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<bits/stdc++.h>
using namespace std;

#define maxPN 100000
#define PNs  1000100
int countPN = 0, countPNs = 0;
int PrimeNum[maxPN], temp;
bool PN[PNs];

bool IsPrimeNumber(int x);

void PrimeN(){
	int i, j;
	PrimeNum[0] = 2;
	PN[2] = true;PrimeNum[countPN++] = 2;
	PN[3] = true;PrimeNum[countPN++] = 3;
//	PN[5] = true;PrimeNum[countPN++] = 5;
	
	for(i = 6; i  < PNs;i+=6){
		if(IsPrimeNumber(i-1)){
			PrimeNum[countPN++] = i-1;
			PN[i-1] = true;//cout<<i-1<<endl;
		}
		if(IsPrimeNumber(i+1)){
			PrimeNum[countPN++] = i+1;
			PN[i+1] = true;//cout<<i+1<<endl;
		}
		countPNs = i+1;//cout<<i-1<<" "<<PN[i-1]<<endl;
	}
}

bool IsPrimeNumber(int x){
	//if(x == 2 || x == 3) return true;
	//if(x <= 4 || x%2 == 0 || x%3 == 0)   return false;
	//if(x < countPNs) return PN[x];
	for(int i = 0; PrimeNum[i] * PrimeNum[i] <= x; i++)
		if(x%PrimeNum[i] == 0)return false;
	return true;
}


int main(){
//	clock_t start, end;
	int n = true, counta;
//	start = clock();
	PrimeN();
	cout<<PrimeNum[countPN-1]<<" "<<countPNs<<endl;
//	cout<<clock()-start<<"ms"<<endl;
	
	while(cin>>n){
		counta = 1;
		
		
		//cout<<(IsPrimeNumber(n)?"yes":"no")<<endl;
	//	start = clock();
		//cout<<IsPrimeNumber(n)<<endl;
		for(int i = 1; i <= n; i+=2){
			if(PN[i])counta++;
		//	cout<<i<<" "<<PN[i]<<endl;
		}
		if(n == 1) counta = 0;
		cout<<counta<<endl;
	//	cout<<clock()-start<<"ms"<<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <vector>
using namespace std;
int main( void )
{
	int n;
	cin >> n;
	
	vector <bool> p(n+1, true);
	p[0] = p[1] = false;
	for ( int i = 4; i <= n; i += 2 )
	{
		p[i] = false;
	}
	for ( int i = 3; i <= n; i += 2 )
	{
		if ( p[i] )
		{
			for ( int j = i+i; j <= n; j += i )
			{
				p[j] = false;
			}
		}
	}

	int kotae = 1;
	for ( int i = 3; i <= n; i += 2 )
	{
		if ( p[i] )
			kotae++;
	}

	cout << kotae << endl;

	return 0;
}

C++: #include <iostream>
#include <math.h>
#define MAX 80000
using namespace std;

int main(){
	int n;
	int prime[MAX],i,j;
	int count;
	bool flag;

	while((cin >> n)!= NULL){
		count = 0;
		for(i = 3; i <= n; i+=2){
			flag = true;
			for(j = 0; j < count; j++){
				if(i % prime[j] == 0){
					flag = false;
					break;
				}
			}
			if(flag == true){
				prime[count++] = i;
				cout<<count<<endl;
			}
		}
		if(n > 1)	count++; //2
		cout << count << endl;
		//delete[] prime;
	}
	return 0;
}

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

using namespace std;
const int MAX = 1000000;
int data[MAX];

void make(){
  fill(data,data+MAX,1);
  
  for(int i = 2; i*i < 1000000; i++){
    if(data[i] == 1){
      for(int j = i*2; j < 1000000; j+=i) data[j] = 0;
    }
  }
}

int main(){
  make();
  int in;
  while(cin >> in){
    
    int sum = 0;
    for(int i = 2; i <= in; i++){
      if(data[i] == 1) sum++;
    }
    
    cout << sum <<endl;
  }
  return 0;
}

C++: #include <iostream>
#include <cmath>
#include <algorithm>
#include <string>
#include <stdio.h>
using namespace std;
int main(){
    int a,t[1000000],u[1000000];
    for(int i=0;i<1000000;i++){
        t[i]=1;
    }
    t[0]=t[1]=u[0]=u[1]=0;
    for(int i=2;i<=1000000;i++){
        if(t[i]==1){
            for(int j=i+i;j<1000000;j+=i)t[j]=0;
            u[i]=u[i-1]+1;
        }else{
            u[i]=u[i-1];
        }
    }
    while(cin>>a){
        cout<<u[a]<<endl;
    }
    return 0;
}

C++: #include<cstdio>
#include<cmath>
using namespace std;
bool book[1000000];
int search[1000000];
int main()
{
	int n,x=0;
	
	int m=floor(sqrt(999999)+0.5);
	for(int i=2;i<=m;i++)
	{
		if(book[i]==false)
		{
			search[i]=++x;
			for(int j=2*i;j<=999999;j+=i)
				book[j]=true;
		}
		else
			search[i]=x;
	}
	for(int i=m+1;i<=999999;i++)
	{
		if(book[i]==false)
			search[i]=++x;
		else
			search[i]=x;
	}
	
	while(~scanf("%d",&n))
	{
		printf("%d\n",search[n]);
	}
	return 0;
}

C++: #include<stdio.h>

int main(void)
{
	int d=2,i,j,s[100000],x;

	while(scanf("%d",d)!=EOF)
	{
		for(i=0;i<=d;i++){
			s[i]=1;}
		x=0;
		if(d<=1){
			printf("%d\n",0);}
		else
		{
			for(i=2;i<=d/2;i++){
			  if(s[i]!=0){
				for(j=i*2;j<d;j+=i)
				    s[j]=0;
			  }
			}
		for(i=0;i<d;i++)
			x+=s[i];
		printf("%d\n",x);
		}
	}
  return 0;
}

C++: #include<stdio.h>
#include<iostream>
#include<algorithm>
#include<vector>
#include<queue>
#include<functional>
 
 
#define rep(i,n) for(int i=0;i<(n);i++)
 
using namespace std;
const int INF = 100000000;
const double EPS = 0.0000001;
typedef pair<int,int> P;

int n;

bool isPrime[100003];

void solve() {
	int ans=0;
	rep(i,n+1) if(isPrime[i]) ans++;
	cout<<ans<<endl;
}

void init() {
	rep(i,100003) isPrime[i]=true;
	isPrime[0]=isPrime[1]=false;

	for(int i=2;i<100003;i++) {
		if(isPrime[i]) {
			for(int j=i+i;j<100003;j+=i) isPrime[j]=false;
		}
	}


}

int main() {
	init();
	while (cin>>n)
	{
		if(n==0) return 0;
		solve();

	}
}

C++: #include<iostream>
#include<string>
#include<math.h>
using namespace std;
int main(){
	int n,a[9];
	a[0]=9592;
	a[1]=17984;
	a[2]=25997;
	a[3]=33860;
	a[4]=41538;
	a[5]=49098;
	a[6]=56543;
	a[7]=63951;
	a[8]=71274;
	while(cin>>n){
		int count=0;
		int i=0;
		while(true){
			if((i+1)*100000<=n)
				count=a[i];
			else
				break;
			i++;
		}
		if(n<100000){
			count=1;
			for(int j=3;j<=n;j++){
			bool t=false;
			for(int k=2;k<1+(double)sqrt((double)j);k++){
				if(j%k==0)
					t=true;
			}
			if(t==false)
				count++;
		    }
		}
		else{
		    for(int j=(i)*100000+1;j<=n;j++){
			    bool t=false;
			    for(int k=2;k<1+(double)sqrt((double)j);k++){
				    if(j%k==0)
				        t=true;
			    }
			    if(t==false)
				    count++;
		    }
		}
		cout<<count<<endl;
	}
	return 0;
}

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

using namespace std;

int main()
{
	
	long long int k,p[100000];
	long long int n;
	p[0]=2;
cin>>n;

		if(n<0){
			break;
		}
	k=0;
	int r=(int)sqrt((float)n);
	for(int i=3;i<=r;i++){
		for(int j=0;j<k+1;j++){
			if(i%p[j]==0){
				break;
			}else if(j==k){
				k++;
				cout<<i<<endl;
				p[k]=i;
			}
		}
	}
	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){
				k++;
				cout<<r<<endl;
			}
		}
	}
	if(k!=0&&k!=1){
		k+=1;
		cout<<k<<endl;
	}else{
		cout<<k<<endl;
	}
}

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

C++: #include <iostream>
#include <vector>
using namespace std;
int main(){
int n;
cin >> n;
vector<int> prime;
prime.push_back(2);//ôðÈ­½ß2ÍÅÉüêÄ¨­
bool flag;
for(int x=3;x<=n;x+=2){
flag=true;
for(int y=0;prime[y]*prime[y]<=x;y++){
if(!(x%prime[y])){
flag=!flag;
break;
}
}
if(flag)prime.push_back(x);
}
cout << prime.size() << endl;
return 0;
}