问题
So I have a function defined that works great at doing merge sort on a linear array if it is implemented by its lonesome, but if I put it into a class it bugs out. I think it's a great example of what I don't quite understand about how classes work; possibly in regards to namespace management(?).
See below:
def sort(array):
print('Splitting', array)
if len(array) > 1:
m = len(array)//2
left = array[:m]
right = array[m:]
sort(left)
sort(right)
i = 0
j = 0
k = 0
while i < len(left) and j < len(right):
if left[i] < right[j]:
array[k] = left[i]
i += 1
else:
array[k] = right[j]
j += 1
k += 1
while i < len(left):
array[k] = left[i]
i += 1
k += 1
while j < len(right):
array[k] = right[j]
j += 1
k += 1
print('Merging', array)
arr = [1,6,5,2,10,8,7,4,3,9]
sort(arr)
Produces the expected correct output:
Splitting [1, 6, 5, 2, 10, 8, 7, 4, 3, 9]
Splitting [1, 6, 5, 2, 10]
Splitting [1, 6]
Splitting [1]
Merging [1]
Splitting [6]
Merging [6]
Merging [1, 6]
Splitting [5, 2, 10]
Splitting [5]
Merging [5]
Splitting [2, 10]
Splitting [2]
Merging [2]
Splitting [10]
Merging [10]
Merging [2, 10]
Merging [2, 5, 10]
Merging [1, 2, 5, 6, 10]
Splitting [8, 7, 4, 3, 9]
Splitting [8, 7]
Splitting [8]
Merging [8]
Splitting [7]
Merging [7]
Merging [7, 8]
Splitting [4, 3, 9]
Splitting [4]
Merging [4]
Splitting [3, 9]
Splitting [3]
Merging [3]
Splitting [9]
Merging [9]
Merging [3, 9]
Merging [3, 4, 9]
Merging [3, 4, 7, 8, 9]
Merging [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
However, I get an error when I attempt to use this function in a class; something to do with namespace managment, I think. See below:
class MergeSort(object):
def __init__(self, array):
self.array = array
def sort(self):
print('Splitting', self.array)
if len(self.array) > 1:
m = len(self.array)//2
left = self.array[:m]
right = self.array[m:]
sort(left)
sort(right)
i = 0
j = 0
k = 0
while i < len(left) and j < len(right):
if left[i] < right[j]:
self.array[k] = left[i]
i += 1
else:
self.array[k] = right[j]
j += 1
k += 1
while i < len(left):
self.array[k] = left[i]
i += 1
k += 1
while j < len(right):
self.array[k] = right[j]
j += 1
k += 1
print('Merging', self.array)
x = MergeSort([1,6,5,2,10,8,7,4,3,9])
x.sort()
Produces the error output:
Splitting [1, 6, 5, 2, 10, 8, 7, 4, 3, 9]
---------------------------------------------------------------------------
NameError Traceback (most recent call last)
<ipython-input-15-89509f86277e> in <module>()
1 x = MergeSort([1,6,5,2,10,8,7,4,3,9])
----> 2 x.sort()
<ipython-input-14-2bba116f00ce> in sort(self)
11 right = self.array[m:]
12
---> 13 sort(left)
14 sort(right)
15
NameError: name 'sort' is not defined
My initial instinct, after google searching around was to change subroutines sort(left) and sort(right) by adding prefixive self., but that generates a positional argument error. Would love a comment or two on what it is that I'm not understanding here. And cheers for good votes if my question is not stupid, and down votes if it is.
回答1:
The reason sort(left) doesn't work is that, as you surmised, you can't call a method on self without specifying self. Leaving that off means it looks for a local or global name sort, doesn't find one, and raises a NameError.
The reason self.sort(left) doesn't work is that the API you defined doesn't work that way. Your class takes the list as a constructor argument, and then takes a sort with no arguments, that operates on the list passed in at construction time. So, you have no way to call your own sort with a different array. If you try self.sort(left), you're passing the wrong number of arguments, just like calling abs(1, 2), and you get the same TypeError.
You have to use your API the way you designed it: Create a new MergeSort sorter object with the new list, then call sort on that new object:
leftsorter = MergeSort(left)
leftsorter.sort()
rightsorter = MergeSort(right)
rightsorter.sort()
回答2:
Replacing the sort(left) and sort(right) components of the sort() function within my class with
leftsorter = MergeSort(left)
leftsorter.sort()
rightsorter = MergeSort(right)
rightsorter.sort()
(Thank you abarnert)
While at the same time removing debugging print statements from the code (thank you Evgany P), and by avoiding reusing a built in function name sort() to avoid confusion, I have a working MergeSort class.
class MergeSort(object):
def __init__(self, array):
self.array = array
def merge_sort(self):
if len(self.array) > 1:
m = len(self.array)//2
left = self.array[:m]
right = self.array[m:]
leftsorter = MergeSort(left)
leftsorter.merge_sort()
rightsorter = MergeSort(right)
rightsorter.merge_sort()
i = 0
j = 0
k = 0
while i < len(left) and j < len(right):
if left[i] < right[j]:
self.array[k] = left[i]
i += 1
else:
self.array[k] = right[j]
j += 1
k += 1
while i < len(left):
self.array[k] = left[i]
i += 1
k += 1
while j < len(right):
self.array[k] = right[j]
j += 1
k += 1
x = MergeSort([3,5,6,2,1,4,10,9,8,7])
x.merge_sort()
x.array
Out[ ]: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Well done everybody!
回答3:
You need to call self.sort() within the class.
A larger problem is that none of your function or class method return anything, just prints, are you satisfied with that?
来源：https://stackoverflow.com/questions/50754601/implementing-merge-sort-function-in-python-class-errors