如何从现有列表列表中创建第一个元素的新列表 [英] How to make a new list of first elements from existing list of lists

问题描述

我在下面有一个列表.我需要使用它来创建仅包含国家/地区名称的新列表.如何循环 x 以获得国家名称列表?

I have a list below. I need to use it to create a new list with only country names. How do I loop x in order to have a list of country names?

x = [["UK", "LONDON", "EUROPE"],
     ["US", "WASHINGTON", "AMERICA"],
     ["EG", "CAIRO", "AFRICA"],
     ["JP", "TOKYO", "ASIA"]]

结果应该像

UK
US
EG
JP

推荐答案

您有两种方法

使用for循环

countries = []

for e in x:
 countries.append(e[0])

或具有列表理解功能，在大多数情况下，这是更好的选择

or with list comprehensions, which would be in most cases the better option

countries = [e[0] for e in x]

此外，如果您的数据源是一个生成器(在这种情况下不是)，或者如果您正在对每个元素进行一些昂贵的处理(在这种情况下也不是)，则可以使用 generator表达式，方法是更改​​方括号()

Furthermore, if your data source is a generator (which it isn't in this case), or if you're doing some expensive processing on each element (not this case either), you could use a generator expression by changing the square brackets [] for parenthesis ()

countries = (e[0] for e in x)

这将按需计算元素，并且如果数据源太长或与列表理解相比，生成器也将减少内存占用.

This will compute on demand the elements, and if the data source is too long or a generator will also reduce the memory footprint compared to a list comprehension.

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