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<iostream>
using namespace std;

bool IsPrime(int n){

    if(n % 2 == 0)
        return false;
 
    for(int i = 3; i * i <= n; i += 2)
         if(n % i == 0)
            return false;
    return true;
}
int CheckNum(int input){
	int count = 1;

	for(int i = 2; i < input; ++i){

		if(IsPrime(i)) ++count;
	}
	return count;
}
int main() {
	int input;

	while(cin >> input)
	cout << CheckNum(input) << endl;
}

C++: #include <iostream>
#include <vector>
#include <numeric>
using namespace std;
int main() {
  int n;
  while (cin >> n) {
    vector<int> v;
    v.resize(n + 1);
    fill(v.begin(), v.end(), 1);
    v[0] = 0;
    v[1] = 0;
    for (int i = 0; i * i < v.size(); ++i) {
      while (v[i] == 0) ++i;
      for (int m = i + 1; m * m < v.size(); ++m) {
        if (m % i == 0) {
          v[m] = 0;
        }
      }
    }
    cout << accumulate(v.begin(), v.end(), 0) << endl;
  }
  return 0;
}

C++: #include<stdio.h>
#include<vector>
using namespace std;
int s[1000000];
int main()
{
	int   i,j,k,n,u;
	s[0]=2;k=1;
	for(i=3;i<100000;i++)
	{for(j=0;j<k;j++)
	if(i%s[j]==0)break;
	if(j==k)s[k++]=i;}
	while(~scanf("%d",&u))
	{for(j=0;j<k;j++)
	if(u<s[j])break;
	printf("%d\n",j);}
	return 0;
}

C++: #include<stdio.h>

int main(){
    int i,j,k,n,f;
    int stack[30],sp=0;
    while(scanf("%d",&stack[sp++])!=EOF);
    for(i=0;i<sp-1;i++){
        n=0;
        for(j=2;j<=stack[i];j++){
            f=0;
            for(k=2;k<j;k++)
                if(j%k==0)f=1;
            if(f==0)n++;
        }
        printf("%d\n",n);
    }
    return 0;
}

C++: #include <bits/stdc++.h>

using namespace std;

#define FOR(i,k,n) for(int i = (k); i < (n); i++)
#define REP(i,n) FOR(i,0,n)
#define INF 114514810
#define ELEM(array) (sizeof (array)/sizeof *(array))
#define MAX_N 100
#define SAFE_FREE(ptr) if( ptr != NULL ){ free(ptr); ptr = NULL; }
typedef unsigned int UINT;
#define ll long long
typedef pair<int, int> P;

bool prime[100005];


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

		int ans = 0;

		REP(i, 100005)prime[i] = true;
		prime[0] = prime[1] = false;
		FOR(i, 2, (int)sqrt(100005))
		{
			if (prime[i] == true)
			{
				FOR(j, 2, (int)sqrt(100005)){
					if (prime[i*j] == true)prime[i*j] = false;
				}
			}
		}
		REP(i, n + 1){
			if (prime[i] == true){
				ans++;
			}
		}
		cout << ans << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<cmath>
using namespace std;
typedef long long ll;
ll prime[1000001];
ll length = 0;
void insert(ll p) { prime[length] = p; length++; return; }
int main(){
	insert(2);
	for (ll i = 3; i < 1000000; i += 2) {
		for (ll j = 0; j < length; j++)
			if (i%prime[j] == 0)goto cont;
			else if (prime[j] > sqrt(i))break;
		insert(i);
	cont:;
	}
	ll n; while (cin >> n) {
		ll ans = 0;
		while (prime[ans] <= n){
			ans++;
		}
		cout << ans << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<vector>
#define MAX 1000000
using namespace std;
int main(){
    vector<int> v(MAX);
    for(int i = 0;i<MAX;i++)
      v[i]=true;

      v[0]=v[1]=false;
    for(int i = 2;i<1000;i++){
      if(v[i]){
        for(int j=i*i;j<MAX;j+=i){
         v[j]=false;
       }
      }
    }
    for(int i = 2;i<MAX;i++)v[i]+=v[i-1];
    while(true){
      int a;
      cin >> a;
      if(cin.eof())break;;
      cout << v[a] << endl;
    }
    return 0;
}

C++: #include <stdio.h>

const int MAX_V = 1000000;
int prime[MAX_V+1];
int a[MAX_V] = {'0'};
int sum[MAX_V+1];
int main() {
  int i, k, v, j, m, n;
  for(i = 2; i <= MAX_V; i++) {
    prime[i] = 1;
  }
  for(i = 2; i*i <= MAX_V; i++) {
    if(prime[i]) {
      for(k = 2 * i; k <= MAX_V; k += i) {
        prime[k] = 0;
      }
    }
  }
  sum[0] = 0;
  for(m = 0; m < MAX_V; m++) {
    sum[m+1] = sum[m] + prime[m];
  }
  while( scanf( "%d", &n ) != EOF ) {
    for( j = 2; j <= n; j++ ) {
      int isprime = prime[j];
      int isprimee = prime[j-2];
    }
    printf( "%d", sum[n+1] );
  }
  return 0;
}

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


bool isPrime(int I)
{
	for(int i = 2; i < I; i ++){
		if(I % i == 0)
			return false;
	}
	return true;
}
void printPrime(int N)
{
	int total = 0;
	for(int i = 2; i <= N; i ++)
	{		
		if(isPrime(i) == true){
			total ++;
		}
	}
	cout<<total<<endl;
}


int main(){
	int n;
	while(cin>>n){
		printPrime(n);
	}
	
}

C++: #include <stdio.h>
#include <math.h>
int sosuu(int x){
    int n,m,sosuu;
    sosuu=x-1;
    for(n=2;n<x;n++){//nは素数を調べる。入力された数字まで１からスライドしていく。
        for(m=2;m<n;m++){//mはnが素数かどうか調べる。素数かどうかをnより小さい数字で数えていく。2からなのは2から素数だから。1はふくめない。
            if(n%m==0){//素数じゃなかったら、つまり割り切れてしまったら。
                sosuu--;
            }
            
            if(n%m==0) break;
        }
    }
    if(x==1||x==0) sosuu=0;
    return sosuu;
}

int main ( void ) {
    int s,result;
    while(scanf("%d",&s)!=EOF){
        result = sosuu(s);
        printf("%d\n",result);
    }
    return 0;
}

C++: #include <cstdio>
#include <cstring>
#define MAXN 1000010
bool is_prime[MAXN];
int prime[MAXN];

void find(int n) {
    memset(is_prime,1,sizeof(is_prime));
    is_prime[0]=is_prime[1]=false;
    prime[0]=0;
    for(int i=2;i<=n;i++) {
        if(is_prime[i]) {
            prime[++prime[0]]=i;
            for(int j=2*i;j<=n;j+=i) is_prime[j]=false;

        }
    }
}


int bifind(int n) {
    int b=1,e=prime[0],mid;
    while(b<=e) {
        mid=(b+e)>>1;
        if(n==prime[mid]) return mid;
        //printf("%d %d %d\n",b,mid,e);
        else if(n>prime[mid]) {b=mid+1;}
        else {e=mid-1;}
    }
    return b-1;
}
int main() {
    int n;
    find(999999);
    //printf("%d\n",prime[0]);
    //printa(prime,prime[0]);
    while(scanf("%d",&n)!=EOF) {
         int pos=bifind(n);
         printf("%d\n",pos);
    }
}

C++: /*#include <stdio.h>
  int main(){
    int a,b,c,d,n,sum=0;
      while(scanf("%d",&n) != EOF){
       for(a=0;a<10;a++){
          for(b=0;b<10;b++){
             for(c=0;c<10;c++){
                for(d=0;d<10;d++){
                    if(a+b+c+d==n)
                        sum++;
                          }
                            }
                             }
                               }
                                printf("%d\n",sum);
                                 sum=0;
                                   }
return 0;
}*/#include <stdio.h>
  int main(){
   int i,j,n,hantei=0;
   int sum=0;
    while(scanf("%d",&n) != 0){
    for(i=2;i<=n;i++){
      for(j=2;j*j<=i;j++){
        if(i%j != 0 || i == j){
            hantei += 0;
               }else{
                  hantei += 1;
                    }
}
if(hantei == 0){
sum++;

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