = 0;
    while( scanf( "%d",&e[k] ) != EOF )k++;
  
    for( int i = 0;i < k;i++ ){
        int n = 10;
        int k = 1;
        while( true ){
            if( e[k] >= n ){
                k++;
                n *= 10;
            } else {
                break;
            }
        }
        printf( "%d\n",k );
    }
    return 0;
}