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)
	}
}

Python: import sys
lines = sys.stdin.readlines()
for line in lines:
    a = int(line)
    b = [False, False]
    for i in range(a-1):
        b.append(True)
    for i in range(a):
        if(b[i]):
            k = i
            while(k <= a-i):
                k += i
                b[k] = False
    #print(b)
    cnt = 0
    for i in b:
        if(i):
            cnt += 1
    print(cnt)

Python: # ?´???°?????°?????¨?????????????????°??????

import sys
import math

prim_no = {2: True}                    # ????????°????´???°??§??????????????????????????????

def is_prime(no):
    if no == 2 or no == 1:
        return True
    
    if no % 2 == 0:
        return False

    if prim_no.get(no) is not None:
        return prim_no.get(no)

    max_check = int(math.sqrt(no))
    for i in range(3, max_check+1, 2):
        if no % i == 0:
            prim_no[no] = False
            return False

    prim_no[no] = True
    return True

def main():
    prim_vals = {}                         # ????????°?????§????´???°????????°
    num_data = []                          # ?????????????????????
    
    while True:
        num = sys.stdin.readline()
        if num is None or num.strip() == '':
            break

        num = int(num.strip())
        num_data.append(num)

    sorted_num_data = sorted(num_data)
    prim_num = {}
    
    for num in sorted_num_data:
        if prim_vals.get(num) is not None:
            cnt = prim_vals.get(num)
        else:
            if num == 1:
                cnt = 0
            else:
                cnt = 0

                if num % 2 == 0:
                    start_num = num -1
                else:
                    start_num = num
                    
                for i in range(start_num, 0, -2):
                    if prim_vals.get(i) is not None:
                        cnt += prim_vals.get(i)
                        break
                    
                    if is_prime(i):
                        cnt += 1
                        
                prim_vals[num] = cnt # ????????°?????§????´???°????????°????????????
        prim_num[num] = cnt

    for num in num_data:
        print(prim_num[num])

if __name__ == '__main__':
    main()
    #print(prim_no)

Python: import sys
while True:
    try:
        s=int(input())+1
    except:
        sys.exit()
    if s<2:
        print(0)
    else:
        list=[True]*s
        list[0]=False
        list[1]=False
        i=2
        while i**2<=s-1:
            j=i*2
            while j<=s-1:
                list[j]=False
                j+=i
            i+=1
        print(list.count(True))

Python: import sys
a=[1]*500000
for i in range(3,999,2):
 if a[i//2]:a[(i*i)//2::i]=[0]*len(a[(i*i)//2::i])
for e in map(int,sys.stdin):print([e-1,sum(a[:(e+1)//2])][e>3])

Python: nums = []
while True:
    try:
        n = int(input())
    except EOFError:
        break
    nums.append(n)

M = max(nums)
primes = []
temp = [i for i in range(2,M+1)]
n = 2
while n*n <= M:
    primes.append(n)
    for i in temp:
        if i%n == 0:
            temp.remove(i)
    n=temp[0]

primes.extend(temp)
for i in nums:
    f = True
    for j,n in enumerate(primes):
        if n > i:
            print(j)
            f = False
            break
    if f:
        print(len(primes))

Python: import sys
results = {}
def fact(n):
    if n in results:
        return results[n]
    ret = [1]
    if n%2==0:
        ret.append(2)
        results[n]=ret
        return ret
    for i in range(3,int(n**0.5)+1,2):
        if n%i==0:
            ret.append(i)
            results[n]=ret
            return ret
    results[n]=ret
    return ret
def isPrime(n):
    if n<=1:
        return False
    if n==2:
        return True
    if fact(n)==[1]:
        return True
    else:
        return False
lines = sys.stdin.readlines()
for line in lines:
    inp = int(line)
    s=0
    for i in range(2,inp+1):
        if isPrime(i):
            s+=1
    print (s)

Python: import sys
for l in sys.stdin:
    s=int(l)
    ccc=0
    for kusa in range(2,s+1):
        bbb=0
        for kuso in range(1,s+1):
            if kusa%kuso==0:
                bbb=bbb+1
        if bbb==2:
            ccc=ccc+1
    print(ccc)

Python: def sieve(n):
    p = [True for i in range(n + 1)]
    p[0] = p[1] = False
    end = int(n ** .5)
    for i in range(2, end + 1):
        if p[i]:
            for j in range(i * i, n + 1, i):
                p[j] = False

    return p

def primes_below(n):
    if n < 2: return 0
    c = 1
    for i in range(3, n + 1, 2):
        if p[i]: c += 1

    return c

p = sieve(1000000)
while 1:
    try:
        n = int(input())
    except EOFError:
        break
    else:
        print(primes_below(n))

Python: import sys
import math

def prime_calc(n):
    rootN = math.floor(math.sqrt(n))
    prime = [2]
    data = [i + 1 for i in range(2,n,2)]
    while True:
        p = data[0]
        if rootN < p:
            return len(prime + data)
        prime.append(p)
        data = [e for e in data if e % p != 0]

def main():
    l = []

    for line in sys.stdin:
        l.append(int(line))

    for line in l:
        print(prime_calc(line))

if __name__ == "__main__":
    main()

Python: import sys, re
from collections import deque, defaultdict, Counter
from math import ceil, sqrt, hypot, factorial, pi, sin, cos, radians
from itertools import permutations, combinations, product
from operator import itemgetter, mul
from copy import deepcopy
from string import ascii_lowercase, ascii_uppercase, digits
from fractions import gcd
from bisect import bisect_left
from heapq import heappush, heappop

def input(): return sys.stdin.readline().strip()
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(): return list(map(int, input().split()))
sys.setrecursionlimit(10 ** 9)
INF = float('inf')
mod = 10 ** 9 + 7

def primes_for(n):
   is_prime = [True] * (n + 1)
   is_prime[0] = False
   is_prime[1] = False
   for i in range(2, n + 1):
       for j in range(i * 2, n + 1, i):
           is_prime[j] = False
   return [i for i in range(n + 1) if is_prime[i]]

prime_list = primes_for(10**6)
is_prime = [0]*10**6
for n in prime_list:
  is_prime[n] = 1

cnt = [0] * 10**6
for i in range(2, 10**6):
  if is_prime[i]:
    cnt[i] = cnt[i-1] + 1
  else:
    cnt[i] = cnt[i-1]

while 1:
  try:
    n = INT()
    print(cnt[n])
  except:
    sys.exit()

Python: import itertools, functools, math, sys

def prime_stream():
    stream = itertools.count(2)
    sieve = lambda x, y: x % y != 0

    while True:
        prime = next(stream)
        stream = filter(functools.partial(sieve, y=prime), stream)
        yield prime

for line in sys.stdin:
    primes = prime_stream()
    num = int(line)
    count = 0
    for i in range(num):
        if next(primes) > num:
            break
        count += 1
    print(count)