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: from math import sqrt
from sys import stdin

def isprime(x) :
    if x == 2 : return x
    elif (x < 2) or (x % 2 == 0) : return False
    else :
        i = 3
        while i <= sqrt(x) :
            if x % i == 0 : return False
            i = i + 2
    return x

for Input in stdin :
    result = 0
    for make in range(2, int(Input) + 1) :
        if isprime(make) != False : result += 1
    print(result)

Python: from __future__ import (division, absolute_import, print_function,
                        unicode_literals)
from sys import stdin

def enum_prime(n):
    if n < 2:
        return []
    if n == 2:
        return [2]
    L = list(range(2, n + 1))
    PL = []
    while True:
        PL.append(L[0])
        L = [i for i in L if i % PL[-1] != 0]
        if L[-1] < PL[-1] ** 2:
            return PL + L

for line in stdin:
    print(len(enum_prime(int(line))))

Python: # coding: utf-8

LIMIT = 999999

prime = [0, 1] + [0 if i % 2 == 0 else 1 for i in range(3, LIMIT + 1)]

for i in range(3, LIMIT + 1):
    if prime[i - 1]:
        for j in range(i ** 2, LIMIT + 1, i):
            prime[j - 1] = 0

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

    print(sum(prime[:n]))

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

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

p=sieve(100000)
while True:
	try:
		n = int(input())
		print(solve(n))
	except EOFError:
		break

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)