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 math
import sys

def prime_calc(n):
    if n < 2:
        return False
    elif n==2 or n==3 or n==5 or n==7:
        return True
    else:
        rootN = math.floor(math.sqrt(n))
        i = 11
        while rootN > i:
            if n % i == 0:
                return False
            else:
                i += 2

    return True

def prime(n):
    cnt = 0
    for i in range(2, n+1):
        ans = prime_calc(i)
        if ans is True:
            cnt = cnt + 1

    return cnt

def main():
    l = []

    for line in sys.stdin:
        l.append(int(line))

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

if __name__ == "__main__":
    main()

Python: import math


def sieve_of_erastosthenes(num):
    if 2 >= num:
        return 1
        
    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:
            return 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

        input_list = sieve_of_erastosthenes(n)

        print(sum(input_list))

Python: 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: prime=[2]
for i in range(3,1000000,2):
    primeq=True
    for p in prime:
        if i%p==0:
            primeq=False
            break
        if i<p*p:break
    if primeq:prime.append(i)
while True:
    try:
        n=int(input())
        ans=0
        for p in prime:
            if p>n:break
            ans+=1
        print(ans)
    except:break

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

# n??\???????´???°?????°???????????¢??°
def prime(n):
    l = range(0,n+1)
    i,k = 0,2
    result = 1

    if n >= 3:
        prime_nums = [2, 3]
    else:
        prime_nums = []

    while i <= n:
        while l[i] > k:
            result = l[i]%k
            k = k+1
            if result == 0:
                break
            if (l[i]-1 == k) and (result != 0):
                prime_nums.append(l[i])

        i, k = i+1, 2

    #return prime_nums
    return len(prime_nums)

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

l = len(array)
for i in array:
    psum = prime(i)
    print(psum)

Python: import sys
a=[0,0,1]+[1,0]*499999
for i in range(3,999,2):
 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

N = 1000000
lst = [1] * N
for i in range(3, int(N**0.5)+1, 2):
    if lst[i] == 1:
        lst[i*i::2*i] = [0] * len(lst[i*i::2*i])
lst = [2] + [i for i in range(3, N, 2) if lst[i] == 1]
print(lst[:100:])
for i in sys.stdin:
    for j in range(len(lst)):
        if lst[j] > int(i):
            print(j)
            break

Python: from math import sqrt,floor
from sys import stdin

def cntPrime(n):
    mx = sqrt(n) + 1
    if n % 2 == 0:
      l = [0, 1] + [1, 0] * (int(n/2) - 1)
    else:
      l = [0, 1] + [1, 0] * (int(n/2) - 1) + [1]
    c = 3
    while c < mx:
        for k in range(c*2, n+1, c):
            l[k-1] = 0
        c = l[:c].index(1) + c
    return sum(l)
for n in stdin:
    n = int(n)
    print(cntPrime(n))

Python: import sys

MAX = 999999
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(n):
  if n == 2:return True
  if n % 2 == 0:return False
  for i in range(3,int(n**0.5)+1,2):
    if n % i == 0:return False
  return True

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

def prime_list(n):
  result = []
  if n >= 2:result.append(2)
  for i in range(3,n+1,2):
    if is_prime(i):result.append(i)
  return result

L2 = prime_list(MAX)

def prime_count_2(n):
  for i,v in enumerate(L2):
    if v > n:return i
  return len(L2)

for n in sys.stdin:
  print(prime_count_2(int(n)))

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

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

for line in sys.stdin:
    n=int(line)
    print(countPrime(n))

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
    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: import math
while True:
    try:
        n=int(input())
        num=list(range(n+1)) #[0,1,,,,n]が出力される
        num[1]=0
        for i in range(n):
            if num[i]>pow(n,0.5):
                break
            if num[i]==0:
                continue
            for j in range(num[i]*2, n+1, num[i]): #num[i]*2からn+1までnum[i]個飛ばしで
                num[j]=0
        print(n+1-num.count(0))
    except:
        break

Python: 
a=int(input())
X=[i for i in range(2,a+1)]
while True:
  X=[i for i in X if i % Y[0] !=0 ]
  if X == Y:
    break
  Y=X[:]

print(len(X))

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(int(line))        

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

if __name__ == "__main__":
    main()

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))