如何在Python中找到位于奇数索引处的元素总和 [英] How to find the sum of elements located at odd indices in Python

问题描述

我知道以前有人在这里问过类似的问题，但我的略有不同.我想编写一个函数，它接受一个正整数列表并返回位于奇数索引处的元素的总和.问题是，我只想使用 for 循环或 while 循环.这是我到目前为止所拥有的:

I know a similar question has been asked here before, but mine is slightly different. I want to write a function that takes a list of positive integers and returns the sum of the elements located at the odd indices. The catch is, I want to use only a for loop or a while loop. Here's what I have so far:

def getSumOdds(aList):
for i in range(0, len(aList)):
    if i%2 == 0:
        pass
    if i%2 != 0:
        sum = sum + aList[i]
        return sum

但是，当我将此代码输入 Python 时，我收到一条错误消息，提示 builtins.UnboundLocalError: local variable 'sum' referenced before assignment.有谁知道找到总和的更好方法或如何修复错误消息?提前致谢.

However, when I enter this code into Python, I get an error saying builtins.UnboundLocalError: local variable 'sum' referenced before assignment. Does anyone know a better way to find the sum or how to fix the error message? Thanks in advance.

推荐答案

您可以使用生成器(或列表推导式):

You can use a generator (or list comprehension for that):

def getSumOdds(aList):
    return sum(aList[i] for i in range(1,len(aList),2))

(如果支持索引(list 支持这个)).

(given indexing is supported (a list supports this)).

或者你可以，比如 @MSeifert 说，使用切片:

or you can, like @MSeifert says, use slicing:

def getSumOdds(aList):
    return sum(aList[1::2])

(鉴于支持切片，并非所有集合都这样做)

(given slicing is supported, not all collections do this)

这就是你所需要的:sum 是一个内置函数.

that's all you need: sum is a builtin function.

如果 aList 是一个生成器(例如你不能访问 i-th 元素)，你可以使用 itertools.islice:

In case aList is a generator (you cannot access the i-th element for instance), you can use itertools.islice:

from itertools import islice

def getSumOdds(aList):
    return sum(islice(aList,1,None,2))

您也可以将此方法用于列表等(只要aList 是可迭代的).

You can also use this method for lists, etc. (as long as aList is iterable).

或者 - 再次支持给定索引 - 您可以将其放入 for 循环中:

Or - again given indexing is supported - you can put it in a for loop:

def getSumOdds(aList):
    result = 0
    for i in range(1,len(aList),2):
        result += aList[i]
    return result

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