随机选数算法

detail：
 1 package cn.edu.buaa.randomSelectAlgo;
 2 
 3 /**
 4  * 等概率从n个数中随机选取m个数,概率为m/n。适用于数据能一次全读入的场景.
 5  * 
 6  */
 7 public class Main_RamdomSelectAlgo {
 8     static int[] data = { 5, 6, 7, 8, 9, 1, 2, 3, 4 };
 9     static int[] selectCount = { 0, 0, 0, 0, 0, 0, 0, 0, 0 };
10     static int loopCount = 1000_000;// 测试次数，为1次时打印结果即为选取结果
11 
12     public static void main(String[] args) {
13         // TODO Auto-generated method stub
14         for (int i = 0; i < loopCount; i++) {
15             randSelect(data, 5);
16         }
17         for (int i = 0; i < selectCount.length; i++) {
18             System.out.print(selectCount[i] + ", ");
19         }
20     }
21 
22     static boolean randSelect(int[] data, int selectNum) {
23         int nSize = data.length;
24         if (nSize < selectNum || selectNum < 0) {
25             return false;
26         }
27         for (int i = 0, iSize = nSize; i < iSize; i++) {// 遍历一遍数据
28             double tmpRamNum = Math.random();
29             if (tmpRamNum <= (double) selectNum / nSize) {
30                 // data[i] is selected
31                 selectCount[i]++;// 被选数被选次数加1，初始为0
32                 selectNum--;
33             }
34             nSize--;
35         }
36         return true;
37     }
38 }
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参考资料：http://club.alibabatech.org/article_detail.htm?articleId=2