Aizu 0009 Prime Number

Prime Number

Write a program which reads an integer n and prints the number of prime numbers which are less than or equal to n. A prime number is a natural number which has exactly two distinct natural number divisors: 1 and itself. For example, the first four prime numbers are: 2, 3, 5, 7.
Input

Input consists of several datasets. Each dataset has an integer n (n ≤ 999999) in a line.

The number of datasets ≤ 30.
Output

For each dataset, prints the number of prime numbers.
Sample Input

10
3
11

Output for the Sample Input

4
2

5

给出一个数n，求n以内素数的个数。我用的埃氏筛法

#include <cstdio>#include <iostream>using namespace std;int prime[1000005];bool is_prime[1000005];int seve(int n){    int p=0;    for(int i=0;i<=n;i++)        is_prime[i]=true;    is_prime[0]=is_prime[1]=false;    for(int i=2;i<=n;i++)    {        if(is_prime[i])        {            prime[p++]=i;            for(int j=2*i;j<=n;j+=i)                is_prime[j]=false;        }    }    return p;}int main(void){    int n;    while(scanf("%d",&n)!=EOF)    {        printf("%d\n",seve(n));    }    return 0;}