Detailed explanation of Python's cmp_to_key() function [partial translation + leetcode topic analysis]

definition

• cmp_to_key() uses key to compare elements
• It's built into the functools module, so functools must be imported to use the function
• Use with tools that accept key functions like min(), max(), sorted(), etc.
• Only accepts one argument that should strictly speaking be a callable
• This function returns a special key that can be used to compare elements
• Used to compare two values ​​and return 1, -1 or 0

Example 1: A program that sorts a list using a key provided by the cmp_to_key() function

import functools


def mycmp(a, b):
	print("comparing ", a, " and ", b)
	if a > b:
		return 1
	elif a < b:
		return -1
	else:
		return 0


print(sorted([1, 2, 4, 2], key=functools.cmp_to_key(mycmp)))

output:

comparing  2  and  1
comparing  4  and  2
comparing  2  and  4
comparing  2  and  2
comparing  2  and  4
[1, 2, 2, 4]

Example 2: Program to print the largest and smallest number from a list using the key provided by the cmp_to_key() function

import functools


def mycmp(a, b):
	print("comparing ", a, " and ", b)
	if a > b:
		return 1
	elif a < b:
		return -1
	else:
		return 0


print(min([45, 78, 813], key=functools.cmp_to_key(mycmp)))
print(max([45, 78, 813], key=functools.cmp_to_key(mycmp)))

output:

comparing  78  and  45
comparing  813  and  45
45
comparing  78  and  45
comparing  813  and  78
813

-------------------------------------------------- -----The following is the topic of leetcode -------------------------------------- ------

Solution:

class Solution:
    def largestNumber(self, nums):
        def cmp_func(x, y):
            if x + y > y + x:
                return 1
            elif x == y:
                return 0
            else:
                return -1
        nums = [str(num) for num in nums]
        nums.sort(key = cmp_to_key(cmp_func), reverse = True)
        return ''.join(nums).lstrip('0') or '0'