Why does Python start at index -1 (as opposed to 0) when indexing a list from the end?
list = ["a", "b", "c", "d"] print(list) # Number 3 is "d" print(list[-4]) # Number -4 is "a"
To explain it in another way, because
-0 is equal to
0, if backward starts from
0, it is ambiguous to the interpreter.
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
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))
~ actually is a math trick of inverse code and complement code, and it is more easy to understand in some situations.
Discussion about whether should use python tricks like
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.
# 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
I have summarized python tricks like this, in case you are interested.