[英]How to repeat each of a Python list's elements n times with itertools only?
I have a list with numbers: numbers = [1, 2, 3, 4]
. 我有一个数字列表:
numbers = [1, 2, 3, 4]
。
I would like to have a list where they repeat n
times like so (for n = 3
): 我想有一个列表,他们重复这样的
n
次(对于n = 3
):
[1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4]
. [1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4]
。
The problem is that I would like to only use itertools
for this, since I am very constrained in performance. 问题是我只想使用
itertools
,因为我的性能非常受限制。
I tried to use this expression: 我试着用这个表达式:
list(itertools.chain.from_iterable(itertools.repeat(numbers, 3)))
But it gives me this kind of result: 但它给了我这样的结果:
[1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4]
which is obviously not what I need. 这显然不是我需要的。
Is there a way to do this with itertools
only, without using sorting, loops and list comprehensions? 有没有办法只使用
itertools
,而不使用排序,循环和列表推导? The closest I could get is: 我能得到的最接近的是:
list(itertools.chain.from_iterable([itertools.repeat(i, 3) for i in numbers]))
, list(itertools.chain.from_iterable([itertools.repeat(i, 3) for i in numbers]))
,
but it also uses list comprehension, which I would like to avoid. 但它也使用列表理解,我想避免。
Since you don't want to use list comprehension, following is a pure (+ zip
) itertools
method to do it - 由于您不想使用列表推导,以下是一个纯(+
zip
) itertools
方法来执行此操作 -
from itertools import chain, repeat
list(chain.from_iterable(zip(*repeat(numbers, 3))))
# [1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4]
Firstly, using functions from itertools
won't necessarily be faster than a list comprehension — you should benchmark both approaches. 首先,使用来自
itertools
函数不一定比列表理解更快 - 你应该对这两种方法进行基准测试。 (In fact, on my machine it's the opposite). (事实上,在我的机器上它是相反的)。
Pure list comprehension approach: 纯列表理解方法:
>>> numbers = [1, 2, 3, 4]
>>> [y for x in numbers for y in (x,)*3]
[1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4]
Using chain.from_iterable()
with a generator expression : 将
chain.from_iterable()
与生成器表达式一起使用 :
>>> from itertools import chain, repeat
>>> list(chain.from_iterable(repeat(n, 3) for n in numbers))
[1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4]
Think you were very close, just rewriting the comprehension to a generator: 认为你非常接近,只是重写对生成器的理解:
n = 3
numbers = [1, 2, 3, 4]
list(itertools.chain.from_iterable((itertools.repeat(i, n) for i in numbers)))
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