查找第一个匹配条件的序列项 [英] Find first sequence item that matches a criterion

问题描述

查找/返回符合特定条件的第一个列表项的最优雅、最有效的方法是什么?

What would be the most elegant and efficient way of finding/returning the first list item that matches a certain criterion?

例如，如果我有一个对象列表，我想获取那些具有 obj.val==5 属性的对象中的第一个对象.我当然可以使用列表理解，但这会导致 O(n)，如果 n 很大，那就太浪费了.一旦满足条件，我也可以使用带有 break 的循环，但我认为可能有更pythonic/优雅的解决方案.

For example, if I have a list of objects and I would like to get the first object of those with attribute obj.val==5. I could of course use list comprehension, but that would incur O(n) and if n is large, it's wasteful. I could also use a loop with break once the criterion was met, but I thought there could be a more pythonic/elegant solution.

推荐答案

如果你的对象没有任何其他索引或排序信息，那么你将不得不迭代直到找到这样的对象:

If you don't have any other indexes or sorted information for your objects, then you will have to iterate until such an object is found:

next(obj for obj in objs if obj.val == 5)

然而，这比完整的列表理解要快.比较这两个:

This is however faster than a complete list comprehension. Compare these two:

[i for i in xrange(100000) if i == 1000][0]

next(i for i in xrange(100000) if i == 1000)

第一个需要 5.75ms，第二个需要 58.3µs(快 100 倍，因为循环短了 100 倍).

The first one needs 5.75ms, the second one 58.3µs (100 times faster because the loop 100 times shorter).

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