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++: #include<iostream>
#include<stdio.h>
#include<math.h>
int p(long x)
{
    long i,j=sqrt(x),k=1;
    for(i=2;i<=(j);i++)
    {
        if(x%i==0){
            k=0;
            //return k;
        break;

        }
    }
    if(k==1)return 1;
    else return 2;
}
using namespace std;
int main()
{
    double a,b,c,i,n,s=0;

    while(cin>>n){
        s=0;
    //a=sqrt(n);
    for(i=2;i<=n;i++)
    {
        if(p(i)==1){s++;}
    }
    cout<<s<<endl;
    }


    return 0;
}

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

int prime[1000000];


bool isPrime(int n) {
  int x = sqrt(static_cast<double>(n));
  for (int i = 3; i <= x; i += 2) {
    if (n % i == 0) {
      return false;
    }
  }
  return true;
}


void get_prime(int max_num) {
  int primecount = 1;
  prime[2] = primecount;
  for (int i = 3; i <= max_num; i += 2) {
    if (isPrime(i))
      prime[i] = ++primecount;
    else
      prime[i] = primecount;
  }
}


int main () {
  int ns[31];
  int i = 0, max_num;
  while (std::cin >> ns[i]) {
    max_num = std::max(max_num, ns[i]);
    if (ns[i] % 2 == 0 && ns[i] != 2)
      ns[i] --;
    i ++;
  }
  get_prime(max_num);
  for (int j = 0; j < i; j ++) {
    std::cout << prime[ns[j]] << std::endl;
  }
  return 0;
}

C++: 
#include <iostream>
#include <cmath>
using namespace std;
int main () 
{
	int limit, count=0;
	
	cout << " Integer : " << endl;
	cin >> limit;
	
    for (int i=2; i<=limit; i++) 
    {
        bool prime=true;
        for (int j=2; j*j<=i; j++)
        {
            if (i % j == 0) 
            {
                prime=false;
                break;    
            }
        }   
        if(prime) {
        	count++;
        }
    }
    cout << "Total : " << count;
    return 0;
}

C++: #include <stdio.h>

int main(){
	int sosuu[100000] = { 2, 3, 5 }, i, x, y = 3,n=0,z;
	
	while (scanf("%d", &n)!=EOF){
		z = 0; 
		for (i = 3; i < n + 1; i += 2){
			for (x = 0; x < y; x++){
				if (i % sosuu[x] == 0){ break; }
				if (x + 1 == y){ sosuu[y] = i, y++;}
			}
		}
		for (i = 0; i < y; i++){ if (sosuu[i] > n)break; z++; }
		printf("%d\n", z);
	}
	return 0;
}

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

#define MAX_NUM (1000000)

static int getPrimeNum(int maxVal);

int main(void)
{
  int n;
  
  while (std::cin >> n) {
    std::cout << getPrimeNum(n) << std::endl;
  }
  
  return 0;
}

static int getPrimeNum(int maxVal)
{
  static int searchList[MAX_NUM] = {0};
  static int primeList[MAX_NUM] = {0};
  
  memset(searchList, 0, sizeof(int) * MAX_NUM);
  memset(primeList, 0, sizeof(int) * MAX_NUM);
  
  double sqMaxVal = sqrt((double)maxVal) + 1;
  
  for (int i = 2; i <= sqMaxVal; i++) {
    if (searchList[i] == 0) {
      searchList[i] = 1;
      primeList[i] = 1;
      for (int j = i + 1; j <= maxVal; j++) {
        if (j % i == 0) {
          searchList[j] = 1;
        }
      }
    }
  }
  
  for (int i = 2; i <= maxVal; i++) {
    if (searchList[i] == 0) {
      primeList[i] = 1;
    }
  }
  
  int count = 0;
  for (int i = 2; i <= maxVal; i++) {
    if (primeList[i] == 1) {
      count++;
    }
  }
  
  return count;
}

C++: #include <stdio.h>

int main(){
	int sosuu[200000] = { 2 }, i, x, y = 1, z = 0;


	while(scanf("%d", &z)!=EOF){//入力した数以下の素数を表示

		for (i = 3; i < z + 1; i += 2){
			for (x = 0; x < y; x++){
				if (i % sosuu[x] == 0){ break; }
				if (x + 1 == y){ sosuu[y] = i, y++; }
			}
		}
		if (z < 2){ y = 0; }
	printf("%d\n", y);//最後に素数の合計数を表示

	for (i = 1; i < y; i++){
		sosuu[i] = 0;
	}y = 1; z = 0;
	}

	return 0;
}

C++: #define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstring>
#define N_MAX 100000

using namespace std;



int main(){
	
	int array[N_MAX],i,j,n, num = 0;
	
	while(cin>>n){
		num = 0;
		for (i=0;i<n;i++)
			array[i]=i+2;
		for (i=0;i<sqrt(n*1.0);i++){
			for (j = (i+2); (i+2) * j <= n; j++)
				array[(i+2) * j - 2] = -1;}

	for (i = 0; i < n-1; i++){
    if (array[i] != -1)
	num++;
	}
	cout<<num<<endl;
	}

	cin>>n;
	
	
	
	
  
	return (0);
}

C++: #include<iostream>

using namespace std;

int main(){
  int n, N;
  int *prime;
  int i,j;
  while(cin >> N){
    n = N + 1; 
    //init
    prime = new int[n];
    for(i=0; i<=n; i++) prime[i] = 0;

    //素数判定
    for(i=2; i*i<=n; i++){
      if(prime[i] == 0){
	prime[i] = 1; //iは素数
	for(j=2; i*j<=n; j++){//倍数ふるい落とし
	  prime[i*j] = -1;//i*jは素数じゃない
	}
      }
    }
    for(i=i; i<=n; i++){
      if(prime[i] == 0) prime[i] = 1;
    }
    
    //output
    int ans=0;
    for(i=2; i<=n; i++){
      if(prime[i] == 1) {
	ans ++;
	//cout << "prime:" << i << endl;;
      }
    }
    cout << ans << endl;

    delete[] prime;
  }
  return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<vector>
#include<set>
#include<string>


int main() {
	int n;
	bool a;
	std::vector<int> prime;
	prime.emplace_back(2);
	for (int i = 3; i < 1000000; i += 2) {
		a = false;
		for (int j = 3; j <= std::sqrt(i); j += 2) {
			if (i%j == 0) {
				a=true;
			}
		}
		if (a == false) {
			prime.emplace_back(i);
		}
	}
	while (std::cin >> n) {
		int ans = 0;
		for (int i = 0; i < prime.size(); i++) {
			if (prime[i] <= n) {
				ans++;
			}
		}
		std::cout << ans << std::endl;
	}
}

C++: #include<iostream>
#include<vector>
#define max 1000010
using namespace std;

bool isp[max];
vector<int> P;

void sieve(){
	for(int i=0;i<max;i++)isp[i]=true;
	for(int i=2;i<max;i++){
		if(isp[i]){
			P.push_back(i);
			for(int j=2*i;j<max;j+=i)isp[j]=false;
		}
	}
	return;
}

int main()
{
	int n;

	sieve();
	while(cin>>n){
		int c=0;
		for(int i=0;i<P.size();i++)if(P[i]<=n)c++;
		cout<<c<<endl;
	}

	return 0;
}

C++: #include <iostream>
#include <bitset>
using namespace std;
bitset<1000000> p;
int main(){
  p.flip();
  p[0] = 0;
  p[1] = 0;
  for(int i = 2; i < 1000; ++i){
    if(p[i]){
      for(int j = 2*i; j < 1000000; j += i) p[j] = 0;
    }
  }
  int n;
  while(cin >> n){
    int ret = 0;
    for(int i = 2; i <= n; ++i) ret += p[i];
    cout << ret << '\n';
  }
  return 0;
}

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

void solve(int x){
	bool C[x];
	
	C[0]=false; C[1]=false;
	for(int i=2; i<=x; i++){
		C[i]=true;
	}
	
	for(int i=2;i<(int)sqrt(x)+1; i++){
		if(C[i]){
			for(int y=i*2; y<=x; y+=i) C[y]=false;
		}
	}
	
	int count=0;
	for(int i=0; i<=x; i++){
		if(C[i]){
			count++;
		}
	}
	
	cout << count << endl;  
}

int main(){
	int n;
	
	while(cin >> n){
		solve(n);
	}
	
	return 0;
}

C++: #include<stdio.h>
#include<math.h>
int prime(int n);

int main()
{
	int k,j,n;
	int store,sum=0;

	while(scanf("%d",&n)!=EOF)
{

			for(j=1;j<=n;j++)
			{
				store=prime(j);
				if(store==1)
					{
						sum++;
					}

			}
			printf("%d\n",sum);
			sum=0;



}


return 0;

}
int prime(int n)
{


	int i;
	float x,k;
	int l;


	x=float(n);
	k=sqrt(x);

	l=int(k);

		if(n==1 || n==0)
			{
				return 0;
			}


		if(n==2)
			{
				return 1;
			}

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

		}

	return 1;


}

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

int n,mem[1000000];
int count;
int now=2;

//?´???°??????0
int main(){
	mem[1]=1;
	for(int i=2;i<=(int)sqrt(1000000.0)+1;i++){
		if(mem[i]==0){
			for(int j=2*i;j<=1000000;j+=i){
				mem[j]=1;
			}
		}
	}
	while(cin>>n){
		count=0;
		for(int i=1;i<n+1;i++){
			if(mem[i]==0) count++;	
		}
		cout<<count<<endl;
	}
	return 0;
}