Go: package main

// https://ja.wikipedia.org/wiki/%E3%82%A8%E3%83%A9%E3%83%88%E3%82%B9%E3%83%86%E3%83%8D%E3%82%B9%E3%81%AE%E7%AF%A9

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() (int, error) {
	return strconv.Atoi(nextString())
}

func main() {
	scanner.Split(bufio.ScanWords)

	for {
		n, err := nextInt()
		if err != nil {
			break
		}
		maybePrime := make([]bool, n+1)
		for i := 0; i <= n; i++ {
			maybePrime[i] = true
		}

		maybePrime[0] = false
		maybePrime[1] = false
		for i := 2; i <= int(math.Sqrt(float64(n)))+1; i++ {
			j := i * 2
			for j <= n {
				maybePrime[j] = false
				j += i
			}
		}

		nPrimes := 0
		for _, p := range maybePrime {
			if p {
				nPrimes++
			}
		}
		// fmt.Println(maybePrime)
		fmt.Println(nPrimes)
	}
}

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

using namespace std;

int main() {
  int n;

  while(!cin.eof()) {
    cin >> n;

    int m = sqrt(n);
    
    vector<bool> h(n+1, true);
    h[0] = false;
    h[1] = false;

    for(int i = 2; i <= m; i++) {
      if(h.at(i)) {
	for(int s = 2; s*i < h.size(); s++) {
	  if(h.at(s*i))
	    h[s*i] = false;
	}
      }
    }
    
    int ans=0;
    for(auto f : h) {
      if(f)
	ans++;
    }

    cout << ans << endl;
  }
  return 0;
}

C++: #include <stdio.h>
int prime[1000000],z;
bool p[1000010];
void pr()
{
    p[2] = 1;
    prime[z] = 2;
    z++;
    for(int i=3; i<1000000; i+=2)
    {
        p[i] = 1;
    }
    for(int i=3; i*i<1000000; i+=2)
    {
        if(p[i])
        {
            for(int j=i+i; j<1000000; j+=i)
            {
                p[j] = 0;
            }
        }
    }
    for(int i=3; i<1000000; i+=2)
    {
        if(p[i])
        {
            prime[z] = i;
            z++;
        }
    }
}

int main()
{
    pr();
    int n,cnt;
    while(~scanf("%d", &n))
    {
        cnt = 0;
        for(int i=0; i<z; i++)
        {
            cnt++;
            if(prime[i] > n)
            {
                cnt--; break;
            }
        }
        printf("%d\n", cnt);
    }
    return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstring>
using namespace std;

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

#define PI 3.14159265358479

typedef long long int ll;
typedef vector<long long int> vi;
typedef vector<long long int>::iterator vit;

int main(){
	vi v(1000010, 1);
	vi copy(1000011, 1);
	ll n, c = 0;
	double rt;

	v[0] = 0;

	while (cin >> n){
		rt = sqrt(n);
		FOR(i, 1, rt){
			if (v[i] == 1){
				for (int j = i + 1; (i + 1)*j <= n; j++){
					v[(i + 1)*j - 1] = 0;
				}
			}
		}
		/*REP(i,n){
			cout << v[i] << "\n";
		}*/
		REP(i, n){
			if (v[i] == 1)c++;
		}
		cout << c << "\n";
	}
}

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

int main(){
	float a[6];
	float x,y;
	while(true){
		cin >> a[0];
		cin >> a[1];
		cin >> a[2];
		cin >> a[3];
		cin >> a[4];
		cin >> a[5];
		if(cin.eof())
			break;
			
		x = (a[4]*a[2]-a[1]*a[5])/(a[0]*a[4]-a[1]*a[3]);
		y = (-a[3]*a[2]+a[0]*a[5])/(a[0]*a[4]-a[1]*a[3]);
		
		cout.precision(3);
		cout << x << " " << y << endl;
	}
	return 0;
}

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

int main()
{
	
	long long int k,p[100000];
	long long int bn,n=-1;
	p[0]=2;
	while(1){
		bn=n;
		cin>>n;
		if(n==bn)
			return 0;
	k=1;
	int r=(int)sqrt((float)n);
	//ツ確ツ認ツ用ツ素ツ青板づ債閉つ篠づ個づ慊づ必ツ要
	//ツ閉つ篠づ慊づづ慊づュツつキ
	for(int i=3;i<r;i++){
		//ツつサツづェツづ慊づづ個素ツ青板づ確ツ認
		for(int j=0;j<k;j++){
			//ツ催甘コツづ慊づ環づェツづ按つゥツづつスツづァツカツウツδ督トツつオツづ、ツ確ツ認ツ用ツ素ツ青板づ個配ツ療アツづ可格ツ納
			if(i%p[j]==0){
				break;
			}else if(j==(k-1)){
				p[k]=i;
				k++;
			}
		}
	}
	long long int r_k=k;
	for(;r<=n;r++){
		for(long int i=0;i<r_k;i++){
			if(r%p[i]==0){
				break;
			}else if(i==r_k-1){
				k++;
			}
		}
	}
	if(n==1){
		k=0;
		cout<<k<<endl;
	}else if(n==2){
		k=1;
		cout<<k<<endl;
	}else if(n==3){
		k=2;
		cout<<k<<endl;
	}else{
	cout<<k<<endl;
	}
	}
}

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

using namespace std;

vector<int> ps;

int isprime(int n){
	vector<int>::iterator it;
	for(it=ps.begin();it!=ps.end();it++){
		if(n%(*it) == 0) return 0;
		if(n <= (*it)*(*it)) break;
	}
	ps.push_back(n);
	return 1;
}

int main(){
	int n;
	ps.push_back(2);
	while(scanf("%d", &n)!=EOF){

		int c=0;
		int last=2;
		for(vector<int>::iterator it=ps.begin();it!=ps.end();it++){
			if((last = *it) <= n) c++;
		}

		for(int i=last+1;i<=n;i++){
			if(i&1 && isprime(i)) c++;
		}

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

C++: #include <stdio.h>
#include <math.h>
#include <stdlib.h>
#define N 999999L
bool num[N];

void furui(){
  long int i;
  int divide;
  int rt = (int) sqrt(N);
  for(i = 0L; i < N; i++){
    num[i] = true;
  }
  num[0] = false;
  num[1] = false;
  for(divide = 2; divide <= rt; divide++){
    if(num[divide] == false) continue;
    for( i = divide; i < N; i+= divide){
      num[i] = false;
    }
    num[divide] = true;
  }
}

int main(void){
  int c, i, j;
  long int k;
  char buf[7];
  long int n;
  long int count;

  furui();

  for(j = 0; j < 30; j++){
    for(i = 0; ;i++){
      c = getchar();
      if(c == EOF || c == '\n'){
        buf[i] = '\0';
        break;
      }
      buf[i] = c;
    }
    n = atol(buf);
    count = 0L;
    for(k = 1L; k <= n; k++){
      if(num[k] == 1) count++;
    }
    std::cout << count << '\n';
    if(c == EOF) break;
  }
  return 0;
}

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

using namespace std;

bool p[10000000];
vector<int> prime;

void init(){
    memset(p,1,sizeof(p));
    p[0] = p[1] = false;
    for(int i = 2; i <= 1000000; i++){
        if(!p[i]) continue;
        for(long long j = 1LL*i*i; j <= 1000000; j += i)
            p[j] = false;
        prime.push_back(i);
    }
}

int CntPrime(int n){
    vector<int>::iterator it = lower_bound(prime.begin(), prime.end(), n);
    if( *it > n) it--;
    return (it - prime.begin() + 1);
}

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

C++: #include<iostream>
 
using namespace std;
 
int main(){
  static bool cmp[1000000]={true};
  static int npr[1000000];
  for(int i=2;i<1000000;i++){
    if(!cmp[i]){
      for(int j=2;j*i<1000000;j++){
    cmp[i*j]=true;
      }
    }
    npr[i]=npr[i-1]+!cmp[i];
  }
  int n;
  while(cin>>n){
    cout<<npr[n]<<endl;
  }
  return 0;
}

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

int main(){
    int n, t;
    while(cin >> n){
      t = 0;
      for(int i = 2;i <= n;++i){    
        for(int j = 2;j < i;++j){
          if(i % j == 0) break;
          if(j == i - 1) ++t;
        }
      }
      if(n >= 2) ++t;
      cout << t << endl;
    }
  return 0;
}

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

const int MAX=1000000;
int N;
vector<int>ip(1000001,1);
int ans=0;




int main(){
	for(int i=2;i<1000;i++){
		if(ip[i]){
			for(int j=0;i*(j+2)<1000000;j++){
				ip[i*(j+2)]=0;
			}
		}
	}
	ip[0]=0;
	ip[1]=0;


	while(cin>>N){

		for(int i=0;i<=N;i++){
			if(ip[i])ans++;
		}
	

		cout<<ans<<endl;
		ans=0;

	}


	
}

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

using namespace std;

const int INF=1000000;

int main()
{
    int pri[INF];
    int res[INF];
    for(int i=0;i<INF;i++)
        pri[i]=0;
    for(int i=2;i*i<INF;i++)
    {
        if(pri[i]==0)
        {
            for(int j=i*2;j<INF;j+=i)
            {
                pri[j]=1;
            }
        }
    }
    int n=0;
    for(int i=2;i<INF;i++)
    {
        if(pri[i]==0)
            n++;
        res[i]=n;
    }
    int C;
    while(cin>>C)
    {
        cout<<res[C]<<endl;
    }
}