编写一个在列表索引之间交替使用正号和负号的函数 [英] Writing a function that alternates plus and minus signs between list indices
问题描述
在我正在处理的作业集中，我遇到了以下问题，我在使用Python-3函数时遇到问题:
In a homework set I'm working on, I've come across the following question, which I am having trouble answering in a Python-3 function:
编写备用函数:int list->需要一个列表的int 数字，并为它们添加交替符号.例如替代 [1,2,3,4] = 1-2 + 3-4 = -2."
"Write a function alternate : int list -> int that takes a list of numbers and adds them with alternating sign. For example alternate [1,2,3,4] = 1 - 2 + 3 - 4 = -2."
完全公开，该问题是在考虑标准ML的情况下编写的，但是我一直在尝试学习Python并遇到了这个问题.我在想它涉及以下方面的组合:
Full disclosure, the question was written with Standard ML in mind but I have been attempting to learn Python and came across the question. I'm imagining it involves some combination of:
拆分列表，
if [i] % 2 == 0:
，然后将交替的正负号串联起来.
and then concatenating the alternate plus and minus signs.
推荐答案
def alternate(l):
return sum(l[::2]) - sum(l[1::2])
取所有偶数索引元素的总和，然后减去所有奇数索引元素的总和.空列表的总和为0，因此它恰好处理了长度为0或1的列表，而没有专门针对这些情况的代码.
Take the sum of all the even indexed elements and subtract the sum of all the odd indexed elements. Empty lists sum to 0 so it coincidently handles lists of length 0 or 1 without code specifically for those cases.
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