Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	for sc.Scan() {
		a, _ := strconv.Atoi(sc.Text())
		fmt.Println(countPrime(a))
	}
}

func countPrime(a int) int {
	primes := make([]bool, a+1)
	n := 0
	for i := 2; i <= a; i++ {
		if primes[i] == false {
			n++
			for j := i * i; j <= a; j += i {
				primes[j] = true
			}
		}
	}
	return n
}

Python: import sys
import math

def isPrime(x):
    if x == 2:
        return True
    if x < 2 or x%2 == 0:
        return False
    for i in range(3,int(math.sqrt(x))+1,2):
        if x%i == 0:
            return False
    return True

if __name__ == '__main__':
    a = []
    for tmp_line in sys.stdin:
        n = (int)(tmp_line)
        a.append(n)
    primes = []
    for x in range(2,max(a)+1):
        if isPrime(x):
            primes.append(x)
    for y in a:
        print(len([x for x in primes if x <= y]))

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])
[print([e-1,sum(a[:(e+1)//2])][e>3])for e in map(int,sys.stdin)]

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

import sys
import math

prim_no = {}                    # ????????°????´???°??§??????????????????????????????

def is_prime(no):
    if no == 2:
        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():
    while True:
        num = sys.stdin.readline()
        if num is None or num.strip() == '':
            break

        num = int(num.strip())
        cnt = 0
        for i in range(2, num + 1):
            if is_prime(i):
                cnt += 1

        print(cnt)

if __name__ == '__main__':
    main()

Python: def isPrime(x):
    if x==2:
        return True
    if x<2 or x%2==0:
        return False
    if pow(2,x-1,x)==1:
        return True
    else:
        return False
while 1:
    try:
        n=input()
        ct=1
        if n==2:
            print(1)
        else:
            for i in xrange(3,n+1,2):
                if isPrime(i):
                    ct+=1
            print(ct)
    except:
        break

Python: import sys
def prime(n):
  l = [2]
  for i in range(3,n+1):
    count = 0
    for j in l:
      if i % j == 0:
        count += 1
    if count == 0:
      l.append(i)
  return l
for line in sys.stdin.readlines():
  n = int(line)
  if n == 1:
    print(0)
  else:
    print(len(prime(n)))

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 lst:
        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

for s in sys.stdin:
    n = int(s)
    lst = eratosthenes(n)
    print(len(lst))

Python: import math
MAX = 1000000
A = [1]*MAX

for i in range(2,int(math.sqrt(MAX))+1):
    if A[i]==1:
        for j in range(2*i,MAX,i):
            A[j]=0
A[0]=0
A[1]=0
for i in range(1,MAX):
    A[i] += A[i-1]

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

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

Python: is_prime = [False,False]

for i in range(2,1000000):
    is_prime.append(True)

for i in range(2,1000000):
    if is_prime[i]:
        for j in range(i*i,1000000,i):
            is_prime[j] = False


n_prime = [0,0]
cnt = int(0)

for i in range(2,1000000):
    if is_prime[i]:
        cnt += 1
    n_prime.append(cnt)

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

Python: def e(n):
    array = [1]*(n+1)
        
    k = 2
    
    while k*k <= n:
        if array[k] == 1:
            i = k
    
            while (i <= n/k):
                array[k*i] = 0
                i += 1
        
        k += 1
    
    # ?´???°(array[i]==1)?????????
    prime = []
    i = 2
    
    while i <= n:
        if array[i] == 1:
            prime.append(i)
        i += 1
    
    return len(prime)


while True:
    try:
        n = int(input())
        print(e(n))
        
    except:
        break

Python: is_prime=[True for i in range(1000000+1)]
is_prime[0]=is_prime[1]=False
for i in range(2,1000):
	if not is_prime[i]:
		continue
	for j in range(2,int(1000000/i)+1):
		is_prime[i*j]=False
cnt=0
primepi=[0 for i in range(1000000)]
for i in range(1000000):
	if is_prime[i]:
		cnt+=1
	primepi[i]=cnt

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

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

Python: import sys

l = [True] * 100000
for i in range(2, 100000):
    if (l[i - 1]):
        for j in range(i ** 2 - 1, 100000, i):
            l[j] = False
for n in sys.stdin:
    print(l[1:int(n)].count(True))

Python: 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:
        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

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 math

m=999999
l=[False]*m
l[1]=True
for i in range(3,m+1):
    if i%2==0:continue
    for e in range(2,int(math.sqrt(i)+1)):
        if i%e==0:
            break
    else:
        l[i-1]=True
    
while True:
    try:n=int(input())
    except:break
    print(len([i for i in l[:n:] if i]))

Python: import math


def sieve_of_erastosthenes(num):
    input_list = [False if i % 2 == 0 or i % 3 == 0 or i % 5 == 0 else True for i in range(num)]
    input_list[0] = input_list[1] = False
    input_list[2] = input_list[3] = input_list[5] = True
    sqrt = math.sqrt(num)

    for serial in range(3, num, 2):
        
        if serial >= sqrt:
            # print([i for i, b in enumerate(input_list) if b == True])
            return sum(input_list)

        for s in range(serial ** 2, num, serial): 
            input_list[s] = False


if __name__ == '__main__':
    while True:
        try:
            n = int(input())
        except:
            break

        print(sieve_of_erastosthenes(n))

Python: import sys
 
 
f = [0, 1] * 500000
 
i = 3
while i <= 1000:
    if f[i] == 1:
        j = i * i
        while j <= 1000000:
            f[j] = 0
            j += i + i
    i += 2
 
f.pop(0)
f.pop(1)
 
for n in sys.stdin:
    i = int(n)
    if i < 2:
        print(0)
    else:
        print(sum(f[:i - 1]))

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])