Go: package main

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

func main() {
	scanner := bufio.NewScanner(os.Stdin)

	for scanner.Scan() {
		num, _ := strconv.Atoi(scanner.Text())
		flag := false
		count := 0

		for i := 2; i <= num; i++ { //1を除く入力値以下の数字
			for j := i - 1; j >= 1; j-- {
				if i%j == 0 && j != 1 {
					flag = true
					break
				}
			}
			if !flag {
				count++
			} else {
				flag = false
			}
		}
		fmt.Println(count)
	}
}

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

int main(){
	int n=0,i,g,k,count=0,keta[6],result,wa3=0,wa4=0,wa8=0;
	double x,y;
	bool p=false;
	while(scanf("%d",&n)==1){
	for(x=0;x<6;x++){
		i=x;
		result=pow(10,x);
		keta[i]=n%result;
	}
	for(i=0;i<6;i++){
		wa3+=keta[i];
		if(i<1) wa4+=keta[i];
		if(i<2) wa8+=keta[i];
	}
	for(g=2;g<=n;g++){
		for(i=2;i<g;i++){
			if(i>n/2+1){
				if(wa3%3==0) break;
				if(wa4%4==0) break;
				if(wa8%8==0) break;
				if(n % i ==0){
				 p=true;
				 break;
				}
			}
			else if(g % i == 0){
				p=true;
				break;
			}
		}
		if(p) p=false;
		else{
			count++;
			p=false;
			}
		}
	printf("%d\n",count);
	count=0;
	wa3=0;
	wa4=0;
	wa8=0;
	}
	return 0;
}

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

int main()
{
	const size_t MAX = 1000000;

	bitset<MAX> primebox;
	primebox.set();
	primebox.reset(0);
	primebox.reset(1);

	for(int i = 2; i < MAX; i++)
	{
		if(!primebox.test(i))
			continue;

		for(int j = i*2; j < MAX; j += i)
			primebox.reset(j);
	}

	size_t n;
	while(cin >> n)
	{
		int found;
		if(n < 2)
			found = 0;
		else if(n == 2)
			found = 1;
		else
		{
			found = 1;
			for(size_t i = 3; i <= n; i += 2)
				if(primebox.test(i))
					found++;
		}

		cout << found << endl;
	}

	return 0;
}

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

using namespace std;
#define MAX_NUM 999999


void do_calc(int *numPrime){
  for(int i=2;i<=MAX_NUM;i++){
    bool flag = false;

    for(int j=2;j<=sqrt(i);j++){
      if(i%j==0){
	flag = true;
	break;
      }
    }
      
    if(flag){
      numPrime[i] = numPrime[i-1];
      continue;
    }else{
      numPrime[i] = numPrime[i-1] + 1;
    }
  }
}
  


int main(){
  int n;
  int numPrime[MAX_NUM];

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

C++: #include<stdio.h>
bool b[4000000];

int main(void)
{
    int a,n,i;
    int count=0;
    int f=1;

    for(i=3;i<4000000;i+=2){
        f=1;
        for(a=2;a*a<=i;a++){
            if(i%a == 0){
                f=0;
                break;

            }
        }
        if(f==1){
            b[i]=1;
        }
    }
    b[2]=1;

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

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

C++: #include<cstdio>
#include<cmath>
#define SIZE 1000000
#define sizE 1000000

int prime[sizE], nprime=0;
int mark[SIZE];

void sieve()
{
    int i,j, limit=sqrt(SIZE*1.0)+2;

    mark[0]=1;
    mark[1]=1;

    for(i=4; i<=SIZE; i+=2)
        mark[i]=1;

    prime[nprime]=2;

    for(i=3; i<=SIZE; i+=2)
    {
           if(!mark[i])
            {
            prime[++nprime] = i;

            if(i<=limit)
            {
                for(j=i*i; j<=SIZE; j+=i*2)
                mark[j]=1;
            }
        }
    }

}

int main()
{
    int n;

    sieve();

    while(scanf("%d", &n)==1)
    {
        int i,k;
        i=k=0;

        while(i++<n)
        {
            if(!mark[i])
            k++;
        }

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

    return 0;
}

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

int main()
{
	bool is_prime[1000000];
	memset(is_prime,false,sizeof(is_prime));
	for(int i=2;i<1000000;i++){
		if(!is_prime[i]){
//			cout<<i<<endl;
			for(int m=i*2;m<1000000;m+=i){
				is_prime[m]=true;
			}
		}
	}
	int n;
	while(cin>>n){
		int ans=0;
		for(int i=2;i<=n;i++){
			if(!is_prime[i]) ans++;
		}
		cout<<ans<<endl;
	}
}

C++: #include <iostream>
#include <vector>
#include <limits.h>
using namespace std;
int main()
{
	int i = 0, maxV = INT_MIN, A[30] = {};
	for (int k; cin >> k; ) {
		A[i++] = k;
		if (maxV < k)
			maxV = k;
	}
	int s = maxV + 1;
	vector<long> vP(s, 0);
	int c = 0, j;
	for (int k = 2; k < s; ++k){
		if (vP[k] == 0){
			vP[k] = ++c;
			for (j = 2; j * k < s; ++j)
				vP[j * k] = -1;
		}
		else if (vP[k] == -1)
			vP[k] = c;		
	}
	for (j = 0; j < i; ++j)
		cout << vP[A[j]] << "\n";
	return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
	int a,n[100000],j,o=0;
	for(int i=2;i<=99999;i++){
		n[i]=1;
		
	}
	n[0]=n[1]=0;
		for( j=2;j<=99999;j++){
			if(n[j]){
				for(int k=j*2;k<=9999;k+=j){
				n[k]=0;
				}
			}
		}
		while(cin>>a){
	for(int k=2;k<=a;k++) if(n[k]) o++;
	cout<<o<<endl;
		}
}

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

int main(){
    int n, cnt;
    bool prime[1000000] = {false, false};

    for(int i = 2;i < 1000000;i++){
        prime[i] = true;
    }
    
    for(int i = 2;i <= sqrt(1000000);i++){
        if(prime[i]){
            for(int j = 2*i;j <= 1000000;j++){
                if(j%i == 0){
                    prime[j] = false;
                }
            }
        }
    }
    
    while(cin >> n){
        cnt = 0;

        for(int i = 2;i <= n;i++){
            if(prime[i]){
                cnt++;
            }
        }
        
        cout << cnt << endl;
    }

    return 0;
}

C++: #include <iostream>

using namespace std;

bool is_prime(int n) {
        if (n < 2) {
                return false;
        }

        int max = n / 2;
        int i;
        for (i=2; i < max; i++) {
                if (n % i == 0) {
                        return false;
                }
        }

        return true;
}

int number_of_primes_under(int n) {
        if (n < 2) {
                return 0;
        }

        int count = 1;
        int i;
        for (i=3; i <= n; i = i + 2) {
                if (is_prime(i)) {
                        count++;
                }
        }

        return count;
}
int main(void) {
        int n;
        while(cin >> n) {
                cout << number_of_primes_under(n);
                cout << endl;
        }

        return 0;
}

C++: #include <iostream>
using namespace std;
bool prime[1000000];
void sieve() {
    fill_n(prime, 1000000, true);
    for (int i=2; i<=1000; ++i) {
        if (prime[i]) {
            for (int j=i*i; j<1000000; j+=i) {
                prime[j] = false;
            }
        }
    }
}
int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    sieve();

    int memo[1000000];
    for (int i=1; i<1000000; ++i) {
        memo[i] = memo[i-1]; 
        if (prime[i]) {
            memo[i] ++; 
        }
    }

    int n;
    while (cin >> n) {
        int cnt = 0;
        for (int i=2; i<=n; ++i) {
            if (prime[i]) {
                cnt ++;
            }
        }
        cout << cnt << "\n";
    }
    return 0;
}

C++: /// http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=0009
/// http://www.tsg.ne.jp/TT/tsg/c/mansaku/P01.html
#include<iostream>
#include<cmath>
using namespace std;

int main(){
    int n;
    while(~scanf("%d", &n)){
        int cnt = 0;
        for(int i = 3; i<=n; i+=2){
            int k= 0;
            for(int j=3; j<sqrt(i); j+=2){
                if(i%j==0){
                    k=1;
                    break;
                }
            }
            if(k==0) cnt++;
        }
        printf("%d\n", cnt);

    }

    return 0;
}

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

int main(void){
int unti[100002] = {0};
fill(unti,unti + 100001,0);
for(int n = 2;n < 100001;n++)
{
if(unti[n] == 0)
{
for(int a = 2;(a * n) < 100001;a++)
{
	unti[a * n]++;
}
}
}
int b;
while(cin >> b )
{
int cnt = (count(unti,unti+b+1,0) - 2);
cout << cnt << endl;
}
return 0;
}