ACM-steps-2.1.2--判别素数-优快云博客

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How many prime numbers

Time Limit: 3000/1000 MS (Java/Others) Memory Limit: 32768/32768 K (Java/Others) Total Submission(s): 8510 Accepted Submission(s): 2716

Problem Description

Give you a lot of positive integers, just to find out how many prime numbers there are.

Input

There are a lot of cases. In each case, there is an integer N representing the number of integers to find. Each integer won’t exceed 32-bit signed integer, and each of them won’t be less than 2.

Output

For each case, print the number of prime numbers you have found out.

Sample Input

3
2 3 4

Sample Output

Author

wangye

Source

HDU 2007-11 Programming Contest_WarmUp

注意的点：

在判别n是否为素数时，只需要循环到sqrt(n),即可。

n=sqrt(n)*sqrt(n);
n=x*y;假设x>sqrt(n),则y<sqrt(n);
循环到sqrt(n)，则不会超时。

#include<iostream>
#include<cmath>
using namespace std;
bool judge(int a)
{
    for(int i=2;i<=sqrt(a);i++)
    {
        if(a%i==0)
        return  false;
    }
    return true;
}
int main()
{
    int a,T,cnt;
    while(cin>>T)
    {
        cnt=0;
    while(T--)
    {
        cin>>a;
        if(judge(a))
        cnt++;
    }
    cout<<cnt<<endl;
    }
    return 0;
}