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<vector>

using namespace std;

int main()
{
	int n;
	int head,fin;
	int temp;
	
	
//	vector<int> isPrime;
	bool isPrime[1000000];
	
	while(cin>>n)
	{
		for(int i = 0; i < n; i++){
//			isPrime.push_back(1);
			isPrime[i] = 1;
		}
//		cout << "Corecced!" << endl;
		isPrime[0] = isPrime[1] = 0;
		temp = 0;
		
		for(;;)
		{
			//TõXgÌæªð©Â¯é
			for(head = temp+1; head < n; head++){
				if(isPrime[head] == 1){
					break;
				}
			}
			//æªðÜÜÈ¢{ÍSÄ¬
			for(int i = head; i*head < n; i++){
				isPrime[i*head] = 0;
			}
			//TõXgÌÅãÖ
			for(fin = n; fin > 0; fin--){
				if(isPrime[fin] == 1){
					break;
				}
			}
		//	printf("head:%d    fin:%d\n",head,fin);
			
			if(fin <= head*head){
				break;
			}
			temp = head;
		}
		
		int cont = 0;
		for(int i = 0; i < n; i++){
			if(isPrime[i] == 1){
				cont++;
		//		cout << "+"<<i << endl;
			}
		}
		printf("%d\n",cont);
	}
}

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

int ans ;
int N ;
bool prime[1000200] ;

void SE ( ) ;

int main ( ) 
{
	memset ( prime , false , sizeof ( prime ) ) ;
	SE ( ) ;
	while ( scanf ( "%d" , &N ) == 1 ) {
		ans = 0 ;
		if ( N >= 2 ) {
			ans++ ;
		}
		for ( int i = 3 ; i <= N ; i += 2 ) {
			if ( prime[i] == false ) {
				ans++ ;
			}
		}
		printf ( "%d\n" , ans ) ;
	}
	return 0 ;
}

void SE ( )
{
	prime[0] = prime[1] = true ;
	for ( int i = 3 ; i < 1004 ; i += 2 ) {
		if ( prime[i] == false ) {
			for ( int j = i * i ; j < 1000200 ; j += i * 2 ) {
				prime[j] = true ;
			}
		}
	}
}

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

int whether(int num);
int count(int num);

int main(){
	int n;
	int a;

	while(1){
		if(feof(stdin)==EOF) break;
		scanf("%d",&n);
		a = count(n);
		printf("%d\n",a); 
	}

	return 0;
}

int whether(int num){
	int s=sqrt(num);
	int i;
	for(i=3; i<=s; i+=2){ if(num%i==0) return 0; }

	return 1;
}

int count(int num){
	int a=1;
	int i;
	if(num <2){ return 0; }
	for(i=3; i<=num; i+=2){
		a += whether(i);
	}

	return a;
}

C++: #include <iostream>
#include <list>
#include <algorithm>

int main(){
	int NUM;
	while(std::cin >> NUM){
		std::list<int> numList,primeList;
		for(int i=2;i<=NUM;++i){
			numList.push_back(i);
		}
		for(int i=numList.front();i*i<NUM;i=numList.front()){
			primeList.push_back(i);
			numList.erase(std::remove_if(numList.begin(),numList.end(),[&](int num){return num%i==0;}),numList.end());
		}
		int cnt=numList.size()+primeList.size();	
		std::cout << cnt << std::endl;
		}
	return 0;
}

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

using namespace std;

int main(int argc, char* argv[])
{
	int i;
	int pos;
	int val;
	int target;
	int sosuList[78499];

	memset(sosuList, 0, sizeof(sosuList));

	sosuList[0] = 2;
	sosuList[1] = 3;
	sosuList[2] = 5;
	sosuList[3] = 7;
	sosuList[4] = 11;

	while (cin >> target) {
		for (pos = 0; sosuList[pos] != 0; pos++) {
			if (sosuList[pos] > target) break;
		}
		for (val = sosuList[pos - 1] + 1 ; val <= target ; val++) {
			for (i = 0; i < pos - 1 ; i++) {
				if (sosuList[i] * 2 > val) { i = pos; break; }
				if (val % sosuList[i] == 0) break;
			}
			if (i >= pos - 1) sosuList[pos++] = val;
		}
		cout << pos << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <climits>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
using namespace std;

bool isprime[1000000];

void solve(int n) {
    int cnt = 0;
    for (int i = 2; i <= n ; ++i) {
        if(isprime[i]) {
            cnt++;
            for (int j = i*2; j <=n ; j = j + i) {
                isprime[j] = false;
            }
        }
    }
    cout << cnt << endl;
}

int main() {
    int n;
    while(cin>>n) {
        for (int i = 0; i <= n; ++i) {
            isprime[i] = true;
        }
        isprime[0] = isprime[1] = false;
        solve(n);
    }


    return 0;
}

C++: #include <stdio.h>

int IsPrime(int n);

int main(void){
    int n, cnt, minID, min=1;
    int list[100000];

    cnt=0;
    for (long i=2; i<1000000; i++){
	//    printf("%d %d\n",i, cnt);
        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;
}

int IsPrime(int n){
    int i;

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

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

int main()
{
	int n;
	const int MAX = 1000000;
	int primecount[MAX];

	for (int i = 2; i < MAX; i++) {
		primecount[i] = 1;
	}

	for (int i = 2; i < MAX; i++) {
		if (primecount[i] == 1) {
			for (int j = i * 2; i < MAX; j += i) {
				primecount[j] = 0;
			}
		}
	}

	for (int i = 1; i < MAX; i++) {
		primecount[i] += primecount[i - 1];
	}

	short i = 1;

	while (cin >> n && i <= 30) {
		cout << primecount[n] << endl;
		i++;
	}

	return 0;
}

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

int main(){
	const int max_num=1100000;
	int prime[max_num];
	int prime2[max_num];
	int min_prime[168]={2 ,3 ,5 ,7 ,11 ,13 ,17 ,19 ,23 ,29 ,31 ,37 ,41 ,43 ,47 ,53 ,59 ,61 ,67 ,71 ,73 ,79 ,83 ,89 ,97 ,101 ,103 ,107 ,109 ,113 ,127 ,131 ,137 ,139 ,149 ,151 ,157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233 ,239 ,241 ,251 ,257 ,263 ,269 ,271 ,277 ,281 ,283 ,293 ,307 ,311 ,313 ,317 ,331 ,337 ,347 ,349 ,353 ,359 ,367 ,373 ,379 ,383 ,389 ,397 ,401 ,409 ,419 ,421 ,431 ,433 ,439 ,443 ,449 ,457 ,461 ,463 ,467 ,479 ,487 ,491 ,499 ,503 ,509 ,521 ,523 ,541 ,547 ,557 ,563 ,569 ,571 ,577 ,587 ,593 ,599 ,601 ,607 ,613 ,617 ,619 ,631 ,641 ,643 ,647 ,653 ,659 ,661 ,673 ,677 ,683 ,691 ,701 ,709 ,719 ,727 ,733 ,739 ,743 ,751 ,757 ,761 ,769 ,773 ,787 ,797 ,809 ,811 ,821 ,823 ,827 ,829 ,839 ,853 ,857 ,859 ,863 ,877 ,881 ,883 ,887 ,907 ,911 ,919 ,929 ,937 ,941 ,947 ,953 ,967 ,971 ,977 ,983 ,991 ,997 };
	int sosuu=0;
	int k=168;

	for(int i=0;i<168;i++){
		prime[i]=min_prime[i];
	}
	for(int i=1001;i<max_num;i+=2){
		sosuu=0;
		for(int j=0;j<168;j++){
			if(i%min_prime[j]==0){
				sosuu++;
			}
		}
		if(sosuu==0){
			prime[k]=i;
			k++;
		}
	}
	int l=0;
	for(int i=0;i<max_num;i++){
		if(i>=prime[l]){
			l++;
		}
		prime2[i]=l;
	}

	int n;

	while(cin>>n){
		cout<<prime2[n]<<endl;
	}

	return 0;
}

C++: #include <iostream>
#include <cmath>
using namespace std;
#define N 10000

int main()
{
	bool prm[N];
	for(int i=0; i<N; i++)
	{
		prm[i]=true;
		int upto= (int) sqrt(N);
		for(int i=2;i<= upto; i++)
		{
			if(prm[i])
				for(int j=2; j*i<N; j++)
				{
				    prm[i*j]=false;
				}
		}
		int n;
		while(cin>>n)
		{
			int count=0;
			for(int i=2; i<=n; i++)
			{
				if(prm[i])
					count++;
			}
			cout<<count<<endl;
		}
	}
	return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<vector>
#include<cmath>
#include<cstdio>
#include<string>
#include<cmath>
typedef long long int ll;


using namespace std;
bool priN[1000000]={};
bool pri[1000000]={};

bool isPrime(int n){
	if(priN[n]==true)return pri[n];
	for(int i=3;i<=sqrt((double)n);i++){
		if(n%i==0){
			priN[n]=true;
			pri[n]=false;
			return false;
		}
	}
	priN[n]=true;
	pri[n]=true;
	return true;
}

int main(){
	while(1){
		int n;
		if( !(cin>>n) )break;
		ll sum=1;
		if (n<2) sum--;
		for(int i=3;i<=n;i+=2){
			if(isPrime(i))sum++;
		}
		cout<<sum<<endl;
	}
	return 0;
}