[英]Creating a Python list from a list of tuples
If I have, for example, a list of tuples such as 例如,如果我有一个元组列表,例如
a = [(1,2)] * 4
how would I create a list of the first element of each tuple? 我将如何创建每个元组的第一个元素的列表? That is,
[1, 1, 1, 1]
. 即
[1, 1, 1, 1]
。
Use a list comprehension : 使用列表理解 :
>>> a = [(1,2)] * 4
>>> [t[0] for t in a]
[1, 1, 1, 1]
You can also unpack the tuple: 您还可以打开元组的包装:
>>> [first for first,second in a]
[1, 1, 1, 1]
If you want to get fancy, combine map
and operator.itemgetter
. 如果您想花哨的话,请结合
map
和operator.itemgetter
。 In python 3, you'll have to wrap the construct in list
to get a list instead of an iterable: 在python 3中,您必须将构造包装在
list
以获得一个列表,而不是一个可迭代的列表:
>>> import operator
>>> map(operator.itemgetter(0), a)
<map object at 0x7f3971029290>
>>> list(map(operator.itemgetter(0), a))
[1, 1, 1, 1]
Two alternatives to phihag's list comprehension: phihag列表理解的两种替代方法:
[x for x, y in a]
from operator import itemgetter
map(itemgetter(0), a)
There are several ways: 有几种方法:
>>> a = [(1,2)] * 4
>>> # List comprehension
>>> [x for x, y in a]
[1, 1, 1, 1]
>>> # Map and lambda
>>> map(lambda t: t[0], a)
[1, 1, 1, 1]
>>> # Map and itemgetter
>>> import operator
>>> map(operator.itemgetter(0), a)
[1, 1, 1, 1]
The technique of using map fell out of favor when list comprehensions were introduced, but now it is making a comeback due to parallel map/reduce and multiprocessing techniques: 引入列表理解后,使用地图的技术就不受欢迎了,但是现在由于并行地图/归约和多处理技术而卷土重来:
>>> # Multi-threading approach
>>> from multiprocessing.pool import ThreadPool as Pool
>>> Pool(2).map(operator.itemgetter(0), a)
[1, 1, 1, 1]
>>> # Multiple processes approach
>>> from multiprocessing import Pool
>>> def first(t):
return t[0]
>>> Pool(2).map(first, a)
[1, 1, 1, 1]
a = [(1,2)] * 4
first_els = [x[0] for x in a]
Assuming you have a list of tuples: 假设您有一个元组列表:
lta = [(1,2), (2,3), (44,45), (37,38)]
access the first element of each tuple would involve subscripting with [0], and visiting each tuple to extract each first element would involve aa list comprehension, which can be assigned to a variable as shown below: 访问每个元组的第一个元素将涉及用[0]下标,并且访问每个元组以提取每个第一个元素将涉及一个列表理解,可以将其分配给一个变量,如下所示:
resultant_list = [element[0] for element in lta]
>>> resultant_list
[1, 2, 44, 37]
I recently found out about Python's zip()
function. 我最近发现了Python的
zip()
函数。 Another way to do what I want to do here is: 做我想在这里做的另一种方法是:
list( zip( *a )[0] )
tup_list = zip( list1, list2 )
interleaves two lists into a list of 2-tuples, but zip( *tup_list )
does the opposite, resulting in a list of a tuple of list1
and a tuple of list2
. tup_list = zip( list1, list2 )
两个列表交织为2个元组的列表,但是zip( *tup_list )
却相反,从而得到list1
元组和list2
元组的列表。
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