如何知道生成的序列最多是一定长度 [英] How to know a generated sequence is at most a certain length

问题描述

我想知道生成的序列是否少于2个条目.

I want to know whether a generated sequence has fewer than 2 entries.

>>> def sequence():
...     for i in xrange(secret):
...         yield i

我效率低下的方法是创建一个列表，并测量其长度:

My inefficient method is to create a list, and measure its length:

>>> secret = 5
>>> len(list(sequence())) < 2
True

很显然，这消耗了整个生成器.

Obviously, this consumes the whole generator.

在我的真实情况下，生成器可能正在穿越大型网络.我想进行检查而不消耗整个生成器，也不构建一个大型列表.

In my real case the generator could be traversing a large network. I want to do the check without consuming the whole generator, or building a large list.

在itertools文档中有一个食谱:

There's a recipe in the itertools documentation:

def take(n, iterable):
    "Return first n items of the iterable as a list"
    return list(islice(iterable, n))

这只会建立一个最大长度为n的列表，这更好.

This only builds a list of max length n, which is better.

所以我可以说:

>>> len(take(2, sequence()) < 2

是否还有一种更Python化，更有效的方式来做到这一点?

Is there an even more pythonic, efficient way to do it?

推荐答案

使用take的解决方案使用islice，构建一个列表并获取其长度:

The solution using take uses islice, builds a list and takes the length of it:

>>> from itertools import islice
>>> len(list(islice(sequence(), 2))
2

为避免创建列表，我们可以使用sum:

To avoid creating the list we can use sum:

>>> sum(1 for _ in islice(sequence(), 2)
2

这大约需要70％的时间:

This takes about 70% of the time:

>>> timeit('len(list(islice(xrange(1000), 2)))', 'from itertools import islice')
 1.089650974650752

>>> timeit('sum(1 for _ in islice(xrange(1000), 2))', 'from itertools import islice')
0.7579448552500647

包装:

>>> def at_most(n, elements):
...     return sum(1 for _ in islice(elements, n + 1)) <= n

>>> at_most(5, xrange(5))
True

>>> at_most(2, xrange(5))
False

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