快排实现时候的一个逻辑错误

　

 1 #include"header_file.h"
 2 using namespace std;
 3 
 4 void swap(int a,int b)
 5 {
 6     int t;
 7     t=a;
 8     a=b;
 9     b=t;
10 }
11 
12 void quick_sort(int a[],int low,int high)
13 {
14     
15     if(low>=high)
16         return;
17         
18     int first;
19     int last;
20     first=low;
21     last=high;
22     
23     int x;
24     x=a[low];
25     
26 
27     while(low<high)
28     {
29         while(low<high&&a[high]>=x)
30         {
31             high--;
32         }    
33         swap(a[low],a[high]);
34         
35         cout<<i<<"      "<<low<<" low"<<"  "<<high<<endl;
36         
37         while(low<high&&a[low]<=x)
38         {
39             low++;
40         }
41         swap(a[high],a[low]);
42         cout<<i<<"      "<<low<<" high"<<"  "<<high<<endl;
43     }
44     if(first<low-1)
45         quick_sort(a,first,low-1);
46     if(last>low+1)
47         quick_sort(a,low+1,last);
48 }
49 
50 int main(void)
51 {
52     int a[7]={4,3,6,7,2,1,5};
53     quick_sort(a,0, sizeof(a) / sizeof(a[0]) - 1);
54     
55     cout<<"test"<<endl;
56     for(int i=0;i<7;i++)
57         cout<<a[i]<<" ";
58 }

执行程序会发现在算法排序的地方无限循环，想了半天才知道是33和41行的位置，不管while循环是否执行都会执行swap语句。逻辑错误！

只能换种方法，改成大家最常见的那种就好了

 1 void quick_sort(int a[],int low,int high)
 2 {
 3     
 4     if(low>=high)
 5         return;
 6         
 7     int first;
 8     int last;
 9     first=low;
10     last=high;
11     
12     int x;
13     x=a[low];
14     
15 
16     while(low<high)
17     {
18         while(low<high&&a[high]>=x)
19         {
20             high--;
21         }    
22         a[low]=a[high];
23         
24     //    cout<<i<<"      "<<low<<" low"<<"  "<<high<<endl;
25         
26         while(low<high&&a[low]<=x)
27         {
28             low++;
29         }
30         a[high]=a[low];
31     //    swap(a[high],a[low]);
32     //    cout<<i<<"      "<<low<<" high"<<"  "<<high<<endl;
33     }
34     a[low]=x;
35     
36     if(first<low-1)
37         quick_sort(a,first,low-1);
38     if(last>low+1)
39         quick_sort(a,low+1,last);
40 }

这里就不是交换了，直接赋值，然后最后给a[low]赋值。

 

很低级的错误

　

posted @ 2016-01-11 17:02  wswang  阅读(257)  评论(0)  编辑  收藏  举报