Go: package main
import (
	"fmt"
	"os"
	"strconv"
	"bufio"
	//"math"
)

var sc = bufio.NewScanner(os.Stdin)

func main(){
	for sc.Scan(){
		n := nextInt()
		fmt.Println(calc(n))
	}
}

func calc(n int)int{
	// エラトステネスで数え上げる
	// 最初から999999まで用意しておいて入力のたびO(1)で出すのとどっちが速いのかはわからん
	eratos := make([]bool, n+1)
	c := 0
	for i:=2; i<len(eratos); i++{
		if eratos[i] == false{
			c++
			for j:=i*i; j<len(eratos); j+=i{
				eratos[j]=true
			}
		}
	}
	return c
}

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

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

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

import sys
import os
import math



def Eratosthenes(n):
    lst = [i for i in range(2, n+1)]
    primes = []
    sqrt_n = n ** 0.5

    while True:
        first = lst.pop(0)
        if first < sqrt_n:
            primes.append(first)
            new_lst = []
            for v in lst:
                if v % first != 0:
                    new_lst.append(v)
            lst = new_lst
        else:
            # ??????
            return primes + [first] + lst

s_list = sys.stdin.readlines()
for s in s_list:
    n = int(s)
    lst = Eratosthenes(n)
    print(len(lst))

Python: import math
import sys

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(line)        

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

if __name__ == "__main__":
    main()

Python: import sys

for input in sys.stdin:
    prime_num = 0
    for i in xrange(int(input)):
        y = i+1
        x = y / 2
        r = 2
        while r <= x:
            if y % r == 0:
                break
            r += 1
        else:
            prime_num += 1
    print(prime_num-1)

Python: import math
def sosu(n) :
    for i in range(3, int(math.sqrt(n))+1, 2) :
        if n % i == 0 :
            return False
    return True

S = [0, 0, 1, 2, 2, 3, 3, 4, 4, 4, 4]
for i in range(11, 1000000) :
    if i % 2 == 0 :
        S.append(S[i-1])
    elif sosu(i) :
        S.append(S[i-1]+1)
    else :
        S.append(S[i-1])


while True :
    try :
        n = int(input())
    except EOFError :
        break
    
    print(S[n])

Python: def prime_judge(n):
    if n==1:
        return False
    elif n==2:
        return True
    elif n%2==0:
        return False
    else:
        sqrt_num=int(n**0.5)+1
        for i in range(3,sqrt_num, 2):
            if n%i==0:
                return False
        return True

prime=[0]
for i in range(1,1000000):
    if prime_judge(i):
        prime.append(prime[-1]+1)
    else:
        prime.append(prime[-1])

while 1:
    try:
        print(prime[int(input())])     
    except:
        break

Python: # PrimeNumber
# エラトステネスの篩

from itertools import product
import math

if __name__ == "__main__":
    while True:
        try:
            # 数値の入力があった
            n = int(input())
            nlist = []
            nlist2 = []
            plist = []

            # for i in range(2, n + 1):
            #    nlist.append(i)  # [2,3,4,5,6,...,n]
            nlist = range(2, n + 1, 1)

            while True:
                #print(nlist, " ", math.sqrt(n))
                sqrtn = math.sqrt(n)
                if nlist[0] > sqrtn:  # 終了条件
                    #plist.append(map(int,nlist))
                    plist.extend(nlist)
                    break
                else:
                    plist.append(nlist[0])

                    #i = 0
                    for j in nlist:
                        if j % nlist[0] != 0:
                            #nlist.pop(i)
                            #nlist.remove(j)
                            nlist2.append(j)
                            #i += 1
                    nlist = nlist2
                    nlist2 = []
            #print("fin: {}".format(plist))
            print(len(plist))

        except EOFError:
            break  # escape from while loop

pass
#

Python: # coding: utf-8
# Here your code !
import math
 
def is_prime_num(in_num):
    cnt = 0
    if in_num == 2:
        return True
    elif in_num <= 1:
        return False
    elif in_num % 2 == 0:
        return False
    else:
        if pow(2, in_num-1, in_num) == 1:
        
            max_num = int(math.sqrt(in_num)+1)
            for j in range(3, max_num, 2):
                if in_num % j == 0:
                    cnt = cnt + 1
                    break
                         
            if cnt == 0:
                #??´????°
                return True
            else:
                return False
        else:
            return False

while True:
    try:
        n = int(input())
        prime_cnt = 0
 
        for i in range(2, n+1):
            if is_prime_num(i):
                #??´????°
                prime_cnt = prime_cnt + 1
                 
 
        print(str(prime_cnt))
    except:
        break

Python: import sys
import math
M = 1000000
B = int(math.sqrt(M)+1)
N = M//6
N1 = N+1 if M % 6 == 5 else N
N2 = N+1 if M % 6 >= 1 else N
l = [1] * N1
m = [1] * N2
for i in range(5,B,6):
    p = (i-5)//6
    k = p+i
    ini = (i*5-1)//6
    if l[p] == 1:
        l[k::i] = [0] * len(l[k::i])
        m[ini-1::i] = [0] * len(m[ini-1::i])
    if m[p] == 1:
        m[k+2::i+2] = [0] * len(m[k+2::i+2])
        l[ini+1::i+2] = [0] * len(l[ini+1::i+2])

for i in sys.stdin:
    n = int(i)
    r =  n-1 if n < 4 else sum(l[0:(n+1)//6])+sum(m[0:(n-1)//6])+2
    print(r)

Python: import numpy
def get_input():
    while True:
        try:
            yield ''.join(input())
        except EOFError:
            break
def primesfrom3to(n):
    sieve = numpy.ones(int(n/2), dtype=numpy.bool)
    for i in range(3,int(n**0.5)+1,2):
        if sieve[int(i/2)]:
            sieve[int(i*i/2)::int(i)] = False
    return 2*numpy.nonzero(sieve)[0][1::]+1
if __name__ == "__main__":
    array = list(get_input())
    for i in range(int(len(array))):
        primeList = list(primesfrom3to(int(array[i])))
        primeList.insert(0,2)
        print(len(primeList))

Python: # coding: utf-8

import sys
import math

max = 11
prime = [2,3,5,7,11]

for line in sys.stdin:
    
    n = int(line)
    
    if n > max:
        for m in range(max + 1, n + 1):
            flag = 1
            
            if m % 2 == 0:
                flag = 0
            else:
                for p in prime:
                    if p > math.sqrt(m):
                        break
                    if m % p == 0:
                        flag = 0
                        break
            
            if flag == 1:
                prime.append(m)
        
        max = n
    
    count = 0
    for i in prime:
        if i > n:
            break
        count += 1
        
    print(count)

Python: plst = [1 for i in range(1000001)]
plst[0] = plst[1] = 0
for i in range(2,1000001):
  if plst[i] == 1:
    for j in range(i * 2,1000001,i):
      plst[j] = 0
while True:
  try:
    print(sum(plst[:int(input()) + 1]))
  except EOFError:
    break

Python: import sys
a=[0,0,1]+[1,0]*5*10**5
for i in range(4,999):
 if a[i]:a[i*2::i]=[0]*len(a[i*2::i])
for e in sys.stdin:print(sum(a[:int(e)+1]))

Python: import sys
m=10**6;a=[0,0]+[1]*m
for i in range(2,999):
 if a[i]>0:
  for j in range(i*2,m,i):a[j]=0
for e in sys.stdin:print(sum(a[:int(e)+1]))