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>

using namespace std;

int n;

bool PrimeJudge(int n)
{
	bool judge = true;
	for(int i=2;i<n;++i)
	{
		if(n%i>1)
		{
			judge = false;
			break;
		}
	}
	return judge;
}

int main()
{
	while(cin>>n)
	{
		int count=0;
		for(int i=1;i<=n;++i)
		{
			if(PrimeJudge(i))
			{
				++count;
			}
		}
		cout<<count<<endl;
	}
}

C++: #include <cstdio>
#include <cstdlib>
#include <iostream>
#include <math.h>

using namespace std; 

int list[1000000]={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, 1009, 1013, 1019, 1021, 1031, 1033, 1039, 1049, 1051, 1061, 1063, 1069};

long int a,b,loop,hasil,input,mid;
long int lo;
long int hi;

 bool Prima(unsigned long n) 
{
    for (unsigned short i = 0; list[i]*list[i] <= n; i++) 
	{
        if (n % list[i] == 0) 
		{
            return false;
        }
    }
 
    return true;
}

int main()
{
	lo=1087;
	a=180;
	while(lo<=100003)
	{
		if(Prima(lo))
		{
			list[a]=lo;
			a++;
		}
		lo+=2;
	}
	//printf("%d",list[78498]);
	while(scanf("%ld",&input)!= EOF)
	{	
	lo=0;
	hi=78497;
	/*while(lo<=hi)
        {
            mid=(lo+hi)/2;
            if (list[mid]<input)
            {
                lo=mid+1; 
            } 
            else if(list[mid]>input)
            {
                hi=mid-1; 
            }
            else 
            {
                break; 
            } 
        }*/
	while(input>=list[lo])
	{
		lo++;
	}
	printf("%d\n",lo);
	}
	return 0; 
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <algorithm>
#include <numeric>
#include <functional>
#include <set>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <cctype>
#include <climits>
#include <fstream>
#include <bitset>
#include <time.h>
#include <assert.h>
#include <random>

#define LL long long
#define VI vector<int>
#define VL vector<long long>
#define FOR(i,a,b) for(int i= (a); i<((int)b); ++i)
#define RFOR(i,a) for(int i=(a); i >= 0; --i)
#define FOE(i,a) for(auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define RALL(c) (c).rbegin(), (c).rend()
#define DUMP(x)  cerr << #x << " = " << (x) << endl;
#define SUM(x) std::accumulate(ALL(x), 0LL)
#define MIN(v) *std::min_element(v.begin(), v.end())
#define MAX(v) *std::max_element(v.begin(), v.end())
#define EXIST(v,x) (std::find(v.begin(), v.end(), x) != v.end())
#define BIT(n) (1LL<<(n))
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end());
#define EPS 1e-14


using namespace std;

// nまでの素数のリストを作成(nを含む) O(N log long N)
vector<int> make_prime_list(int n) {
    vector<bool> is_prime(n + 1, true);
    is_prime[0] = false;
    is_prime[1] = false;

    vector<int> prime_list;
    for (int i = 2; i < n + 1; ++i) {
        if (!is_prime[i]) { continue; }
        prime_list.emplace_back(i);

        for (int j = i; j < n + 1; j += i) {
            is_prime[j] = false;
        }
    }
    return prime_list;
}

int main(int argc, char *argv[]) {
    cin.tie(0);
    ios::sync_with_stdio(false);

    vector<int> prime_list = make_prime_list(1000100);

    int n;
    while(cin >> n) {
        int ans = 0;
        for(int i = 0;;++i)  {
            if(prime_list[i] <= n){
                ans++;
            }
            else {
                break;
            }
        }

        cout << ans << endl;
    }

    return 0;
}

C++: #include<cstdio>
using namespace std;
bool prime[1000000];
#define max 1000000
int main(){
	prime[1] = true;
	for(int i = 2; i < max; i++){
		if(!prime[i]){
			for(int j = i * i; j < max; j+=i){
				prime[j] = true;
			}
		}
	}
	int n;
	while(scanf("%d",&n) != EOF){
		int ans = 0;
		for(int i = 1; i <= n; i++){
			if(!prime[i])ans++;
		}
		printf("%d\n",ans);
	}
	return 0;
}

C++: #include<iostream>
#include<iomanip>
#include<algorithm>
#include<cctype>
#include<cmath>
#include<string>
#include<set>
#include<map>
#include<queue>
#include<vector>
using namespace std;

#define MAX 1000000

static bool gosei[MAX];

void b(){
	for(int i=2;gosei[i]==false&&i<MAX;i++)
		for(int j=2,k;(k=i*j)<MAX;j++)
			gosei[k]=true;
}

int c(int n){
	int cnt=0;
	for(int i=2;i<=n;i++)
		if(!(gosei[i]))cnt++;
	return cnt;
}

int main() {
	b();
	int n;
	while(cin>>n)
		cout<<c(n)<<endl;
	return 0;
}

C++: #include<cstdio>
int main(){
    int n;
    while(scanf("%d",&n)!=EOF){
        int x=2;
        for(int j=3;j<n;j+=2){
            bool a=true;
            for(int i=3;i*i<n;i+=2){
                if(j%i==0){
                    a=false;
                }
            }
            if(a)x++;
        }

        printf("%d\n",x);
    }

    return 0;
}

C++: #include <iostream>

int main() {
	int n;

	/**
	 * 0: No data
	 * 1: Prime number
	 * 2: Non prime number
	 */
	int db[1000000] = {2, 2, 1, 1};

	while (std::cin >> n) {
		int primes = 0; // n以下の素数の数
		int now;

		/* dbを走査 */
		loop:
		for (int i = 0; i <= n; i++)
		switch (db[i]) {
			case 1:
				primes++;
				break;
			case 0:
				now = i;
				goto next_phase;
		}

next_phase:
	/* 足りてなければ必要なだけごにょごにょ */
		for (int j = now; j <= n; j++) {
			bool flag = false; // はずれの目印
			for (int k = 2; k < j; k++)
				if (!(j % k)) {
					flag = true;
					break;
				}
			if (!flag) {
				db[j] = 1;
				primes++;
			} else
				db[j] = 2;
		}
		std::cout << primes << std::endl;
	}
}

C++: #include <iostream>
#include <cmath>
using namespace std;
int prime(int);
int main(){
    int a,sum;
    int b[999999];
    b[0]=0;
    for(int i=1;i<999999;i++){
        b[i]=prime(i);
    }
    while ( cin >> a){
        sum =0;
        for (int i=0;i<a;i++){
            sum +=b[i];
        }
        cout << sum-1 << endl;
    }
}


int prime(int x){
    if(x==2 || x==3 || x==5 || x==7)
        return 1;
    if(x%2==0 || x%3==0)
        return 0;
    for (int i=5;i<=sqrt(x)+1;i+=6){
        if(x%i==0 || x%(i+2)==0)
            return 0;
    }
    return 1;
}

C++: #include<cstdio>
int solve(int n){
	for(int i=2;i*i<=n;i++){
		if(n % i == 0){
			return 0;
		}
	}
	return 1;
}

int main(){
	int n;
	while(scanf("%d",&n) != EOF){
		int cnt = 0;
		for(int i=2;i<=n;i++){
			cnt += solve(i);
		}
		printf("%d\n",cnt);
	}
	return 0;
}