从末尾索引列表时，为什么Python从索引-1(而不是0)开始? [英] Why does Python start at index -1 (as opposed to 0) when indexing a list from the end?

问题描述

list = ["a", "b", "c", "d"]
print(list[3]) # Number 3 is "d"

print(list[-4]) # Number -4 is "a"

推荐答案

用另一种方式解释它，因为-0等于0，如果从0开始向后，则它对于解释器来说是模棱两可的.

To explain it in another way, because -0 is equal to 0, if backward starts from 0, it is ambiguous to the interpreter.

如果您对-感到困惑，并且正在寻找另一种更容易理解的向后索引的方法，则可以尝试~，它是前向的一面镜子:

If you are confused about -, and looking for another way to index backwards more understandably, you can try ~, it is a mirror of forward:

arr = ["a", "b", "c", "d"]
print(arr[~0])   # d
print(arr[~1])   # c

~的典型用法类似于交换镜像节点"或在排序列表中找到中位数":

The typical usages for ~ are like "swap mirror node" or "find median in a sort list":

"""swap mirror node"""
def reverse(arr: List[int]) -> None:
    for i in range(len(arr) // 2):
        arr[i], arr[~i] = arr[~i], arr[i]

"""find median in a sort list"""
def median(arr: List[float]) -> float:
    mid = len(arr) // 2
    return (arr[mid] + arr[~mid]) / 2

"""deal with mirror pairs"""
# verify the number is strobogrammatic, strobogrammatic number looks the same when rotated 180 degrees
def is_strobogrammatic(num: str) -> bool:
    return all(num[i] + num[~i] in '696 00 11 88' for i in range(len(num) // 2 + 1))

~实际上是逆代码和补码的数学技巧，在某些情况下更容易理解.

~ actually is a math trick of inverse code and complement code, and it is more easy to understand in some situations.

讨论是否应使用~之类的python技巧:

Discussion about whether should use python tricks like ~:

在我看来，如果它是您自己维护的代码，则由于可能具有很高的可读性和可用性，因此您可以使用任何技巧来避免潜在的错误或更轻松地实现目标.但是在团队合作中，避免使用太聪明"的代码，可能会给您的同事带来麻烦.

In my opinion, if it is a code maintained by yourself, you can use any trick to avoid potential bug or achieve goal easier, because of maybe a high readability and usability. But in team work, avoid using 'too clever' code, may bring troubles to your co-workers.

例如，这是 Stefan Pochmann 中的一个简洁代码，用于解决

For example, here is one concise code from Stefan Pochmann to solve this problem. I learned a lot from his code. But some are just for fun, too hackish to use.

# a strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down)
# find all strobogrammatic numbers that are of length = n
def findStrobogrammatic(self, n):
    nums = n % 2 * list('018') or ['']
    while n > 1:
        n -= 2
        # n < 2 is so genius here
        nums = [a + num + b for a, b in '00 11 88 69 96'.split()[n < 2:] for num in nums]
    return nums

如果您感兴趣的话，我已经总结了 Python技巧.

I have summarized python tricks like this, in case you are interested.

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