Python3实现自定义比较排序/运算符

自定义比较排序/运算符

Python3和Python2相比有挺多变化。

在Python2中可以直接写一个cmp函数作为参数传入sort来自定义排序，但是Python3取消了。

在这里总结一下Python3的自定义排序的两种写法，欢迎补充。

我们以二维空间中的点来作为待排序的数据结构，我们希望能先比较x后再比较y。

class Pos:
    def __init__(self, x = 0, y = 0):
        self.x = x
        self.y = y
 
    def __str__(self):
        return ('(％s, ％s)' ％ (self.x, self.y))
 
    __repr__ = __str__

1.cmp函数

第一种方法我们还是以重写cmp或lambda表达式的形式，和Python2很类似

注意，此方法用sorted是不能成功排序的

只是要借助functools

import functools
def cmp(a, b):
    return a.x-b.x if a.x != b.x else a.y-b.y  # x y均按照从小到大的顺序
 
if __name__ == '__main__':
 
    test_list = [Pos(5, 1), Pos(2,5), Pos(2, 4)]
    # test_list.sort(key=functools.cmp_to_key(lambda a,b: a.x-b.x if a.x != b.x else a.y-b.y))
    test_list.sort(key=functools.cmp_to_key(cmp))
    # sorted(test_list, key=functools.cmp_to_key(cmp))  #    亲测此方法不能成功排序
    print(test_list)  # 输出结果 [(2, 4), (2, 5), (5, 1)]

2.重写类方法

Python2中可以直接重写__cmp__方法来实现比较，但是Python3中已经取消了.

Python3中需要细分每一个比较运算符.

__lt__: <
__gt__: >
__ge__: >=
__eq__: ==
__le__: <=

实现如下

class Pos:
   def __init__(self, x = 0, y = 0):
       self.x = x
       self.y = y

def __str__(self):
       return ('(％s, ％s)' ％ (self.x, self.y))

def __lt__(self, other):
       print('lt: ' + str(self))
       return self.x < other.x if self.x != other.x else self.y < other.y

def __gt__(self, other):
       print('gt: ' + str(self))
       return self.x > other.x if self.x != other.x else self.y > other.y

def __ge__(self, other):
       print('ge: ' + str(self))
       return self.x >= other.x if self.x != other.x else self.y >= other.y

def __eq__(self, other):
       print('eq: ' + str(self))
       return self.x == other.x and self.y == other.y

def __le__(self, other):
       print('le: ' + str(self))
       return self.x <= other.x if self.x != other.x else self.y <= other.y

__repr__ = __str__

我们实践一下

if __name__ == '__main__':

if Pos(5,1) <= Pos(2,4):
       print('True!')
   if Pos(5,1) == Pos(2,4):
       print('True!')
   if Pos(5,1) > Pos(2,4):
       print('True!')
# 输出
# le: (5, 1)
# eq: (5, 1)
# gt: (5, 1)
# True!

最后我们回到排序

if __name__ == '__main__':

test_list = [Pos(5, 1), Pos(2,5), Pos(2, 4)]
   test_list.sort()
   print(test_list)

test_list.sort(reverse=True)
   print(test_list)

# 输出
# lt: (2, 5)
# lt: (2, 4)
# [(2, 4), (2, 5), (5, 1)]
# lt: (2, 5)
# lt: (2, 4)
# [(5, 1), (2, 5), (2, 4)]

Python3实现各种排序方法

# coding=gbk
import random
from array import array
def swap(lyst,i,j):
   temp = lyst[i]
   lyst[i] = lyst[j]
   lyst[j] = temp
#选择排序，复杂度O(n^2)
def selectionSort(lyst):
   i = 0
   while i < len(lyst) - 1:
       minIndex = i
       j = i + 1
       while j < len(lyst):
           if lyst[j] < lyst[minIndex]:
               minIndex = j
           j += 1
       if minIndex != i:
           swap(lyst,minIndex,i)
       i += 1
#冒泡排序，复杂的O(n^2)
def bubbleSort(lyst):
   n = len(lyst)
   while n > 1:
       i = 1
       while i < n:
           if lyst[i] < lyst[i-1]:
               swap(lyst,i,i-1)
           i += 1
       n -= 1
#冒泡排序优化改进最好情况
def bubbleSortWithTweak(lyst):
   n = len(lyst)
   while n > 1:
       swapped = False
       i = 1
       while i < n:
           if lyst[i] < lyst[i-1]:
               swap(lyst,i,i-1)
               swapped = True
           i += 1
       if not swapped: return
       n -= 1
#插入排序，复杂的O(n^2)
def insertionSort(lyst):
   i = 1
   while i < len(lyst):
       itemToInsert = lyst[i]
       j = i - 1
       while j >= 0:
           if itemToInsert < lyst[j]:
               lyst[j+1] = lyst[j]
               j -= 1
           else:
               break
       lyst[j+1] = itemToInsert
       i += 1
#快速排序,最好情况，复杂的O(n*(log2 n))，最坏情况，复杂的O(n^2)
def quicksort(lyst):
   quicksortHelper(lyst,0,len(lyst)-1)
def quicksortHelper(lyst,left,right):
   if left < right:
       pivotLocation = partition(lyst,left,right)
       quicksortHelper(lyst,left,pivotLocation-1)
       quicksortHelper(lyst,pivotLocation+1,right)
def partition(lyst,left,right):
   middle = (left+right) // 2
   pivot = lyst[middle]
   lyst[middle] = lyst[right]
   lyst[right] = pivot
   boundary = left
   for index in range(left,right):
       if lyst[index] < pivot:
           swap(lyst,index,boundary)
           boundary += 1
   swap(lyst,right,boundary)
   return boundary
#合并排序
def mergeSort(lyst):
   copyBuffer = [0]*(len(lyst))
   mergeSortHelper(lyst,copyBuffer,0,len(lyst)-1)
def mergeSortHelper(lyst,copyBuffer,low,high):
   if low < high:
       middle = (low+high)//2
       mergeSortHelper(lyst,copyBuffer,low,middle)
       mergeSortHelper(lyst,copyBuffer,middle+1,high)
       merge(lyst,copyBuffer,low,middle,high)
def merge(lyst,copyBuffer,low,middle,high):
   i1 = low
   i2 = middle + 1
   for i in range(low,high+1):
       if i1 > middle:
           copyBuffer[i] = lyst[i2]
           i2 += 1
       elif i2 > high:
           copyBuffer[i] = lyst[i1]
           i1 += 1      
       elif lyst[i1] < lyst[i2]:
           copyBuffer[i] = lyst[i1]
           i1 += 1      
       else :
           copyBuffer[i] = lyst[i2]
           i2 += 1
   for i in range(low,high+1):
       lyst[i] = copyBuffer[i]
def main(size = 20,sort = mergeSort):  
   lyst = []
   for count in range(size):
       lyst.append(random.randint(1,size+1))
   print(lyst)
   sort(lyst)
   print(lyst)
if __name__ == "__main__":
   main()

来源：https://blog.csdn.net/as1171799253/article/details/83685615