[英]QuickSort in C++ will not finish sorting
Could someone please help me and tell me why quick sort algorithm will not sort the final element? 有人可以帮我,告诉我为什么快速排序算法不能对最终元素进行排序吗? When I input: 6 89 2 78 12 19 99 43 7 63 I get an ALMOST sorted: 2 6 7 12 78 19 43 99 63 89
当我输入时:6 89 2 78 12 19 99 43 7 63我得到了排序的ALMOST:2 6 7 12 78 19 43 99 63 89
I have tried to figure this out my self, but when I work my way thought the code at some point I get lost doing a desk check. 我试图弄清楚自己的意思,但是当我以自己的方式工作时,以为代码在某个时候会迷失于进行桌面检查。
#include <iostream>
using namespace std;
// function prototypes
void quickSort(int arrayList[], int left, int right);
int choosePivot(int arrayList[], int listSize);
int partition(int array[], int left, int right);
void printArray(int theArray[], int Size);
// void swap(int value1, int value2);
int main()
{
int myList[] = {6, 89, 2, 78, 12, 19, 99, 43, 7, 63};
printArray(myList, 10);
quickSort(myList, 0, 9);
printArray(myList, 10);
//int myList2[] = { 7, 4, 9, 10, -9 };
//printArray(myList2, 5);
//quickSort(myList2, 0, 5);
//printArray(myList2, 5);
cin;
getchar;
getchar;
return 0;
}
void quickSort(int arrayList[], int left, int right)
{
//if (left < right)
if(right > left)
{
int p = partition(arrayList, left, right);
printArray(arrayList, 10);
quickSort(arrayList, left, p-1);
quickSort(arrayList, p + 1, right);
}
}
// left (index value) - left most part of the array we are working on
// right (index value) - right most part of the array we are working on
int partition(int array[], int left, int right)
{
//int pivot = array[left]; // I will have to write a function to find the
// optimum pivot point, this is the naive solution
int pivot = array[(left+right)/2];
int i = left;
int j = right;
int temp;
while (i < j)
{
//cout << "in the loop" ;
while (array[i] < pivot)
i++;
while (array[j] > pivot)
j--;
if (i < j)
{
temp = array[i];
array[i] = array[j];
array[j] = temp;
i++;
j--;
}
}
return i;
}
void printArray(int theArray[], int Size)
{
for (int i = 0; i < Size; i++)
{
cout << theArray[i] << " ";
}
cout << endl ;
}
You have a bug in your partition()
function (I take it that it's Hoare partition algorithm implementation). 您的
partition()
函数中存在一个错误(我认为这是Hoare分区算法的实现)。 You just need to remove this code: 您只需要删除以下代码:
i++;
j--;
after swapping values. 交换值后。
Here is the corrected partition()
function code: 这是更正后的
partition()
函数代码:
int partition(int array[], int left, int right)
{
int pivot = array[(left + right) / 2];
int i = left;
int j = right;
while (i < j) {
while (array[i] < pivot)
i++;
while (array[j] > pivot)
j--;
if (i < j) {
int temp;
/* Swap array[i] and array[j] */
temp = array[i];
array[i] = array[j];
array[j] = temp;
}
}
return i;
}
Even with this bug fixed, you should create a good unit test for your quicksort
implementation, because there may be some other subtle bugs (it's very hard to write bug-less implementation of partition()
from scratch). 即使已修复此错误,也应为您的
quicksort
实现创建一个好的单元测试,因为可能还会存在其他一些细微的错误(很难从头开始编写没有bug的partition()
实现)。 Check for edge cases, corner cases and boundary cases in your unit test . 在单元测试中检查边缘情况,拐角情况和边界情况 。
I'm not sure how much this would help but when you get your pivot of 我不确定这有多大帮助,但是当您掌握
(left + right) / 2
you are doing (0+9) / 2 which is 4. But the thing is, the middle should be 5 if the size of the array is 10. 您正在执行(0 + 9)/ 2,即4。但是事实是,如果数组的大小为10,则中间应该为5。
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