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Accepted
Sam Flynn
Asked: 2023-08-18 08:08:47 +0800 CST

为什么空列表也能起作用？

  • 772

为什么向binary_search函数传递一个空列表？毕竟，它并不是真正的空。

from typing import List


def binary_search(nums: List[int], item: int):
    print("binary_search nums", nums)
    low = 0
    high = len(nums) - 1

    while low <= high:
        middle = (low + high) // 2

        if nums[middle] == item:
            return middle
        if nums[middle] > item:
            high = middle - 1
        if nums[middle] < item:
            low = middle + 1
    return None


def smallest_find(nums: List[int]):
    smallest = nums[0]
    smallest_index = 0

    for i in range(1, len(nums)):
        if nums[i] < smallest:
            smallest = nums[i]
            smallest_index = i
    return smallest_index


def selection_sort(nums: List[int]):
    sorted_nums = []

    for _ in range(len(nums)):
        smallest_index = smallest_find(nums)
        sorted_nums.append(nums.pop(smallest_index))
    return sorted_nums

import random

while True:
    nums = [random.randint(-100, 100) for _ in range(20)]
    item = 24
    if item in nums:
        print(nums)
        print(selection_sort(nums))
        print(binary_search(selection_sort(nums), item))
        break

结果：

[20, 84, -65, -76, -46, -7, 19, 48, -55, 31, 62, -86, 16, 24, 67, -87, -65, -57, 12, 61]
[-87, -86, -76, -65, -65, -57, -55, -46, -7, 12, 16, 19, 20, 24, 31, 48, 61, 62, 67, 84]
binary_search nums []
None

为什么向binary_search函数传递一个空列表？毕竟，它并不是真正的空。

Python 3.8.10

python-3.x

1 个回答

  1. Best Answer

    您的第二次调用selection_sort实际上返回一个空列表，这就是为什么binary_search得到一个空列表。

    原因是这一行：

    sorted_nums.append(nums.pop(smallest_index))

    你正在从日常工作中pop汲取所有价值观。这是清空（并覆盖）输入列表numsselection_sortnums

    您可以通过制作以下内容的浅拷贝来避免这种情况nums：

    from typing import List


def binary_search(nums: List[int], item: int):
    print("binary_search nums", nums)
    low = 0
    high = len(nums) - 1

    while low <= high:
        middle = (low + high) // 2

        if nums[middle] == item:
            return middle
        if nums[middle] > item:
            high = middle - 1
        if nums[middle] < item:
            low = middle + 1
    return None


def smallest_find(nums: List[int]):
    smallest = nums[0]
    smallest_index = 0

    for i in range(1, len(nums)):
        if nums[i] < smallest:
            smallest = nums[i]
            smallest_index = i
    return smallest_index


def selection_sort(nums: List[int]):
    sorted_nums = []
    nums_ = nums[:] # shallow copy
    for _ in range(len(nums_)):
        smallest_index = smallest_find(nums_)
        sorted_nums.append(nums_.pop(smallest_index))
    return sorted_nums

import random

while True:
    nums = [random.randint(-100, 100) for _ in range(20)]
    item = 24
    if item in nums:
        print(nums)
        print(selection_sort(nums))
        print(binary_search(selection_sort(nums), item))
        break

    输出：

    [41, -59, 38, -50, 25, -62, 58, -81, 7, 24, -93, 41, 92, 3, 65, -61, 47, -54, 72, 65]
[-93, -81, -62, -61, -59, -54, -50, 3, 7, 24, 25, 38, 41, 41, 47, 58, 65, 65, 72, 92]
binary_search nums [-93, -81, -62, -61, -59, -54, -50, 3, 7, 24, 25, 38, 41, 41, 47, 58, 65, 65, 72, 92]
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