hdoj-2161-Primes

Description
Write a program to read in a list of integers and determine whether or not each number is prime. A number, n, is prime if its only divisors are 1 and n. For this problem, the numbers 1 and 2 are not considered primes.

Input
Each input line contains a single integer. The list of integers is terminated with a number<= 0. You may assume that the input contains at most 250 numbers and each number is less than or equal to 16000.

Output
The output should consists of one line for every number, where each line first lists the problem number, followed by a colon and space, followed by “yes” or “no”.

Sample Input

1
2
3
4
5
17
0

Sample Output

1: no
2: no
3: yes
4: no
5: yes
6: yes

炒鸡基础的数论
判断素数

#include<cstdio>#include<cstring>#include<iostream>using namespace std;int vis[20000];void sieve(){    memset(vis,0,sizeof(vis));    for(int i=2;i<=20000;i++)    {        if(!vis[i])        for(int j=i*i;j<=20000;j+=i)        {            vis[j]=1;        }}}int main(){    int n,Case;    Case=0;    while(scanf("%d",&n)!=EOF)    {        if(n<=0) break;        Case++;        sieve();        if(n==1)  printf("%d: no\n",Case);        else if(n==2) printf("%d: no\n",Case);        else {            if(vis[n]==1) printf("%d: no\n",Case);            else printf("%d: yes\n",Case);        }    }    return 0;}