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>

const int MAX_V = 1000000;
int prime[MAX_V+1];
int main() {
    int i, k, 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;
            }
        }
    }
    while(scanf("%d", &v) != EOF) {
        int cnt=0;
        for(i=2; i<=v; i++){
            if(prime[i]==1){
                cnt++;
            }
        }
        printf("%d\n",cnt);
    }
}

C++: #include "bits/stdc++.h"

using namespace std;

typedef vector<int> vi;
typedef pair<int,int> pii;
typedef long long ll;

#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
#define all(a)  (a).begin(),(a).end()
#define pb push_back

bool isp[1000000]={};


void makeEra(){
    for(int i=2;i<=1000;i++){
        if( isp[i] ){
            for(int j=2;j<1000000/i;j++){
                isp[i*j]=false;
            }
        }
    }
}


int main(){
    rep(i,1000000)isp[i]=true;
    isp[0]=isp[1]=false;
    makeEra();
    
    int n;
    while(cin>>n){
        int c = 0;
        rep(i,n+1){
            if(isp[i])c++;
        }
        cout<<c<<endl;
    }
}

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

bool isprime(int n){
	if(n==2 || n==3)
		return true;
	else{
		for(int i=2; i<=sqrt(n); i++){
			if(n%i==0)
				return false;
		}
		return true;
	}
}

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

C++: #include <stdio.h>

const int MAX_V = 1000000;
int prime[MAX_V+1];
int sum[MAX_V+2];
void init(){
    int i, k, 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;
            }
        }
    }   
}

void accumulate(){
   for(int i=1; i<MAX_V+2; i++){
        if(i==1){
            sum[i]=0;
        }else{
            sum[i]=sum[i-1]+prime[i];
        }
    }
}


int main(){
    init();
    accumulate();
    int num;
    while(scanf("%d", &num) != EOF){
        printf("%d\n", sum[num]);
    }
    return 0;
}

C++: #include <iostream>
using namespace std;
bool isPrime[1000000];
void calcPrime(){
  fill(isPrime, isPrime+1000000, true);
  isPrime[0] = isPrime[1] = false;

  for(int i = 4; i < 1000000; i+=2){
    isPrime[i] = false;
  }
  
  for(int i = 3; i*i < 1000000; i+=2){
    if(isPrime[i] == false)continue;
    for(int j = i+i; j < 1000000; j+=i){
      isPrime[j] = false;
    }
  }
}
int main(){
  calcPrime();
  int n;
  while(cin >> n){
    int cnt = 0;
    for(int i = 2; i <= n; i++){
      if(isPrime[i])cnt++;
    }
    cout << cnt << endl;
  }
  return 0;
}

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

int main(){
  bool p[1000000];
  int num[1000000];
  int c,n;

  for(int i=0;i<1000000;i+=2)p[i] = false;
  for(int i=1;i<1000000;i+=2)p[i] = true;
  p[1] = false;
  p[2] = true;

  c = 3;
  while(c < 1000){
    for(int i=3*c;i<1000000;i+=2*c)p[i] = false;
    do{c+=2;}while(!p[c]);
  }

  c = 0;
  for(int i=0;i<1000000;i++){
    if(p[i])c++;
    num[i] = c;
  }

  while(cin >> n)cout << num[n] << endl;
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <algorithm>
#include <sstream>
#include <cmath>
#include <set>
#include <iomanip>
#include <deque>
#include <limits>
using namespace std;
typedef long long ll;
 
#define REP(i,n) for(int (i)=0;(i)<(int)(n);(i)++)
#define RREP(i,n) for(int (i)=(int)(n)-1;i>=0;i--)
#define FOREACH(i,Itr) for(auto (i)=(Itr).begin();(i)!=(Itr).end();(i)++)
#define REMOVE(Itr,n) (Itr).erase(remove((Itr).begin(),(Itr).end(),n),(Itr).end())
#define UNIQUE(Itr) sort((Itr).begin(),(Itr).end()); (Itr).erase(unique((Itr).begin(),(Itr).end()),(Itr).end())
#define LBOUND(Itr,val) lower_bound((Itr).begin(),(Itr).end(),(val))
#define UBOUND(Itr,val) upper_bound((Itr).begin(),(Itr).end(),(val))
 
struct PrimeNumbers {
    vector<bool> sieve;
    vector<int> prime;
    PrimeNumbers(int n) : sieve(n+1,true) {
        sieve[0] = sieve[1] = false;
        for(long long i=2; i<sieve.size(); i++){
            if(sieve[i]){
                prime.push_back((int)i);
                for(long long j=i*i; j<sieve.size(); j+=i) {
                    sieve[j] = false;
                }
            }
        }
    }
    int size(void) { return (int)prime.size(); }
    int & operator [] (int n) { return prime[n]; }
    bool operator == (int n) { return sieve[n]; }
    bool operator != (int n) { return sieve[n]; }
};

int main() {

    int t;
    while(cin >> t) {
        PrimeNumbers p(t);
        cout << p.size() << endl;
    }
    return 0;
}

C++: #include <iostream>
#include <cmath>
#define N 999999
using namespace std;

int arr[N];

void Eratosthenes(){
	for(int i = 0; i < N; i++){
		arr[i] = 1;
	}
	for(int i = 2; i < sqrt(N); i++){
		if(arr[i]){
			for(int j = 0; i * (j + 2) < N; j++){
				arr[i *(j + 2)] = 0;
			}
		}
	}
	return;
}

int main(){
	int i,j=0,n,pn;
	while(cin>>n){
		pn=0;
		Eratosthenes();
		for(i=2 ; i<=n ; i++){
			if(arr[i]==1) pn++;
		}
		cout << pn << endl;
		
	}
	return 0;
}

C++: int main(){
int n;
while(cin >> n){
int a[n+1];
int b;
int c;
b=1;
while(b<n+1){
a[b]=0;
b=b+1;}
a[1]=1;
int d;
d=0;
b=2;
while(b<n+1){
if(a[b]==0){d=d+1; c=2; while(1){if(c*b>n){break;} a[c*b]=1; c=c+1;}}
b=b+1;}
cout << d;}

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

using namespace std;

int main()
{
    long int arr[999999];
    int p=0;
    int i,b,a;
    for(a=2;a<=999999;a++){

                b=0;
                for(i=2;i<=sqrt(a);i++){
                    if(a%i==0){
                    b=1;
                    break;
                    }
                }
                if(b==0){
                arr[p]=a;
                p++;
                }
            }
    int num,cot;
    while(scanf("%d",&num)!=EOF){
            cot=0;
            for(i=0;i<=999999;i++){
                if(arr[i]<=num)
                    cot++;
                else
                    break;
            }
            cout << cot << endl;
    }
    return 0;
}

C++: #include <iostream>

int main() {
    int n;
    while (std::cin >> n) {
        int primes = 0; // n以下の素数の数
        for (int i = 2; i <= n; i++) {
            bool flag = false; // はずれの目印
            for (int j = 2; j < i; j++)
                if (!(i % j)) {
                    flag = true;
                    break;
                }   
            if (!flag)
                 primes++;
        }
        std::cout << primes << std::endl;
     }   
}