Go: package main

// https://ja.wikipedia.org/wiki/%E3%82%A8%E3%83%A9%E3%83%88%E3%82%B9%E3%83%86%E3%83%8D%E3%82%B9%E3%81%AE%E7%AF%A9

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() (int, error) {
	return strconv.Atoi(nextString())
}

func main() {
	scanner.Split(bufio.ScanWords)

	for {
		n, err := nextInt()
		if err != nil {
			break
		}
		maybePrime := make([]bool, n+1)
		for i := 0; i <= n; i++ {
			maybePrime[i] = true
		}

		maybePrime[0] = false
		maybePrime[1] = false
		for i := 2; i <= int(math.Sqrt(float64(n)))+1; i++ {
			j := i * 2
			for j <= n {
				maybePrime[j] = false
				j += i
			}
		}

		nPrimes := 0
		for _, p := range maybePrime {
			if p {
				nPrimes++
			}
		}
		// fmt.Println(maybePrime)
		fmt.Println(nPrimes)
	}
}

C++: #include <stdio.h>
#include <math.h>
#include <stdlib.h>
#define N 999999
bool num[N];

void furui(){
  long int i;
  int divide;
  int rt = (int) sqrt(N);
  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 = 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] == 1) count++;
    }
    printf("%ld\n", count);
    if(c == EOF) break;
  }
  return 0;
}

C++: #include <iostream>
using namespace std;
// エラトステネスのふるい
int main() {
  int num = 1000;
  int n, prime[num];
  while (cin >> n) {
    for (int i = 0; i < num; i++) {
      prime[i] = 0;
    }
    prime[0] = 1;
    prime[1] = 1;
    
    int count = 0;
    for (int i = 0; i <= n; i++) {
      if (prime[i] == 0) {
	count++;
	for (int j = i; j <= n; j += i) {
	  prime[j] = 1;
	}
      }
    }
    cout << count << endl;
  }
  return 0;
}

C++: #include <iostream>

using namespace std;
int main(){
	bool n[1000000];
	for(int i=0; i<1000000; i++){ n[i]=1; }
	n[0]=0;
	n[1]=0;

	for(int i=2; i<1000000; i++){
		if(n[i]==0){ continue; }
		if(n[i]==1){
			for(int j=i*2; j<1000000; j+=i){ n[j]=0;	}
		}
	}

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

	return 0;
}

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

int main(){
int n,a;
int pri[999999];
pri[0]=2;
int ans=1;
while(cin>>n){
for(int i=3;i<n+1;i++){
for(int j=0;j<ans&&pri[j]*pri[j]<i;j++){
a=i%pri[j];
if(a==0){
break;}
if(j==ans-1){
pri[ans]=i;
ans++;
break;
}
}}
cout<<ans<<endl;}}

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

int main(void) {
    int N=999999;
    int p[N] , s[N];
    p[0]=0 , s[0]=0;
    for (int i=1; i<N; i++){
        p[i] = 1;
        s[i] = 0;
    }
    for (int i=1; i<N; i++){
        if (p[i]==1){
            int n=i+1 , m=2*n;
            while (m<=N){
                p[m-1] = 0;
                m += n;
            }
        }
    }
    
    for (int i=1; i<N; i++){
        s[i] = s[i-1]+p[i];
    }
    int n;
    while (1){
        cin >> n;
        if (cin.eof())
            break;
        cout << s[n-1] << endl;
    }
    return 0;
}

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

int main() {
int b[1000000];
for (int k=0;k<1000001;k++){
	b[k]=0;
}
b[2]=1;
b[3]=1;
b[4]=0;
b[5]=1;
b[6]=0;
b[7]=1;

for (int p=11;p<1000001;p+=2){
int i,a=0 ;
	i=3;
	while(i<sqrt(p)){
		if(p%i==0){
			break;
	}i=i+2;
	}
	if (i>=sqrt(p)){
		a=1;
	}
	b[p]=a;
}
int c[1000000];
for (int l=0;l<1000001;l++){
	c[l+1]=c[l]+b[l+1];
}
int r;
while(cin>>r){
	cout<<c[r]<<endl;
}
	return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
	int N,count=0,k;
	while(k<20){
		cin >> N;
		count = 0;
		if(N<0)	break;
	for(int x=2;x<=N;x++){
		int flag = 0;
		for(int y=2;y<x;y++){
			if((x%y)==0)	flag = 1;
		}
		if(flag == 0)	count++;
	}
	cout << count << endl;
		k++;
	}
	return 0;
}

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

#define MAX 10000000
int a[MAX];

int main()
{
	int n;
	a[0] = a[1] = 0;
	a[2] = 1;
	a[3] = 2;
	for (int i = 4; i < MAX; i++)
	{
		bool ok = true;
		for (int j = 2; j < sqrt(i) + 0.1; j++)
		{
			if (i % j == 0)
			{
				a[i] = a[i - 1];
				ok = false;
				break;
			}
		}
		if (ok)
			a[i] = a[i - 1] + 1;
	}

	while (cin >> n)
		cout << a[n] << endl;
	
	return 0;
}

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

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

using namespace std;

int pri(int n){
	int x = 0;
	int sum;
	
	for(int i = 1;i <= n;i++){
		sum = 0;
		for(int j = 1; j <= i; j++){
			if( i % j == 0) sum += j;
			if(sum == i + 1) x++;
		}
	}
	return x;
}

int main(){
	int n, x;
	while(scanf("%d", &n) != EOF)
	
	cout << pri(n) << endl;
	
	return 0;
}

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

const int MAX = 1000000;

int main() {
	bool prime[MAX + 1];
	int sum[MAX + 1];
	fill(prime, prime + MAX + 1, true);
	fill(sum, sum + MAX + 1, 0);
	prime[0] = prime[1] = false;
	for(int i = 2; i <= MAX; i++) {
		sum[i] = sum[i - 1];
		if(prime[i]) {
			sum[i]++;
			for(int j = 2 * i; j <= MAX; j += i)
				prime[j] = false;
		}
	}
#if 0
	for(int i = 1; i < 20; i++) {
		cout << i << " " << sum[i] << endl;
	}
#endif
	for(int n; cin >> n;)
		cout << sum[n] << endl;
	return 0;
}

C++: #define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <cstring>
#include <algorithm>
#include <limits.h>
#include <cmath>
#include <time.h>
#define REP(i,a,b) for(i=a;i<b;i++)
#define rep(i,n) REP(i,0,n)
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
#define SQR(X) ((X)*(X))

/* Eratosthenes(int prime[],int n)は
nまでの素数をprimeに、2,3,と入れていきます */
/* 実行後arr[i]はiが素数の時1になります */
static ull prime[1000000];
ull Eratosthenes(ull N){
  static ull arr[15000005];
  for(ull i = 0; i < N; i++){
    arr[i] = 1;
  }
  for(ull i = 2; i < sqrt((double)N); i++){
    if(arr[i]){
      for(ull j = 0; i * (j + 2) < N; j++){
        arr[i *(j + 2)] = 0;
      }
    }
  }
  ull a = 0;
  for(ull i = 2; i < N; i++){
    if(arr[i]){
      prime[a++] = i;
    }
  }
  return a;
}

int test = 0;
/* ここからが本編 */
int main(void)
{
  int i,j,k,l;
  int rep = 100000;
  int n;
  while(cin >> n) {
    cout << Eratosthenes(n+1) << endl;
  }
  return 0;
}

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 <iostream>

using namespace std;

int isprime(int x){
	if(x < 2){
		return false;
	}else if(x==2){
		return true;
	}else{
		for(int i=3;i<x/2;i=i+2){
			if(x%i==0){
				return false;
			}
		}
		return true;
	}
}

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