自定义比较排序运算符

时间:2022-05-23 as1171799253 人气:0

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()

以上为个人经验，希望能给大家一个参考，也希望大家多多支持。

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