Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

func nextInt() int {
	i, e := strconv.Atoi(scanner.Text())
	if e != nil {
		panic(e)
	}
	return i
}

func check(n int) int {
	eratos := make([]bool, n+1)
	cnt := 0
	for i := 2; i < len(eratos); i++ {
		if !eratos[i] {
			cnt++
			for j := i*i; j < len(eratos); j += i {
				eratos[j] = true
			}
		}
	}
	return cnt
}

func main() {
	for scanner.Scan() {
		n := nextInt()
		fmt.Println(check(n))
	}
}

Python: import sys
import math

lines = str(sys.stdin.read()).strip().split("\n")

for line in lines:
    line = int(line)
    pList = []
    sList = list(reversed(list(range(3, line + 1))[::2]))
    while sList[-1] < int(math.ceil(math.sqrt(line))) + 1:
        pList.append(sList.pop())
        for p in pList:
            sList = list(filter(lambda i: i % p != 0, sList))
    pList.extend(sList)
    pList.append(2)
    print(len(pList))

Python: limit = int(input(""))
def primeNumber(number):
    count = 0
    for i in range(2,number+1):
        if (number%i==0):
            count = count + 1
    if (count==1):
        return True
    else:
        return False

sum = 0
for i in range(2,limit+1):
    if (primeNumber(i)):
        sum = sum + 1

print(sum)

Python: def pri(n):
    s=[True for _ in range(n+1)]
    i=2
    while i**2<=n:
        if s[i]:
            j=i*2
            while j<=n:
                s[j]=False
                j+=i
        i+=1

    tab=[i for i in range(n+1) if s[i] and i>=2]
    return(tab)


while True:
    try:
        print(len(pri(int(input()))))
    except:
        break

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

while True:
    try:
        n = int(raw_input())
        if n < 2:
            print(0)
            continue
        list1 = range(2, n + 1)
        list2 = []
        while True:
            i = list1.pop(0)
            list2.append(i)
            for x in list1:
                if x % i == 0:
                    list1.remove(x)
            if len(list1) == 0:
                break
            if i**2 > sorted(list1,lambda n1, n2:n2 - n1)[0]:
                break
        print(len(list2 + list1))
    except EOFError:
        break

Python: def primes(n):
    is_prime = [True] * (n + 1)
    is_prime[0] = False
    is_prime[1] = False
    for i in range(2, int(n**0.5) + 1):
        if not is_prime[i]:
            continue
        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]]

if __name__ == "__main__":
    while True:
        try:
            n = int(input())
            print(len(primes(n)))
        except:
            break

Python: import sys
def isPrime(i):
    if i<2:
        return False
    else:
        for j in range(2,i):
            if j*j>i:
                break
            elif i%j==0:
                return False
        return True

def countPrime(i):
    cnt=0
    for x in range(2,i+1):
        if isPrime(x) == True:
            print(x)
            cnt+=1
    return cnt

while True:
    try:
        n=int(input())
        cnt=0
        for x in range(2,n+1):
            if isPrime(x)==True:
                cnt+=1
        print(cnt)
    except:
        break

Python: # PrimeNumber

from itertools import product
import math

if __name__ == "__main__":
    while True:
        try:
            n = int(input())
            nlist = []
            nlist2 = []
            plist = []

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

            while True:
                #print(nlist, " ", math.sqrt(n))
                if nlist[0] > math.sqrt(n):  # 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: import sys

def prime(n):
	if n == 2: return True
	if n < 2 or n & 1 == 0: return False
	return pow(2, n-1, n) == 1

a = [prime(i) for i in range(1000000)]

for s in sys.stdin:
	i = int(s)
	print(a[:i+1].count(True))

Python: from math import sqrt
def pm(n):
    l = list(range(3,n+1)[0::2])
    p = 3
    i = 0
    p = l[i]
    m = sqrt(n)+1
    while p <= m:
        l = l[:i+1] + [n for n in l[i+1:] if n%p != 0]
        i += 1
        p = l[i]
    return len(l) + 1

from sys import stdin
for i in stdin:
    print(pm(int(i)))

Python: def make_prime():
	prime=[0 for i in range(10**6)]
	used=[0 for i in range(10**6)]
	for i in range(2,10**6):
		if used[i]==1:
			prime[i]=prime[i-1]
			continue
		prime[i]=prime[i-1]+1;
		for j in range(i,10**6,i):
			used[j]=1
	return prime

def main():
	prime=make_prime()
	while True:
		try: n=int(input())
		except EOFError: break
		print(prime[n])

if __name__=="__main__":
	main()

Python: import sys
import math

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

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

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

Python: import sys

ary=[]
for i in sys.stdin:
    ary.append(int(i))

def isp(n):
    a = 1
    for i in range(2, int(n ** 0.5) + 1):
        if not n % i:
            a = 0
            break
    return a

for i in ary:
    s = 0
    for i in range(2, 12):
        if isp(i):
            s += 1
    print(s)

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

import sys


def eratosthenes_sieve(n):
    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


def prime(n):
    return eratosthenes_sieve(n)

def main():
    inp = [int(n) for n in sys.stdin]
    for n in inp:
        n_pn = len(prime(n))
        print(n_pn)

if __name__ == '__main__':
    main()

Python: 
def isp(n):
    a = 1
    for i in range(2, (n + 1) // 2 + 2):
        if not n % i:
            a = 0
            break
    return a


num = int(input())
if num == 2:
    print(1)
elif num == 3:
    print(2)
else:
    s = 2
    for x in range(2, num + 1):
        if isp(x):
            s += 1
    print(s)

Python: n = 999999
c = [1 for i in range(n)]
c[0] = 0
i = 2
while i**2 <= n:
    j = i*2
    while j <= n:
        c[j - 1] = 0
        j += i
    i += 1

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

Python: 
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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))