题意：

给一个数n，问1~n内有多少个素数

题目：

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 and 7.

Input

Input consists of several datasets. Each dataset has an integer n (1 ≤ n ≤ 999,999) in a line.

The number of datasets is less than or equal to 30.

Output

For each dataset, prints the number of prime numbers.

Sample Input

10
3
11

Output for the Sample Input

4
2
5

分析：

打个素数筛就行

AC模板：

#include<stdio.h>
#include<string.h>
#include<map>
#include<algorithm>
using namespace std;
const int M=1e6+10;
int n;
int dp[M],book[M];
map<int,int>mp;
void init()
{for(int i=2; i<M; i++){if(!book[i]){mp[i]=1;for(int j=i; j<M; j+=i)book[j]=1;}}for(int i=1; i<M; i++){dp[i]=dp[i-1];if(mp[i])dp[i]++;}
}
int main()
{init();while(~scanf("%d",&n)){printf("%d\n",dp[n]);}return 0;
}

备战ccpc分站赛ing ，题目分析简略，见谅，转载请注明出处。。。。。