快速排序函数不会产生预期的输出，但分区函数会产生（python）

Question

我必须在学校项目的“切片”对象上实现快速排序算法，切片对象是一个元组：

• 一个“数据”字段（要排序的整个 numpy 数组）
• 'left' 和 'right' 字段（表示数组中切片子部分的索引）

分区函数代码如下：

def partition (s, cmp, piv=0):
"""
Creates two slices from *s* by selecting in the first slice all
elements being less than the pivot and in the second one all other
elements.

:param s: A slice of is a dictionary with 3 fields :
          - data: the array of objects,
          - left: left bound of the slide (a position in the array),
          - right: right bound of the slice.
:type s: dict
:param cmp: A comparison function, returning 0 if a == b, -1 is a < b, 1 if a > b
:type cmp: function
:return: A couple of slices, the first slice contains all elements that are
         less than the pivot, the second one contains all elements that are
         greater than the pivot, the pivot does not belong to any slice.
:rtype: tuple

>>> import generate
>>> import element
>>> import numpy
>>> def cmp (x,y):
...    if x == y:
...       return 0
...    elif x < y:
...       return -1
...    else:
...       return 1
>>> t = numpy.array([element.Element(i) for i in [4,2,3,1]])
>>> p = {'left':0,'right':len(t)-1,'data':t}
>>> pivot = 0
>>> p1,p2 = partition(p,cmp,pivot)
>>> print(p1['data'][p1['left']:p1['right']+1])
[1 2 3 4]
>>> print(p2['data'][p2['left']:p2['right']+1])
[]
"""
#convenience variables to make the function easily readable.
left = s['left']
right = s['right']
swap(s['data'],right,piv)

j = left
for i in range(left,right+1):
    if (cmp(s['data'][i] , s['data'][right]) != 1 ):
        swap(s['data'],i,j)
        j += 1
s1 = {'data':s['data'],'left':left,'right':j-1}
s2 = {'data':s['data'],'left':j+1,'right':right}

return s1,s2

该函数的 doctests 不会失败，但是在调用 quicksort 递归函数时，分区函数没有按预期运行。 quicksort 函数的代码如下：

def quicksort_slice (s, cmp):
"""
A sorting function implementing the quicksort algorithm

:param s: A slice of an array, that is a dictionary with 3 fields :
          data, left, right representing resp. an array of objects and left
          and right bounds of the slice.
:type s: dict
:param cmp: A comparison function, returning 0 if a == b, -1 is a < b, 1 if a > b
:type cmp: function
:return: Nothing

>>> import generate
>>> import element
>>> import numpy
>>> def cmp (x,y):
...    if x == y:
...       return 0
...    elif x < y:
...       return -1
...    else:
...       return 1
>>> t = numpy.array([element.Element(i) for i in [4,5,1,2,3,8,9,6,7]])
>>> p = {'left':0,'right':len(t)-1,'data':t}
>>> quicksort_slice(p,cmp)
>>> print(p['data'])
[1 2 3 4 5 6 7 8 9]
"""

if s['left'] < s['right'] :
    s1,s2 = partition(s,cmp)
    quicksort_slice(s1,cmp)
    quicksort_slice(s2,cmp)

这是此函数的 doctests 产生的输出：

File "C:\Users\Marion\Documents\S2\ASD\TP\tp2_asd\src\sorting.py", line 148, in __main__.quicksort_slice
Failed example:
    print(p['data'])
Expected:
    [1 2 3 4 5 6 7 8 9]
Got:
    [8 2 1 3 7 4 9 5 6]

我试图插入调试打印语句来观察对分区函数的递归调用，它似乎没有正确交换值，但我不知道如何解决这个问题。 任何帮助将非常感激。

编辑：正如下面的一条评论所指出的，我只是忘记更新 piv 的值，在第一次调用该函数后它并不总是 0。 我也替换了：

for i in range(left,right+1):
if (cmp(s['data'][i] , s['data'][right]) != 1 ):
    swap(s['data'],i,j)
    j += 1

和

for i in range(left,right):
    if (cmp(s['data'][i] , s['data'][right]) != 1 ):
        swap(s['data'],i,j)
        j += 1
swap(s['data'],right,j)

Answer 1

之前尝试关闭此线程，然后才意识到我需要一个答案来执行此操作。 解决方案由 jasonharper 提供。 正如 jasonharper 所指出的，我“总是使用索引 0 处的元素作为枢轴 - 即使 0 不在切片内”。