从交替的侧面循环列表
问题描述
给定一个列表
a = [0,1,2,3,4,5,6,7,8,9]
我怎样才能得到
b = [0,9,1,8,2,7,3,6,4,5]
也就是说,产生一个新的列表,其中每个连续的元素都是从原始列表的两侧交替取出的?
解决方案>>>[a[-i//2] if i % 2 else a[i//2] for i in range(len(a))][0, 9, 1, 8, 2, 7, 3, 6, 4, 5]
解释:
此代码从 a<的开头 (
p>a[i//2]
) 和结尾 (a[-i//2]
) 选择数字/code>,交替(if i%2 else
).总共选择了 len(a)
个数字,因此即使 len(a)
是奇数,这也不会产生不良影响.[-i//2 for i in range(len(a))]
产生 0, -1, -1, -2, -2, -3, -3, -4, -4, -5
,[ i//2 for i in range(len(a))]
产生 0, 0, 1, 1, 2, 2, 3, 3, 4, 4
,i%2
在 False
和 True
之间交替,
所以我们从 a
中提取的索引是:0, -1, 1, -2, 2, -3, 3, -4, 4, -5
.
我对 pythonicness 的评价:
这个单线的好处是它很短并且显示出对称性(+i//2
和 -i//2
).
但不好的是,这种对称性具有欺骗性:
有人可能会认为 -i//2
与 i//2
相同,只是符号翻转了.但在 Python 中,整数除法返回底数 的结果,而不是向零截断.所以 -1//2 == -1
.
此外,我发现通过索引访问列表元素的方式比迭代更少.
Given a list
a = [0,1,2,3,4,5,6,7,8,9]
how can I get
b = [0,9,1,8,2,7,3,6,4,5]
That is, produce a new list in which each successive element is alternately taken from the two sides of the original list?
解决方案>>> [a[-i//2] if i % 2 else a[i//2] for i in range(len(a))]
[0, 9, 1, 8, 2, 7, 3, 6, 4, 5]
Explanation:
This code picks numbers from the beginning (a[i//2]
) and from the end (a[-i//2]
) of a
, alternatingly (if i%2 else
). A total of len(a)
numbers are picked, so this produces no ill effects even if len(a)
is odd.
[-i//2 for i in range(len(a))]
yields 0, -1, -1, -2, -2, -3, -3, -4, -4, -5
,
[ i//2 for i in range(len(a))]
yields 0, 0, 1, 1, 2, 2, 3, 3, 4, 4
,
and i%2
alternates between False
and True
,
so the indices we extract from a
are: 0, -1, 1, -2, 2, -3, 3, -4, 4, -5
.
My assessment of pythonicness:
The nice thing about this one-liner is that it's short and shows symmetry (+i//2
and -i//2
).
The bad thing, though, is that this symmetry is deceptive:
One might think that -i//2
were the same as i//2
with the sign flipped. But in Python, integer division returns the floor of the result instead of truncating towards zero. So -1//2 == -1
.
Also, I find accessing list elements by index less pythonic than iteration.
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