数据库实现原理#2（获取第N个值）

获取数组中的第N个值,仍使用代码说明,算法参考快速排序的思想,详见代码注释.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "../pub/pub.h"
//tmparr : 待处理的数组
//counter : 数组元素个数
//n : 从小到大排列,第n个数字(从0开始计数)
static int quicksort_nth_recursion(int tmparr[],int counter,int n)
{
  //DEBUG : 数组信息
  //print_array(tmparr,counter);
  //任意取一个值,找到该值的位置
  int pos = 0;
  int randompos = rand() % counter;
  //把选定的值移到第一个位置
  swap(tmparr,0,randompos);
  for(int i=1;i < counter;i++)
  {
    //从1开始遍历,如遍历的元素小于选定的值,则把pos加一并吧该值移到pos所在的位置
    //循环完成后,小于随机选择值的数会在pos的左边,大于等于选择值的在pos的右边
    if(tmparr[i] < tmparr[0])
    {
      //如果遍历
      swap(tmparr,++pos,i);
    }
  }
  //printf("value = %d,counter = %d,target = %d,pos = %d\n",tmparr[0],counter,n,pos);
  //把选定的值移到它该在的地方
  swap(tmparr,pos,0);
  if(pos == n)
    return tmparr[pos];
  //递归处理
  if(pos < n)//在右边的数组中
    quicksort_nth_recursion(tmparr+(pos+1),counter-(pos+1),n-(pos+1));
  else//在左边的数组中
    quicksort_nth_recursion(tmparr,pos,n);
}
void main(void)
{
  //参数:第1个/中间/最后一个
  int pos[3] = {1,0,0};
  int result = 0,counter = 0;
  int arr[] = {4,10,25,100,53,103,50,40,77,9,5,1,65,19,60,51,500};
  counter  = sizeof(arr)/sizeof(int);
  pos[1] = (counter+1)/2;//中位数
  pos[2] = counter;//最大值
  for(int i = 0;i < 3;i++)
  {
    result = quicksort_nth_recursion(arr,counter,pos[i]-1);
    printf("---------- The No.%d result is %d ----------\n",pos[i],result);
  }
  printf("------------------------------------------------------------\n");
  int arr2[1<<16];
  srand(38838);
  for(int i=0;i < 1<<16;i++)
  {
    arr2[i] = rand();
  }
  counter  = sizeof(arr2)/sizeof(int);
  pos[1] = (counter+1)/2;
  pos[2] = counter;
  for(int i = 0;i < 3;i++)
  {
    result = quicksort_nth_recursion(arr2,counter,pos[i]-1);
    printf("---------- The No.%d result is %d ----------\n",pos[i],result);
  }
}

运行输出

helloworld@DESKTOP-BRAEUTR /d/yunpan/Work/Z-SRC/sort
$ /d/tmp/test.exe
---------- The No.1 result is 1 ----------
---------- The No.9 result is 50 ----------
---------- The No.17 result is 500 ----------
------------------------------------------------------------
---------- The No.1 result is 0 ----------
---------- The No.32768 result is 16541 ----------
---------- The No.65536 result is 32767 ----------