按元素列表排序元组列表

I want to sort a list of tuples by their second elements.

我想按照第二个元素对元组列表进行排序。

Example input:

示例输入：

[("Bob",3),("Terry",1)]

Example output:

输出示例：

[("Terry",1)("Bob",3)]

3 个解决方案

#1

Another cool trick is to use on from Data.Function:

另一个很酷的技巧是从Data.Function使用on：

import Data.Function (on)
import Data.List (sortBy)

sortBy (compare `on` snd) [...]

Not much different than comparing but a nice trick from time to time.

与比较差别不大，但不时是一个很好的伎俩。

#2

You can use sortBy and comparing:

您可以使用sortBy并比较：

sortBy :: (a -> a -> Ordering) -> [a] -> [a]
comparing :: (Ord b) => (a -> b) -> a -> a -> Ordering

In this case, we want to compare by the second element. You can use comparing snd to get a function that can compare two tuples by their second element.

在这种情况下，我们想要通过第二个元素进行比较。你可以使用比较snd来获得一个可以通过第二个元素比较两个元组的函数。

#3

Consider a "regular" sort

考虑“常规”排序

sort xs = ... a < b ...

Such sorts must make use of compare, or its friends such as <. So if you have implemented such a thing already, then instead of just compare a b or a < b, you can instead do compare (snd a) (snd b) or snd a < snd b.

这种分类必须使用比较，或其朋友，如<。所以如果你已经实现了这样的事情，那么你可以改为比较（snd a）（snd b）或者snd a snd b而不是仅仅比较b或a 。

sort xs = ... snd a < snd b ...

Of course if you get smart, you'll abstract out the "accessor", and make it an additional input to the sorting function:

当然，如果你变得聪明，你将抽象出“访问者”，并使其成为排序功能的额外输入：

sortComparingOn f xs = ... f a < f b ...

You might even abstract out the comparator altogether:

你甚至可能完全抽象出比较器：

sortBy cmp xs = ... a `cmp` b ...

sortBy is provided in Data.List, as ehird mentioned.

sortBy中提供了sortBy，如第三篇所述。

#1