Go: package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strconv"
)

var sc = bufio.NewScanner(os.Stdin)

func main() {
	var primeCount int
	for sc.Scan() {
		input := nextInt()
		for i := 0; i <= input; i++ {
			if IsPrime(i) {
				primeCount++
			}
		}
		fmt.Printf("%d\n", primeCount)
		primeCount = 0
	}
}

func IsPrime(num int) bool {
	switch {
	case num == 2:
		return true
	case num < 2 || num%2 == 0:
		return false
	}
	sqrt := math.Sqrt(float64(num))
	for i := 3; i <= int(sqrt); i += 2 {
		if num%i == 0 {
			return false
		}
	}
	return true
}

func next() string {
	return sc.Text()
}

func nextInt() int {
	a, _ := strconv.Atoi(next())
	return a
}

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

using namespace std;

int main()
{
    int n, flag, c = 1;
    long long a[99999];
    for(int i=2; i<=99999; i++)
    {
        flag = 0;
        for(int j=2; j<=sqrt(i); j++)
        {
            if(i % j == 0)
            {
                flag = 1;
            }
        }
        if(flag == 0)
        {
            a[i] = 1;
        }
        else
        {
            a[i] = -1;
        }
    }

    while(scanf("%d",&n) != EOF)
    {
        int m = 0;
        for(int i=2; i<=n; i++)
        {
            if(a[i] == 1)
            m++;
        }

        cout<<m<<endl;
    }
}

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

bool prime(int m){
	if(m < 2)return false;
	if(m == 2)return true;
	if(m%2 == 0)return false;
	for(int i = 3; i*i <= m; i += 2){
		if(m%i == 0)return false;
	}
		return true;
}

int main(){
	int n;
	
	while(cin >> n){
		int a=0;
		for(int i = 1; i <= n; ++i){
			if(prime(i))++a;
	}
		cout << a << '\n';
	}
}

C++: #include <stdlib.h>
#include <string>
#include <vector>
#include <algorithm>
#include <iostream>


/*
	HOME-  Volume0-0009
	Submit Statistics
	 
	PCK 2003
	Prime Number
	Time Limit : 1 sec, Memory Limit : 32768 KB
	English version is here
	Prime Number

	6 ÈºÌ³Ì® n ðüÍµAn ÈºÌfª¢­Â é©ðoÍ·évOðì¬µÄº³¢B½¾µAfÆÍ 1 Æ©ª©gÅµ©èØêÈ¢³Ì®Ì¤¿ 1 ðÌ¼¢½àÌð¢¢Ü·Bá¦Î 10 ÈºÌfÍA2, 3, 5, 7 Å·B
	Input

	¡Ìf[^Zbgª^¦çêÜ·Bef[^ZbgÉ n ªPsÉ^¦çêÜ·BüÍÌÅãÜÅµÄº³¢B
	Output

	ef[^Zbg²ÆÉAn ÈºÌfÌÂðPsÉoÍµÄº³¢B
	Sample Input

	10

	Output for the Sample Input

	4

*/

using namespace std;


int main()
{
	string s;
	
	while (getline(cin, s))
	{
		int n;
		sscanf(s.c_str(), "%ld", &n);
		
		vector<int> v;
		v.reserve(n);
		
		for (int i = 0; i <= n; i++)
		{
			v.push_back(1);
		}
		v[0] = 0;
		v[1] = 0;
		
		for (int i = 2; i <= n; i++)
		{
			for (int k = i + 1; k <= n; k++)
			{
				if (0 == (k % i))
				{
					v[k] = 0;
				}
			}
		}
		
		int x = 0;
		for (vector<int>::iterator it = v.begin(); it != v.end(); it++)
		{
			if (*it)
			{
				x++;
			}
		}
		
		printf("%d\n", x);
	}

	return 0;
}

C++: #include <iostream>
using namespace std;
const int len = 20;

int main()
{
    int arr[30] = {0},n,count=0;
     cin>>n;
    for (int i = 2; i < n; i++)
    {
        for (int j = i * i; j < n; j+=i)
        {
            arr[j - 1] = 1;
        }
    }
    for (int i = 1; i <n; i++)
    {
        if ((arr[i - 1] == 0)&&(i>1))
            count++;
    }
    std::cout <<count<<endl;
}

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

typedef unsigned int Int;

Int sqrt_prm(const Int& target, Int x = 0) {
	while (x*x < target) {
		x++;
	}
	return x;
}

int main() {
	while (not std::cin.eof()) {
		Int target;
		std::cin >> target;

		std::vector<Int> primes = {};
		Int sqrt = 1;
		for (Int n = 3; n <= target; n+=2) {
			bool flag = true;

			sqrt = sqrt_prm(n, sqrt);
			for (auto&& p = primes.begin(); not(*p > sqrt or p == primes.end()); p++) {
				if (n % *p == 0) {
					flag = false;
					break;
				}
			}

			if (flag) {
				primes.push_back(n);
			}
		}

		std::cout << primes.size() + (target >= 2 ? 1 : 0) << std::endl;
	}
}

C++: 
#include<iostream>
#include<string>
#include<cmath>
#include<vector>
#include<stack>
#include<queue>
#include<algorithm>
#include<complex>
using namespace std ;
typedef vector<int> vi ;
typedef vector<vi> vvi ;
typedef vector<string> vs ;
typedef pair<int, int> pii; 
typedef long long ll ;

#define loop(i,a,b) for(int i = a ; i < b ; i ++)
#define rep(i,a) loop(i,0,a)
#define all(a) (a).begin(),(a).end()

int main(void){
  static bool cmp[10000000] = {true};
  static int npr[10000000];
  loop(i,2,10000000){
    if(!cmp[i]){
      for(int j = 2 ; j * i < 10000000 ; j ++){
	cmp[i*j]=true;
      }
    }
    npr[i]=npr[i-1] + !cmp[i];
  }
  int n ;
  while(cin>>n){
    cout<<npr[n]<<endl;
  }
}

C++: #include <algorithm>
#include <bitset>
#include <cassert>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <unordered_set>
#include <unordered_map>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)
#define FOR(i,b,e) for (int i=(int)(b); i<(int)(e); i++)
#define ALL(x) (x).begin(), (x).end()

bool pr[1000000];

template <size_t N>
void psieve(bool (&p)[N]) {
  for (int i = 0; i < N; i++)
    p[i] = true;
  p[0] = p[1] = false;
  for (int i = 2; i * i < N; i++) {
    if (!p[i])
      continue;
    for (int j = i * i; j < N; j += i)
      p[j] = false;
  }
}

int solve(int n) {
  int r = 0;
  REP (i, n+1) r += pr[i];
  return r;
}

int main() {
  ios::sync_with_stdio(false);
  psieve(pr);
  int n;
  while (cin >> n) {
    cout << solve(n) << endl;
  }
  return 0;
}

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

int main()
{
	int n = 0, x = 0;
	int countdata[1000];
	while (cin >> n) {
		int count = 0;
		for (int a = 2; a <= n; a++) {
			int count2 = 0;
			for (int b = 2; b <= a / 2; b++) {
				if (a%b != 0)
					count2++;
			}
			if (count2==a/2-1)
				count++;
		}
		countdata[x] = count;
		x++;
	}
	for (int a = 0; a < x; a++) {
		cout << countdata[a] << endl;
	}
}

C++: // 2012/11/07 Tazoe

#include <iostream>
using namespace std;

int main()
{
	bool is_prime[1000000];

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

	for(int i=2; i*i<1000000; i++){
		if(is_prime[i]){
			for(int j=i+i; j<1000000; j+=i){
				is_prime[j] = false;
			}
		}
	}

	int n_prime[1000000];

	int cnt = 0;
	for(int i=0; i<1000000; i++){
		if(is_prime[i]){
			cnt++;
		}

		n_prime[i] = cnt;
	}

	while(true){
		int n;
		cin >> n;

		if(cin.eof())
			break;

		cout << n_prime[n] << endl;
	}

	return 0;
}

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

using namespace std;

int main() {
	int n;

	while (cin >> n) {
		int counter = 0;
		
		if(n < 1000){
			for (int i = 2; i <= n; i++) {
				int kari = 1;
				for (int j = 2; j < i; j++) {
					kari = i % j;
					if(kari == 0) {
						break;
					}
				} if (kari != 0) {
					counter++;
				}
			}
		}else if(n < 10000){
			counter = counter + 168;
			for (int i = 1000; i <= n; i++) {
				int kari = 1;
				for (int j = 2; j < i; j++) {
					kari = i % j;
					if(kari == 0) {
						break;
					}
				} if (kari != 0) {
					counter++;
				}
			}
		} else if(n < 100000){
			counter = counter + 1229;
			for (int i = 10000; i <= n; i++) {
				int kari = 1;
				for (int j = 2; j < i; j++) {
					kari = i % j;
					if(kari == 0) {
						break;
					}
				} if (kari != 0) {
					counter++;
				}
			}
		} else {
			counter = counter + 9592;
			for (int i = 100000; i <= n; i++) {
				int kari = 1;
				for (int j = 2; j < i; j++) {
					kari = i % j;
					if(kari == 0) {
						break;
					}
				} if (kari != 0) {
					counter++;
				}
			}
		}
		cout << counter << endl;
	}

	return 0;
}