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<iostream>
#include<stdio.h>
#include<math.h>
int prime(int x)
{
    int i,j=1;
    int l;
    l=sqrt(x);
    for(i=2;i<=l+1;i++)
    {
        if(x%i==0)
        {
            j=0;
            break;

        }
    }
    //if(j==0)return 0;
    if(x==2||j==1) return 1;


}
using namespace std;
int main()
{
    int n;//=999999;
    //cout<<n<<endl;
    int j,i;int s=0;
    while(cin>>n)
    {
       s=0;

        for(i=2;i<=n;i++)
        {
            if(prime(i)==1){s++;}//cout<<i<<endl;}
        }
        cout<<s<<endl;

    }



    return 0;
}

C++: #include<stdio.h>
int a[1000000];
int main(void)
{
	int s,d,f,g,h,i,j;
	a[1]=0;
	for(i=2;i<=999999;i++){
		a[i]=1;
	}
	for(i=2;i*i<=1000000;i++){
		if(a[i]!=0){
			s=2;
			while(i*s<=999999){
				a[i*s]=0;
				s++;
			}
		}
	}
/*	for(i=1;i<=100000;i++){
		if(a[i]==1){
			printf("%d\n",i);
		}
	}*/
	while(scanf("%d",&f)!=EOF){
		g=0;
		for(i=1;i<=f;i++){
		if(a[i]==1){
			g++;
		}
	}
	printf("%d\n",g);
	}
	return 0;
}

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


vector<int> primes;
int ref_max=2;

int makePrimenumbers(int a){
	if(a<ref_max)return 0;

	temp = ref_max;
	ref_max=a;
	int i,k,isp=1;

	for(k=temp;k<ref_max;k++){
		for(i=0;i<primes.size();i++){
			if(k%primes[i]==0){
				isp=0;
				break;
			}
		}
		if(isp==1){
			primes.push_back(k);
		}
		isp=1;
	}
	return 0;
}

int countprime(int a){
	int i;
	for(i=0;i<prime.size();i++)
		if(a<prime[i])
			break;
	return (i);
}

int main(void)
{
	int result;
	while(scanf("%d",&a)!=EOF){
		makePrimenumbers(a);
		result = countprime(a);
		printf("%d\n",result);
	}
  return 0;
}

C++: #include <iostream>
#include <string>
#include <map>
#include <vector>
#include <set>
#include <iomanip>
#include <fstream>
#include <cstdint>
#include <cmath>
#include <algorithm>
#include <utility>
#include <numeric>
using namespace std;

#define REP(i,f,n) for(int i=(f); i < (n); ++i)
#define PER(i,f,n) for(int i=(n-1); i >= f; --i)

//----------------------------------
//----------------------------------

constexpr int64_t CALCNUM = 2000000;

bool primes[CALCNUM];

signed main()
{
	for(int i = 0; i < CALCNUM; i++)
		primes[i] = true;
    ios::sync_with_stdio(false);
	vector<int> prime;

	for(int i = 2; i <CALCNUM ; i++)
	{
		if(primes[i])
		{
			prime.push_back(i);
			for(int j = 2 * i; j < CALCNUM; j += i)
				primes[j] = false;
		}
	}
	int n;
	while(cin >> n)
	{
		for(int i = 0; i < prime.size(); ++i)
			if(prime[i] > n){
				cout << i <<'\n'; break;
			}
	}
}

C++: #include <stdio.h>

int main(void)
{
	int n;
	int ans;
	char prime[1000000];
	int i, j;
	int p;
	
	while (scanf("%d", &n) != EOF){
	
		for (i = 0; i < n + 1; i++){
			prime[i] = 1;
		}
		prime[0] = prime[1] = 0;
		
		for (i = 2, p = 0; i <= n + 1; i++){
			if (prime[i]){
				for (j = 2 * i; j <= n; j += i){
					prime[j] = 0;
				}
			}
		}
		for (i = 0, p = 0;i <= n; i++){
			if (prime[i] == 1){
				p++;
			}
		}
		
		ans = p;
		printf("%d\n", ans);
		}
	
	return (0);
}

C++: #pragma  warning (disable:4996)
#include<iostream>
#include<cstdio>
#include<queue>
#include<cstring>
#include<string>
#include<cmath>
#include<algorithm>
#define MAX 10000000
using namespace std;
int amount[MAX];
int vis[MAX];
int prime[MAX];
int main(){
	memset(vis, 0, sizeof(vis));
	memset(prime, 0, sizeof(prime));
	prime[0] = prime[1] = 0;
	
	for (int i = 2; i <= MAX; i++){
		if (!vis[i]){

			prime[i] = 1;
			for (int j = i + i; j <= MAX; j += i){
				vis[j] = 1;
				prime[j] = 0;

			}
		}
	}
	int sum = 0;
	for (int i = 2; i <= MAX; i++){
			if (prime[i]){
				sum++;
			}
			amount[i] = sum;
		}

	int n;
	cin >> n;
		cout << amount[n]<< endl;
	return 0;
}

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

int main()
{
	int n;

	while(std::cin >> n)
	{
		std::vector<int> Sosuu(1);

		Sosuu[0] = 2;

		for( int i=2; i<n; ++i )
		{
			size_t j=0;

			for( ; j<Sosuu.size(); ++j )
			{
				if(i%Sosuu[j]==0)
				{
					break;
				}
			}
			if(j==Sosuu.size())
			{
				Sosuu.push_back(i);
			}
		}

		std::cout << Sosuu.size() << '\n';
	}
}

C++: #include <stdio.h>
#include <math.h>
#include <stdlib.h>
#define N 999999L
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 <bits/stdc++.h>
using namespace std;

int check[1000000 + 5];

void sieve()
{
check[1] = 1;
for(int i = 4; i <= 1000000; i += 2)
check[i] = 1;
for(int i = 3; i <= 1000; i += 2)
for(int j = i * i; j <= 1000000; j += i + i)
check[j] = 1;
for(int i = 1; i <= 1000000; i++)
check[i] += check[i - 1];
}

int main()
{
sieve();
int n;
while(cin >> n)
cout << n - check[n] << endl;
return 0;
}

C++: #include <cstdio>
#include <vector>
#include <cmath>
#include <ctime>

using namespace std;

int main()
{
	int n;
	int len;
	vector<int> prime;
	
	//unsigned long t = time(NULL);
	prime.push_back(2);
	len = 1;
	for (int i = 3; i < 1000000; i += 2){
		int j;
		for (j = 0; j < len && prime[j] < i / 2; j++){
			if (i % prime[j] == 0){
				break;
			}
		}
		if (j >= prime.size() || prime[j] >= i / 2){
			prime.push_back(i);
			len++;
		}
	}
	//printf("%lu\n", time(NULL) - t);
	while (scanf("%d", &n) != EOF){
		int count;
		
		for (count = 0; prime[count] <= n; count++){//;
			//printf("%d\n", prime[count]);
			;
		}
		printf("%d\n", count);
	}
	
	return 0;
}

C++: #include<cstdio>
#include<string.h>
using namespace std;
int pNum;
bool IsPrime[1000010];
int Prime[1000010];
int GetPrime(int n){
    memset(IsPrime,true,sizeof(IsPrime));
    IsPrime[0]=IsPrime[1]=false;
    pNum=0;
    for(int i=2;i<=n;i++){
        if(IsPrime[i])Prime[++pNum]=i;
        for(int j=1;j<=pNum&&Prime[j]*i<=n;j++){
            IsPrime[Prime[j]*i]=false;
            if(i%Prime[j]==0)break;
        }
    }
    return pNum;
}
int main(){
    int n;
    while(scanf("%d",&n)!=EOF){
        printf("%d\n",GetPrime(n));
    }
    return 0;
}

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

vector <int> prime;
bool is_checked[1000010] = {false};

int main() {
	// Eratosthenes
	is_checked[0] = true; is_checked[1] = true;
	for(int i = 2; i < 1000000;i++){
		if (is_checked[i]) continue;
		prime.push_back(i);
		for(int j = 1;;j++) {
			if(i*j > 1000000) break;
			is_checked[i*j] = true;
		}
	}

	int n;
	while(cin >> n) {
		int count = 0;
		for (int i = 0;;i++) {
			if (prime[i] > n) break;
			count++;
		}
		cout << count << endl;
	}
	return 0;
}