Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		input := nextInt()
		fmt.Println(IsPrime(input))
	}
}

func IsPrime(num int) int {
	eratos := make([]bool, num+1)
	c := 0
	for i := 2; i < len(eratos); i++ {
		if eratos[i] == false {
			c++
			eratosCalc(i, eratos)
		}
	}
	return c
}

func eratosCalc(primeNum int, eratos []bool) {
	for i := primeNum * primeNum; i < len(eratos); i += primeNum {
		eratos[i] = true
	}
}

func next() string {
	return sc.Text()
}

func nextInt() int {
	a, _ := strconv.Atoi(next())
	return a
}

Python: import sys
import math

N = 1000000
primes = [1] * (N//2+1)
primes[0] = 1

for i in range(3,int(math.sqrt(N))+1,2):
    if primes[i//2]:
        primes[(i*i)//2::i] = [0] * len(primes[(i*i)//2::i])
    primes[i//2] += primes[i//2-1]

for i in range(int(math.sqrt(N))+1,N+1,2):
    primes[i//2] += primes[i//2-1]


for i in sys.stdin:
    n = int(i)
    if n == 1:
        print(0)
    elif n == 2:
        print(1)
    else:
        print(primes[(n-1)//2])

Python: # coding: utf-8

import math

def sieve_of_erastosthenes(num):

    if 2 > num:
        return 0

    serial_number_list = [r for r in range(2, num + 1)]
    multiple_list = []
            
    while math.sqrt(num) >= serial_number_list[0]:
        i = 1

        serial_number = serial_number_list[0]
  
        multiple_list.append(serial_number)
                    
        while serial_number_list[len(serial_number_list) - 1] >= serial_number * i:
    
            if serial_number * i in serial_number_list:
                
                serial_number_list.pop(serial_number_list.index(serial_number * i))

            i += 1

    return len(multiple_list + serial_number_list)

if __name__ == '__main__':
    input_list = []
    
    while True:
        try:
            print(sieve_of_erastosthenes(int(input())))
        except:
            break

Python: # -*- coding: utf-8 -*-

import sys
from itertools import accumulate

def input(): return sys.stdin.readline().strip()
def list2d(a, b, c): return [[c] * b for i in range(a)]
def list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)]
def list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]
def ceil(x, y=1): return int(-(-x // y))
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(N=None): return list(MAP()) if N is None else [INT() for i in range(N)]
def Yes(): print('Yes')
def No(): print('No')
def YES(): print('YES')
def NO(): print('NO')
sys.setrecursionlimit(10 ** 9)
INF = float('inf')
MOD = 10 ** 9 + 7

def eratosthenes_sieve(n):
    """ 素数列挙(エラトステネスの篩) """

    table = [0] * (n + 1)
    prime_list = []
    for i in range(2, n + 1):
        if table[i] == 0:
            prime_list.append(i)
            for j in range(i + i, n + 1, i):
                table[j] = 1
    return prime_list

N = 1000000
primes = eratosthenes_sieve(N)
A = [0] * 1000000
for p in primes:
    A[p] = 1
acc = list(accumulate(A))

while True:
    try:
        a = INT()
        print(acc[a])
    except:
        # 入力が取れなかったら終了
        break

Python: # coding: utf-8
# Here your code !
#coding:utf-8
import math
import sys

def flatten(L):
    if isinstance(L, list):
        if L == []:
            return []
        else:
            return flatten(L[0]) + flatten(L[1:])
    else:
        return [L]

def Eratos(data):

    p_list = [] # ????????????
    tmp = []
    num = int(math.sqrt(len(data)))+1
    for i in data:
        i=data[0]
        if i>num:
            break
        else:
            p_list.append(i)
            for j in data:
                if j%i == 0:
                    data.remove(j)
        
    p_list.append(data)
    return p_list

N = input()
a = []
lists = []
for line in sys.stdin:
    # a.append(input())
    # print(line)
    x= list(range(2,(int(line)+1)))
    print(len(flatten(Eratos(x))))

# r_list = list(range(2,(x+1)))
# ans = []
# ans = Eratos(r_list)

# print(len(flatten(ans)))

Python: import sys
def main():
    for number in sys.stdin:
        p = 0
        number = int(number)
        for num in range(1, number+1,2):
            a = int(num**0.5)
            bl = True
            for k in range(2, a+1):
                if num % k == 0:
                    bl = False
                    break
            if bl:
                p += 1
        print(p)
    return None


if __name__ == '__main__':
    main()

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: L = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541, 547, 557, 563, 569, 571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643, 647, 653, 659, 661, 673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751, 757, 761, 769, 773, 787, 797, 809, 811, 821, 823, 827, 829, 839, 853, 857, 859, 863, 877, 881, 883, 887, 907, 911, 919, 929, 937, 941, 947, 953, 967, 971, 977, 983, 991, 997]

def is_prime_2(n):
  a = int(n ** 0.5)
  for i in L:
    if i > a:return True
    if n % i == 0:return False
  return True

def prime_count(n):
  result = 0
  if n >= 2:result+=1
  for i in range(3,n+1,2):
    if is_prime_2(i):result+=1
  return result

while 1:
  print(prime_count(int(input())))