随机选数算法

detail：

package cn.edu.buaa.randomSelectAlgo;

/**
 * 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.
 * 
 */
public class Main_RamdomSelectAlgo {
    static int[] data = { 5, 6, 7, 8, 9, 1, 2, 3, 4 };
    static int[] selectCount = { 0, 0, 0, 0, 0, 0, 0, 0, 0 };
    static int loopCount = 1000_000;// 测试次数，为1次时打印结果即为选取结果

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        for (int i = 0; i < loopCount; i++) {
            randSelect(data, 5);
        }
        for (int i = 0; i < selectCount.length; i++) {
            System.out.print(selectCount[i] + ", ");
        }
    }

    static boolean randSelect(int[] data, int selectNum) {
        int nSize = data.length;
        if (nSize < selectNum || selectNum < 0) {
            return false;
        }
        for (int i = 0, iSize = nSize; i < iSize; i++) {// 遍历一遍数据
            double tmpRamNum = Math.random();
            if (tmpRamNum <= (double) selectNum / nSize) {
                // data[i] is selected
                selectCount[i]++;// 被选数被选次数加1，初始为0
                selectNum--;
            }
            nSize--;
        }
        return true;
    }
}

 

参考资料：http://club.alibabatech.org/article_detail.htm?articleId=2